Structure and function of Cerebratulus lacteus neurotoxin B-IV: tryptophan-30 is critical for function while lysines-18, -19, -29, and -33 are not required

The Cerebratulus lacteus B-toxins are a family of polypeptide neurotoxins known to bind to crustacean voltage-sensitive sodium channels. We have previously shown that in the most abundant homolog, toxin B-IV, Arg-17 in the N-terminal helix and a positive charge at position 25 in the loop region are essential for function. In this report, we target a tryptophan residue at position 30, as well as lysine residues found in both the N-terminal helix and loop regions by polymerase chain reaction mutagenesis, to determine their contributions to toxin activity. Substitution of Trp-30 with a serine causes a more than 40-fold reduction in specific toxicity, whereas replacement by tyrosine and phenylalanine is well tolerated. The secondary structures of both these muteins are identical to that of the wild-type toxin as determined by circular dichroism spectroscopy. Thermal denaturation experiments also show that their conformational stabilities are intact. These results demonstrate that an aromatic residue at this position is required for toxin function. Charge neutralizing substitutions of Lys-18 and Lys-19 located in the N-terminal helix have very little effect on toxicity, suggesting the nonessentiality of these residues. Similar results are also obtained for the charge neutralizing muteins for Lys-29 and Lys-33 in the loop region. Interestingly, reduction experiments demonstrate that both K29N and W30S are more sensitive to reducing agent than wild-type B-IV, raising the possibility that the loop sequence may modulate toxin stability.     

Charge pair interactions stabilizing ferredoxin-ferredoxin reductase complexes. Identification by complementary site-specific mutations

Ferredoxin reductase (Fd-reductase) supplies electrons to mitochondrial steroid hydroxylase cytochrome P450 enzymes via a [2Fe-2S] ferredoxin. Chemical labeling studies with bovine Fd-reductase have implicated Lys-243 as important in binding to bovine ferredoxin (Hamamoto, I., Kazutaka, K., Tanaka, S., and Ichikawa, Y. (1988) Biochim. Biophys. Acta 953, 207-213). We have used site-directed mutagenesis to examine the role of charged residues in this region of human Fd-reductase in ferredoxin binding. Mutant proteins were expressed in Escherichia coli and were assayed for activity by ferredoxin-mediated electron transfer to cytochrome c. Replacement of Lys-242 (homologous to Lys-243 in bovine Fd-reductase) with Gln and replacement of Arg-241 with Ser had little effect (2.7- and 3.6-fold increased Km, respectively). In contrast, mutants at positions 239 and 243 (R239S and R243Q) exhibited markedly lower affinity for ferredoxin (17.5- and 1,600-fold increased Km, respectively). Studies were also carried out with two ferredoxin charge mutants shown previously to have lowered affinity for Fd-reductase (Coghlan, V. M., and Vickery, L. E. (1991) J. Biol. Chem. 266, 18606-18612). Comparisons of the binding of ferredoxin mutants D76N and D79N to Fd-reductase mutants R239S and R243Q suggest that Arg-239 and Arg-243 of Fd-reductase each interact directly with both Asp-76 and Asp-79 of ferredoxin during formation of the complex between the two proteins. These results support the hypothesis that specific electrostatic interactions involving this region are important in stabilizing the ferredoxin-Fd-reductase complex.     

Cysteine-scanning mutagenesis of helix IV and the adjoining loops in the lactose permease of Escherichia coli: Glu126 and Arg144 are essential. off

Cys-scanning mutagenesis has been applied to the remaining 45 residues in lactose permease that have not been mutagenized previously (from Gln100 to Arg144 which comprise helix IV and adjoining loops). Of the 45 single-Cys mutants, 26 accumulate lactose to > 75% of the steady state observed with Cys-less permease, and 14 mutants exhibit lower but significant levels of accumulation (35-65% of Cys-less permease). Permease with Phe140-->Cys or Lys131-->Cys exhibits low activity (15-20% of Cys-less permease), while mutants Gly115-->Cys, Glu126-->Cys and Arg144-->Cys are completely unable to accumulate the dissacharide. However, Cys-less permease with Ala or Pro in place of Gly115 is highly active, and replacement of Lys131 or Phe140 with Cys in wild-type permease has a less deleterious effect on activity. In contrast, mutant Glu126-->Cys or Arg144-->Cys is inactive with respect to both uphill and downhill transport in either Cys-less or wild-type permease. Furthermore, mutants Glu126-->Ala or Gln and Arg144-->Ala or Gln are also inactive in both backgrounds, and activity is not rescued by double neutral replacements or inversion of the charged residues at these positions. Finally, a mutant with Lys in place of Arg144 accumulates lactose to about 25% of the steady state of wild-type, but at a slow rate. Replacement of Glu126 with Asp, in contrast, has relatively little effect on activity. None of the effects can be attributed to decreased expression of the mutants, as judged by immunoblot analysis. Although the activity of most of the single-Cys mutants is unaffected by N-ethylmaleimide, Cys replacement at three positions (Ala127, Val132, or Phe138) renders the permease highly sensitive to alkylation. The results indicate that the cytoplasmic loop between helices IV and V, where insertional mutagenesis has little effect on activity [McKenna, E., et al. (1992) Proc. Natl. Acad. Sci. U.S.A. 89, 11954-11958], contains residues that play an important role in permease activity and that a carboxyl group at position 126 and a positive charge at position 144 are absolutely required.     

Structure of neurotoxin B-IV from the marine worm Cerebratulus lacteus: a helical hairpin cross-linked by disulphide bonding

B-IV is a 55-residue, crustacean-selective, neurotoxin secreted by Cerebratulus lacteus, a large marine worm found along the northeastern coast of North America. The 3D structure of this molecule in aqueous solution has been determined by 1H NMR spectroscopy at 600 MHz. The molecule has a well-defined helical hairpin structure, with the branches of the hairpin linked by four disulphide bonds. The disulphide connectivities were established from the NMR data to be 1-8/2-7/3-6/4-5, which differed from those determined previously by chemical means, where 1-7 and 2-8 connectivities were found. Each branch of the hairpin is largely alpha-helical, with the helices in the N and C-terminal branches encompassing residues 11 to 23 and 34 to 49, respectively. The loop connecting the branches of the hairpin contains two inverse gamma-turns centred on residues 24 and 25, a type I beta-turn at residues 28 to 31 and a type II beta-turn at residues 30 to 33. Arg17, -25 and -34, which are important for activity, are all on the same face of the molecule, while Trp30, which is also important for activity, is on the opposite face. Structure comparisons show that the B-IV structure is quite similar to those of Rop (ColE1 repressor of primer) and the heat-stable enterotoxin B from Escherichia coli. These structural similarities are discussed in relation to possible mechanisms of action of B-IV.     

Structure and function in rhodopsin: rhodopsin mutants with a neutral amino acid at E134 have a partially activated conformation in the dark state

The Glu-134-Arg-135 residues in rhodopsin, located near the cytoplasmic end of the C helix, are involved in G protein binding, or activation, or both. Furthermore, the charge-neutralizing mutation Glu-134 to Gln-134 produces hyperactivity in the activated state and produces constitutive activity in opsin. The Glu/Asp-Arg charge pair is highly conserved in equivalent positions in other G protein-coupled receptors. To investigate the structural consequences of charge-neutralizing mutations at Glu-134 and Arg-135 in rhodopsin, single spin-labeled side chains were introduced at sites in the cytoplasmic domains of helices C (140), E (227), F (250), or G (316) to serve as "molecular sensors" of the local helix bundle conformation. In each of the spin-labeled rhodopsins, a Gln substitution was introduced at either Glu-134 or Arg-135, and the electron paramagnetic resonance spectrum of the spin label was used to monitor the structural response of the helix bundle. The results indicate that a Gln substitution at Glu-134 induces a photoactivated conformation around helices C and G even in the dark state, an observation of potential relevance to the hyperactivity and constitutive activity of the mutant. In contrast, little change is induced in helix F, which has been shown to undergo a dominant motion upon photoactivation. This result implies that the multiple helix motions accompanying photoactivation are not strongly coupled and can be induced to take place independently. Gln substitution at Arg-135 produces only minor structural changes in the dark- or light-activated conformation, suggesting that this residue is not a determinant of structure in the regions investigated, although it may be functionally important.     

Identification of a residue involved in transition-state stabilization in the ATPase reaction of DNA gyrase

Examination of the X-ray crystal structure of the 43 kDa N-terminal domain of the DNA gyrase B protein (GyrB) shows that the majority of the interactions with bound ATP are made with subdomain 1 (residues 2-220). However, two residues from subdomain 2, Gln335 and Lys337, interact with the gamma-phosphate of ATP. The proposed roles for these residues include nucleotide binding, transition-state stabilization, and triggering protein conformational changes. We have used site-directed mutagenesis to convert Gln335 to Asn and Ala and Lys337 to Gln and Ala in the N-terminal domain of GyrB. Two of the resultant mutant proteins, GyrB43(Q335A) and GyrB43(K337Q), were shown to be correctly folded, and their interactions with ATP have been analyzed in detail. The Q335A protein is apparently unchanged with regard to nucleotide binding and hydrolysis, whereas the K337Q protein shows a modest decrease in nucleotide binding and a drastic reduction in ATPase activity. This is manifested by a approximately 10(3)-fold decrease in kcat. When the two mutations were moved into full-length GyrB, the Q335A mutation again showed little or no effect on activity, whereas the K337Q mutation had undetectable supercoiling and ATPase activities. We conclude that Gln335 is dispensable for ATP binding and hydrolysis by the gyrase B protein, whereas Lys337 has a critical role in the ATPase reaction and is likely to be a key residue in transition-state stabilization.     

Roles of conserved arginine residues in the metal-tetracycline/H+ antiporter of Escherichia coli

Seven arginine residues are conserved in all the tetracycline/H+ antiporters of Gram-negative bacteria. Four (Arg67, -70, -71, and -127) of them are located in the putative cytoplasmic loop regions and three (Arg31, -101, and -238) in the putative periplasmic loop regions [Eckert, B., and Beck, C. F. (1989) J. Biol. Chem. 264, 11663-11670]. These arginine residues were replaced by alanine, lysine, or cysteine one by one through site-directed mutagenesis. None of the mutants showed significant alteration of the protein expression level. The mutants resulting in the replacement of Arg31, Arg67, Arg71, and Arg238 with either Ala, Cys, or Lys retained tetracycline resistance levels comparable to that of the wild type. Among them, only the Arg238 --> Ala mutant showed very low transport activity in everted membrane vesicles, probably due to the instability of the mutant protein. The replacement of Arg70 and Arg127 with Ala or Cys resulted in a drastic decrease in the drug resistance and almost complete loss of the transport activity, while the Lys replacement mutants retained significant resistance and transport activity, indicating that the positively charged side chains at these positions conferred the transport function. On the other hand, neither the Ala, Cys, nor Lys replacement mutant of Arg101 exhibited any drug resistance or transport activity. As for the reactivity of the Cys replacement mutants, only two (Arg71 --> Cys and Arg101 --> Cys) were not reactive with NEM, the other five mutants being highly or moderately reactive. The reactivity of the cysteine-scanning mutants around Arg101 with NEM revealed that Arg101 is located in transmembrane helix IV. It is not likely that Arg101 confers the protein folding through a salt bridge with a transmembrane acidic residue because no double mutants involving Arg101 --> Ala and the replacement of one of three transmembrane acidic residues (Asp15, Asp84, and Asp285) showed the recovery of any tetracycline resistance or transport activity. The effect of tetracycline on the [14C]NEM binding to the combined mutants S65C/R101A and L97C/R101A suggests that Arg101 may cause a substrate-induced conformational change of the putative exit gate of TetA(B).     

Cysteine-scanning mutagenesis around transmembrane segment III of Tn10-encoded metal-tetracycline/H+ antiporter

Each amino acid in the putative transmembrane helix III and its flanking regions (from Gly-62 to Tyr-98) of the Tn10-encoded metal-tetracycline/H+ antiporter (Tet(B)) was individually replaced with Cys. Out of these 37 cysteine-scanning mutants, the mutants from G62C to R70C and from S92C to Y98C showed high or intermediate reactivity with [14C]N-ethylmaleimide (NEM) except for the M64C mutant. On the other hand, the mutants from R71C to S91C showed almost no reactivity with NEM except for the P72C mutant. These results confirm that the transmembrane helix III is composed of 21 residues from Arg-71 to Ser-91. The majority of Cys replacement mutants retained high or moderate tetracycline transport activity. Cys replacements for Gly-62, Asp-66, Ser-77, Gly-80, and Asp-84 resulted in almost inactive Tet(B) (less than 3% of the wild-type activity). The Arg-70 --> Cys mutant retained very low activity due to a mercaptide between Co2+ and a SH group (Someya, Y., and Yamaguchi, A. (1996) Biochemistry 35, 9385-9391). Three of these six important residues (Ser-77, Gly-80, and Asp-84) are located in the transmembrane helix III and one (Arg-70) is located in the flanking region. These four functionally important residues are located on one side of the helical wheel. Only two of the residual 31 Cys mutants were inactivated by NEM (S65C and L97C). Ser-65 and Leu-97 are located on the cytoplasmic and periplasmic loops, respectively, in the topology of Tet(B). The degree of inactivation of these Cys mutants with SH reagents was dependent on the volume of substituents. In the presence of tetracycline, the reactivity of the S65C mutant with NEM was significantly increased, in contrast, the reactivity of L97C was greatly reduced, indicating that the cytoplasmic and periplasmic loop regions undergo substrate-induced conformational change in the mutually opposite direction.     

NMDA-receptor antagonist requirements in conantokin-G

A series of variants of the neuroactive 17-residue gamma-carboxyglutamate-(Gla)-containing polypeptide, conantokin-G (con-G), were synthesized with the intention of determining those features that were important for its N-methyl-D-aspartate (NMDA) receptor-targeted antagonist activity and for adoption of its divalent cation-dependent alpha-helical conformation. Employing the binding of [3H]dizolcipine (MK-801) as an assay for open receptor ion channels in rat brain membranes, which displays inhibition by con-G (IC50 = 0.48 microM), it was found that replacement by an Ala residue of Gla4 led to complete inactivation of the peptide, whereas a similar replacement of Gla3 resulted in a 20-fold decreased potency. Ala substitutions for Gla10 and Gla14 did not substantially affect [3H]MK-801 binding. This same substitution at Gla7 appeared to slightly enhance binding. Ala replacements of non-Gla residues demonstrated that four of them, viz. Glu2, Leu5, Gln9, and Ile12, possessed at least 200-fold decreases in inhibitory potency, whereas similar replacements at Gly1, Leu11, and Arg13 resulted in peptides with 8- to 12-fold increases in the IC50 values. The remaining amino acid residues tested in the single Ala replacement series showed no significant changes in the inhibitory characteristics of wild-type con-G. Additional studies with carboxyl-terminal truncated peptides revealed that the carboxyl-terminal 4 amino acids were unimportant for this activity. There was no strict correlation of inhibition of [3H]MK-801 binding with the ability of these peptides to form cation-dependent alpha-helices. Peptides with notably low alpha-helical content in the presence of these cations were lacking at least one, or both, of Gla10 and Gla14. Con-G[Gla3,4,7,10,14E] and con-G[Gla7,10,14E] were the only peptides that remained in a completely random conformation upon metal ion addition.     

The substrate-binding site in the lactose permease of Escherichia coli

Site-directed N-ethylmaleimide labeling was studied with Glu-126 and/or Arg-144 mutants in lactose permease containing a single, native Cys residue at position 148 in the substrate-binding site. Replacement of either Glu-126 or Arg-144 with Ala markedly decreases Cys-148 reactivity, whereas interchanging the residues, double-Ala replacement, or replacement of Arg-144 with Lys or His does not alter reactivity, indicating that Glu-126 and Arg-144 are charge-paired. Importantly, although alkylation of Cys-148 is blocked by ligand in wild-type permease, no protection whatsoever is observed with any of the Glu-126 or Arg-144 mutants. Site-directed fluorescence with 2-(4-maleimidoanilino)-naphthalene-6-sulfonic acid (MIANS) in mutant Val-331 --> Cys was also studied. In marked contrast to Val-331 --> Cys permease, ligand does not alter MIANS reactivity in mutant Glu-126 --> Ala/Val-331 --> Cys, Arg-144 --> Ala/Val-331 --> Cys, or Arg-144 --> Lys/Val-331 --> Cys and does not cause either quenching or a shift in the emission maximum of the MIANS-labeled mutants. However, mutation Glu-126 --> Ala or Arg-144 --> Ala and, to a lesser extent, Arg-144 --> Lys cause a red-shift in the emission spectrum and render the fluorophore more accessible to I-. The results demonstrate that Glu-126 and Arg-144 are irreplaceable for substrate binding and suggest a model for the substrate-binding site in the permease. In addition, the findings are consistent with the notion that alterations in the substrate translocation pathway at the interface between helices IV and V are transmitted conformationally to the H+ translocation pathway at the interface between helices IX and X.     

Low-level laser therapy in osteoarticular diseases in geriatric patients]

The formation of the antibody variable domain binding unit (Fv) is the net result of three competing assembly reactions. The affinities of concurrent homologous interactions of heavy and light chain variable domains limits the heterologous interaction leading to productive formation of the Fv. To address the possible role of light chain dimerization in this phenomenon, the Gln38 residue at the dimer interface of an immunoglobulin light chain variable domain (VL) was replaced by charged amino acids. The effects of these mutations on VL homodimer formation were monitored by small-zone size exclusion HPLC and the affinities of interaction were determined by computer simulation. Reduced VL homodimerization was observed in three of the four mutants, Q38R, Q38D and Q38K. The association constants for the Q38R and Q38D homodimers were 1.2 x 10(4) and 3.2 x 10(3) M(-1), respectively. This corresponded to a 20-75-fold reduction in the homodimer association constant relative to the wild-type VL, which had an association constant of 2.4 x 10(5) M(-1). Surprisingly, the fourth charge mutant, Q38E, had a higher association constant than the wild-type VL. The potential for charged residues to facilitate heterodimeric assembly of immunoglobulin domains was also tested. Heterodimerization was observed between the Q38D and Q38R V(L)s, but with an association constant of 4.7 x 10(4) M(-1), approximately fivefold lower than that obtained for homodimerization of the native V(L). In addition, replacement of the neutral, solvent-accessible Gln38 residue with either Asp or Arg was found to be significantly destabilizing. These results suggest that charged residues could be introduced at immunoglobulin domain interfaces to guide heterodimer formation and to minimize unfavorable competing homologous associations. Nonetheless, these apparently simple modifications may also result in unintended consequences that are likely to depend upon structural features of particular variable domains.     

Definition of optimal substrate recognition motifs of Ca2+-calmodulin-dependent protein kinases IV and II reveals shared and distinctive features

The substrate recognition determinants of Ca2+-calmodulin-dependent protein kinase (CaMK) IV and CaMKIIalpha were investigated using peptide substrates modeled on the amino acid sequence encompassing Ser-9 of synapsin I. For both kinases, hydrophobic residues (Leu or Phe) at the -5 position, are well tolerated, whereas non-hydrophobic residues (Arg, Ala, or Asp) decrease Vmax/Km by 55- to >4000-fold. At the -3 position, substitution of Ala for Arg leads to decreases of 99- and 343- fold in Vmax/Km for CaMKIV and CaMKIIalpha, respectively. For both kinases, the nature of the residues occupying the -4, -1, and + 4 positions exerts relatively little influence on phosphorylation kinetics. CaMKIV and CaMKIIalpha respond differently to substitutions at the -2 and +1 positions. Substitution of Arg at the -2 position with non-basic residues (Gln or Ala) leads to 6-fold decreases in Vmax/Km for CaMKIV, but 17-28-fold increases for CaMKIIalpha. Additionally, peptides containing Leu, Asp, or Ala at the +1 position are phosphorylated with similar efficiencies by CaMKIV, whereas the Leu-substituted peptide is preferred by CaMKIIalpha (by a factor of 5.8-9.7-fold). Thus, CaMKIV and CaMKIIalpha preferentially phosphorylate substrates with the motifs: Hyd-X-Arg-X-X-Ser*/Thr*, and Hyd-X-Arg-NB-X-Ser*/Thr*-Hyd, respectively, where Hyd represents a hydrophobic, X any, and NB a non-basic amino acid residue. The different specificities of the two kinases may contribute to their targeting to distinct physiological substrates during Ca2+-dependent cellular events.     

Human sulfite oxidase R160Q: identification of the mutation in a sulfite oxidase-deficient patient and expression and characterization of the mutant enzyme

Sulfite oxidase catalyzes the terminal reaction in the degradation of sulfur amino acids. Genetic deficiency of sulfite oxidase results in neurological abnormalities and often leads to death at an early age. The mutation in the sulfite oxidase gene responsible for sulfite oxidase deficiency in a 5-year-old girl was identified by sequence analysis of cDNA obtained from fibroblast mRNA to be a guanine to adenine transition at nucleotide 479 resulting in the amino acid substitution of Arg-160 to Gln. Recombinant protein containing the R160Q mutation was expressed in Escherichia coli, purified, and characterized. The mutant protein contained its full complement of molybdenum and heme, but exhibited 2% of native activity under standard assay conditions. Absorption spectroscopy of the isolated molybdenum domains of native sulfite oxidase and of the R160Q mutant showed significant differences in the 480- and 350-nm absorption bands, suggestive of altered geometry at the molybdenum center. Kinetic analysis of the R160Q protein showed an increase in Km for sulfite combined with a decrease in kcat resulting in a decrease of nearly 1,000-fold in the apparent second-order rate constant kcat/Km. Kinetic parameters for the in vitro generated R160K mutant were found to be intermediate in value between those of the native protein and the R160Q mutant. Native sulfite oxidase was rapidly inactivated by phenylglyoxal, yielding a modified protein with kinetic parameters mimicking those of the R160Q mutant. It is proposed that Arg-160 attracts the anionic substrate sulfite to the binding site near the molybdenum.     

Effects of Asp-369 and Arg-372 mutations on heme environment and function in human endothelial nitric-oxide synthase

Eight polar amino acid residues in the putative substrate-binding region from Thr-360 to Val-379 in human endothelial nitric-oxide synthase (eNOS) (Thr-360, Arg-365, Cys-368, Asp-369, Arg-372, Tyr-373, Glu-377, and Asp-378) were individually mutated. Only two of these residues, Asp-369 and Arg-372, were found to be essential for enzyme activity. A further series of mutants was generated by replacing these two residues with various amino acids and the mutant proteins were expressed in a baculovirus system. Mutant eNOS had a very low L-citrulline formation activity with the exception of D369E and R372K, which retained 27% and 44% of the wild-type enzyme activity, respectively. Unlike the wild-type enzyme, all mutants except D369E, R372K, and R372M had a low spin heme (Soret peak at 416 nm). All the Asp-369 mutants had higher Kd values for L-arginine (1-10 mM) than wild-type eNOS (0.4 microM) and an unstable heme-CO complex, and except for D369E, had a very low (6R)-5,6,7, 8-tetrahydro-L-biopterin (BH4) content. In contrast, each of Arg-372 mutants retained a considerable amount of BH4, had a moderate reduction in L-arginine affinity, and had a more stable heme-CO complex. 1-Phenylimidazole did not bind to wild-type eNOS heme, but bound to all Asp-369 and Arg-372 mutants (Kd ranged from 10 to 65 microM) except R372K. Heme spin-state changes caused by binding of 3, 5-lutidine appeared to depend on both charge and size of the side chains of residues 369 and 372. Furthermore, all Asp-369 and Arg-372 mutants were defective in dimer formation. These results suggest that residues Asp-369 and Arg-372 in eNOS play a critical role in oxygenase domain active-site structure and activity.     

Identification of a polar region in transmembrane domain 6 that regulates the function of the G protein-coupled alpha-factor receptor

The alpha-factor pheromone receptor (Ste2p) of the yeast Saccharomyces cerevisiae belongs to the family of G protein-coupled receptors that contain seven transmembrane domains (TMDs). Because polar residues can influence receptor structure by forming intramolecular contacts between TMDs, we tested the role of the five polar amino acids in TMD6 of the alpha-factor receptor by mutating these residues to nonpolar leucine. Interestingly, a subset of these mutants showed increased affinity for ligand and constitutive receptor activity. The mutation of the most polar residue, Q253L, resulted in 25-fold increased affinity and a 5-fold-higher basal level of signaling that was equal to about 19% of the alpha-factor induced maximum signal. Mutation of the adjacent residue, S254L, caused weaker constitutive activity and a 5-fold increase in affinity. Comparison of nine different mutations affecting Ser254 showed that an S254F mutation caused higher constitutive activity, suggesting that a large hydrophobic amino acid residue at position 254 alters transmembrane helix packing. Thus, these studies indicate that Gln253 and Ser254 are likely to be involved in intramolecular interactions with other TMDs. Furthermore, Gln253 and Ser254 fall on one side of the transmembrane helix that is on the opposite side from residues that do not cause constitutive activity when mutated. These results suggest that Gln253 and Ser254 face inward toward the other TMDs and thus provide the first experimental evidence to suggest the orientation of a TMD in this receptor. Consistent with this, we identified two residues in TMD7 (Ser288 and Ser292) that are potential contact residues for Gln253 because mutations affecting these residues also cause constitutive activity. Altogether, these results identify a new domain of the alpha-factor receptor that regulates its ability to enter the activated conformation.     

Role for Pro-13 in directing high-affinity binding of anthopleurin B to the voltage-sensitive sodium channel

Anthopleurin A (ApA) and B (ApB) are 49-amino acid polypeptide toxins from the Pacific sea anemone Anthopleura xanthogrammica that interfere with inactivation of voltage-gated sodium channels. ApA, which differs from ApB in seven of the 49 amino acids, displays markedly enhanced isoform selectivity compared with ApB, acting preferentially on cardiac over neuronal sodium channels. Previous studies in this lab have indicated the importance of two unique charged residues in ApB, Arg-12 and Lys-49, in this toxin's ability to discriminate between neuronal and cardiac sodium channels. Likewise, a double mutant (R12S/K49Q) recently characterized in this lab (Khera et al., 1995) displays a greatly reduced affinity for neuronal channels, essentially restoring the discriminatory ability of ApA. When the remaining five residues unique to ApB are individually converted to those of ApA, only ApB (Pro-13) shows a major effect, reducing the affinity of the new mutant toxin (P13V) against both channel isoforms approximately 10-fold. This effect is most likely the result of a conformational rearrangement within the surrounding cationic cluster which includes Arg-12 and -14, as well as Lys-49. However, when placed into the context of the double mutant R12S/K49Q a unique effect is observed: the new triple mutant (R12S/P13V/K49Q) is no longer able to discriminate effectively between channel isoforms. Its affinity for the neuronal sodium channel is significantly enhanced compared to either P13V or to the double mutant R12S/K49Q. These results are consistent both with our proposed model (Khera et al., 1995) and with the recently reported solution structure of ApB, which implicate the cationic cluster in both affinity and channel isoform selectivity. We suggest that the P13V mutation results in a shift in the relative orientation of cationic residues within the large flexible loop between residues 9-18, thus strengthening their interactions with target sequences of the neuronal sodium channel.     

Mutational analysis of invariant arginines in the IIAB(Man) subunit of the Escherichia coli phosphotransferase system

The mannose transporter of bacterial phosphotransferase system mediates uptake of mannose, glucose, and related hexoses by a mechanism that couples translocation with phosphorylation of the substrate. It consists of the transmembrane IIC(Man)-IID(Man) complex and the cytoplasmic IIAB(Man) subunit. IIAB(Man) has two flexibly linked domains, IIA(Man) and IIB(Man), each containing a phosphorylation site (His-10 and His-175). Phosphoryl groups are transferred from the phosphoryl carrier protein phospho-HPr to His-10, hence to His-175 and finally to the 6' OH of the transported hexose. Phosphate-binding sites and phosphate-catalytic sites frequently contain arginines, which by their guanidino group can stabilize phosphate through hydrogen bonding and electrostatic interactions. IIB(Man) contains five arginines which are invariant in the homologous IIB subunits of Escherichia coli, Klebsiella pneumoniae and Bacillus subtilis. The IIA domains have no conserved arginines. The five arginines were replaced by Lys or Gln one at a time, and the mutants were analyzed for transport and phosphorylation activity. All five IIB mutants can still be phosphorylated at His-175 by the IIA domain. R172Q is completely inactive with respect to glucose phosphotransferase (phosphoryltransfer from His-175 to the 6' OH of Glc) and hexose transport activity. R168Q has no hexose transport and strongly reduced phosphotransferase activity. R204K has no transport but almost normal phosphotransferase activity. R304Q has only slightly reduced transport activity. R190K behaves like wild-type IIAB(Man). Arg-168, Arg-172, and Arg-304 are part of the hydrogen bonding network on the surface of IIB, which contains the active site His-175 and the interface with the IIA domain (Schauder, S., Nunn, R.S., Lanz, R., Erni, B. and Schirmer, T. (1998) J. Mol. Biol. 276, 591-602) (Protein Data Bank accession code 1BLE). Arg-204 is at the putative interface between IIB(Man) and the IIC(Man)-IID(Man) complex.     

Human lysosomal acid lipase/cholesteryl ester hydrolase and human gastric lipase: site-directed mutagenesis of Cys227 and Cys236 results in substrate-dependent reduction of enzymatic activity

Chemical modification studies and site-directed mutagenesis experiments have provided evidence that human lysosomal acid lipase/cholesteryl ester hydrolase (HLAL), human gastric lipase (HGL), and rat lingual lipase (RLL) are serine esterases. Loss of HLAL and HGL activity was also observed in the presence of sulfhydryl-reactive substances, suggesting that cysteines are likewise essential for substrate hydrolysis. To study the functional role of the HLAL and HGL cysteine residues, we replaced these amino acids with alanine by site-directed mutagenesis. Substitutions at positions 227 and 236, alone or together, drastically reduced hydrolytic activity in a substrate-dependent manner while the other mutants were not affected to any great extent. HLAL(Cys227-->Ala), HLAL(Cys236-->Ala), and HLAL(Cys227-->Ala, Cys236-->Ala) were essentially inactive against cholesteryl oleate, but retained about 23-39%, 28-37%, and 13-17% of catalytic activity for both triolein and tributyrin, respectively. The data obtained with the corresponding HGL mutants confirmed the importance of residues 227 and 236 in maintaining enzymatic activity towards long- and short-chain triglycerides. In order to assess the contribution of the eight amino acids delimited by Cys227 and Cys236 to lipolysis, we generated HLAL replacement mutants containing the corresponding residues 228-235 of HGL or RLL. Both HLAL chimeras were catalytically active towards all three substrates analyzed, indicating that these amino acids do not determine HLAL substrate specificity. Deletion of the eight-amino acid alpha-helix as well as disruption of its hydrophobic surface, in contrast, abolished enzymatic activity. Our studies suggest that Cys227, Cys236, and the amphipathic helix formed by residues 228-235 are essential for HLAL- and HGL-mediated neutral lipid catabolism.     

Determinants of the inhibitory action of purified 14-kDa phospholipases A2 on cell-free prothrombinase complex

It has been suggested (Kini, R. R., and Evans, H. J. (1987) J. Biol. Chem. 262, 14402-14407) that the anticoagulant activity of members of the 14-kDa phospholipase A2 (PLA2) family depends on the presence of basic residues within a variable surface region (residues 54-77) distinct from both the conserved catalytic machinery and surface sites mediating the antibacterial action of these enzymes (see Weiss, J., Inada, M., Elsbach, P., and Crowl, R. M. (1994) J. Biol. Chem. 269, 26331-26337). To further define the determinants of the anticoagulant activity of PLA2, we have analyzed the inhibitory effects of purified native and recombinant PLA2 on cell-free prothrombinase. Both native and recombinant wild-type pig pancreas (net charge -1) and human "secretory" PLA2 (net charge +15) produced similar dose-dependent inhibition of prothrombinase activity that was significantly less potent than a toxic PLA2 purified from snake venom. Site-specific mutations that either increased or decreased PLA2 activity toward bactericidal/permeability-increasing protein-treated Escherichia coli by up to 50-fold had no effect on antiprothrombinase activity. In contrast, substitution of Arg for Asp-59/Gly for Ser-60 in the pig PLA2 increased antiprothrombinase activity by 5-10-fold without affecting catalytic activity toward a range of phospholipid substrates or antibacterial activity. Comparison of antiprothrombinase activity of catalytically active and inactive forms of the PLA2 and under a range of phospholipid conditions revealed that the potent antiprothrombinase activity of native toxic venom PLA2 and of the D59R.S60G mutant pancreatic PLA2 reflect combined catalytic and noncatalytic actions, the latter apparently dependent on basic residues at discrete surface sites in the enzyme.