Interaction between proteins containing homeodomains associated to leucine zippers from sunflower

A strategy based on the use of PCR with one degenerate oligonucleotide deduced from conserved sequences and lambda gt10 primers was used to isolate homeobox containing sequences from sunflower stem and root cDNA libraries. Six different partial cDNAs coding for the first 48 amino acids of homeodomains and amino terminal sequences were analyzed and found to be members of the HD-Zip superfamily, which contain a homeobox linked to a leucine zipper coding region. A full-length cDNA clone, Hahb-10, was isolated and characterized. The leucine zipper portions of Hahb-10 and of the previously reported Hahb-1 have been utilized to construct fusions with the N-terminal domain of the lambda repressor. These fusions were tested for their ability to bind to lambda promoters in vivo. The expression of a protein containing an active dimerization domain, but not capable of DNA binding, exerts a dominant negative effect on the ability of repressor-zipper fusions to bind to its target DNA. From these experiments, it was concluded that Hahb-1 and -10, when co-expressed, form preferentially homodimers. Exchange of conserved threonines and leucines at positions a1 and d1 of both zippers reduces dimerization efficiency and allows the formation of heterodimers, suggesting that these residues are, among others, determinants of the specificity of interaction, most likely through changes in hydrophobic packing interactions at the dimer interface. The results imply that a great number of interacting molecular entities compose this protein superfamily which is presumably involved in regulating plant developmental responses.     

IQGAP1, a calmodulin-binding protein with a rasGAP-related domain, is a potential effector for cdc42Hs

Proteins that associate with the GTP-bound forms of the Ras superfamily of proteins are potential effector targets for these molecular switches. A 195 kDa protein was purified from cell lysates by affinity chromatography on immobilized cdc42Hs-GTP and a corresponding cDNA was isolated. Sequence analysis revealed localized identities to calponin, the WW domain, unconventional myosins and to the rasGAP-related domain (GRD) contained in IRA, NF-1, SAR1 and rasGAP. p195 was found to be identical to IQGAP1, a protein previously reported to bind ras. Purified recombinant p195/IQGAP1 bound to and inhibited the GTPase activity of cdc42Hs and rac whereas no interaction with ras was detected. The C-terminal half of IQGAP1 containing the GRD bound to cdc42 and rac in a GRD-dependent fashion, but a smaller fragment containing only the GRD did not. Cdc42 was also co-immunoprecipitated from cell lysates with antibody specific to p195/IQGAP1. Calmodulin also co-immunoprecipitated with p195/IQGAP1 and was found to associate with fragments containing the IQ domain. Expression of a cDNA fragment encoding the GRD inhibited the CDC24/CDC42 pathway in yeast, but no effect on ras was observed. In mammalian cells, both endogenous and ectopically expressed p195/IQGAP1 were localized to lamellipodia and ruffling cell membranes, where co-localization with actin was apparent. These results suggest that IQGAP1 is an effector target for cdc42Hs and may mediate the effects of this GTPase on cell morphology.     

Identification of conserved amino acids in the human granulocyte-macrophage colony-stimulating factor receptor alpha subunit critical for function. Evidence for formation of a heterodimeric receptor complex prior to ligand binding

A superfamily of growth factor and cytokine receptors has recently been identified, which is characterized by four spatially conserved cysteine residues, a tryptophan-serine motif (WSXWS) in the extracellular domain, and a proline-rich cytoplasmic domain. The high affinity human granulocyte-macrophage colony-stimulating factor (GM-CSF) receptor (hGM-CSFR) consists of two subunits, alpha (hGM-CSFR alpha) and beta (hGM-CSFR beta), both of which are members of the receptor superfamily. In this study, we prepared mutations in conserved amino acids of the receptor subunit necessary for GM-CSF binding (hGM-CSFR alpha) and analyzed mutant receptors for low affinity binding, internalization, and high affinity binding when complexed with the beta subunit. Mutations in the cytoplasmic domain did not affect GM-CSF binding or receptor internalization. Mutation of a single conserved serine residue within the WSXWS motif diminishes cell surface receptor expression but not ligand binding. Mutation of either the second or third conserved cysteine residue of hGM-CSFR alpha resulted in complete loss of low affinity binding; however, co-expression of the cysteine 2 mutant with hGM-CSFR beta yielded a high affinity receptor complex. Since neither the cysteine 2 mutant nor the beta subunit can bind ligand alone, this result suggests that hGM-CSFR alpha and hGM-CSFR beta exist in a preformed heterodimeric protein complex on the plasma membrane.     

NF1 (neurofibromatosis type 1)

Several distinct Ras GTPase activating proteins (GAPs) from mammals, including Ras GAP of 120 kDa (GAP1) and NF1, stimulate the intrinsic GTPase activity of normal Ras, but not oncogenic Ras mutants (Trahey and McCormick, 1987). That is the reason why normal Ras remains predominantly in the inactive GDP-bound form (D-Ras), whereas oncogenic Ras remains constitutively in the active GTP-bound form (T-Ras). NF1 is a tumor suppressor of 2818 amino acids whose disruption or deletion causes brain tumors called neurofibromatosis type 1 by elevating the T-Ras level. T-Ras activates several distinct oncogenic effectors, including Ser/Thr kinase Raf, GAP1, P1-3 kinase, PKC-zeta and Ra1 GDS. Interestingly, the binding of T-Ras to either GAPs or these oncogenic effectors requires the same effector domain I (residues 32-40) of T-Ras molecule. In other words, these GAPs and effectors compete for binding to T-Ras. Using a series of N- and C-terminal deletion mutants of NF1, we identified a 78 amino acid fragment (NF78, residues 1441-1518) as the minimum GAP domain, and a 56 amino acid fragment (NF 56, residues 1441-1496) as the minimum Ras-binding domain. Furthermore, we identified the Raf fragment of 81 amino acids (Raf81, residues, 51-131) as the minimum Ras-binding domain with a high affinity. We found that (i) these NF1 fragments and Raf81 compete for binding to T-Ras, and that (ii) over-expression of these NF1 or Raf fragments strongly suppresses the malignant transformation caused by oncogenic Ras mutants. Thus, these agents offer a unique opportunity to control the proliferation of T-Ras-associated tumors that represent more than 30% of all human carcinomas including neurofibromatosis type 1.     

Identification of residues in the first domain of human Fc alpha receptor essential for interaction with IgA

The FcR family contains multiple receptors for Igs, of which the most distantly related ( approximately 20%) is the IgA receptor (human Fc alpha R), being more homologous ( approximately 35%) to another family of killer-inhibitory receptor-related immunoreceptors with a 19q13.4 chromosomal location in humans. This study of the Fc alpha R demonstrated that, like several IgG receptors, Fc alpha R is a low affinity receptor for Ab (Ka approximately 106 M-1). Rapid dissociation of the rsFc alpha R:IgA complex (t1/2 approximately 25 s) suggests that monomer IgA would bind transiently to cellular Fc alpha Rs, while IgA immune complexes could bind avidly. Mutagenesis of histidyl 85 and arginyl 82, in the FG loop of domain 1, demonstrated that these residues were essential for the IgA-binding activity of Fc alpha R, while arginyl 87 makes a minor contribution to the binding activity of the receptor. This site is unusual among the Fc receptors (Fc gamma RII, Fc gamma RIII, and Fc epsilon RI), in which the ligand binding site is in domain 2 rather than domain 1, but like Fc alpha R, the FG loop comprises part of the ligand binding site. The putative F and G strands flanking the Fc alpha R ligand binding site are highly homologous in the other killer-inhibitory receptor-related immunoreceptors, suggesting they comprise a conserved structural element on which divergent FG loops are presented and participate in the specific ligand interactions of each of these receptors.     

Transmembrane signaling by the interleukin-2 receptor: progress and conundrums

Activation of the multicomponent interleukin-2 receptor (IL-2R) complex leads to a rapid increase in tyrosine phosphorylation of a number of cellular proteins including the IL-2R beta and IL-2R gamma chains of the IL-2R and the RAF-1 serine threonine kinase. In addition, phosphatidylinositol 3-kinase (PI-3K) protein and activity can be immunoprecipitated with anti-phosphotyrosine and anti-IL-2R beta antibodies from IL-2-activated but not resting T lymphocytes. We have demonstrated that the SH2 (SRC homology 2) domains of the 85 kDa subunit of PI-3K are sufficient to mediate binding of the PI-3K complex to tyrosine phosphorylated, but not non-phosphorylated IL-2R beta, suggesting that tyrosine phosphorylation is an integral component of the activation of PI-3K by the IL-2R. Since none of the members of the IL-2R complex contains an intrinsic tyrosine kinase domain, IL-2-induced tyrosine phosphorylation must be the consequence of activation of intracellular tyrosine kinases. SRC family members including lck, lyn and fyn have been demonstrated to associate with IL-2R beta through binding of the kinase domain to the acidic domain of IL-2R beta. However, we have demonstrated that the serine rich (SD) region of the cytosolic domain of IL-2R beta is also required for association of a tyrosine kinase with the IL-2R complex and that IL-2 can induce proliferation and tyrosine phosphorylation in cell lines which lack the known SRC family kinases expressed by T lymphocytes. Thus members of other kinase families besides SRC may also be involved in mediating IL-2 signal transduction. Biochemical studies and studies of cells expressing mutant IL-2 receptors indicate that IL-2-induced tyrosine kinase activation initiates a complex signaling cascade. The cascade includes SRC family kinase members such as lck, fyn, and lyn, activation of Raf-1 and PI-3K, and ras, and increased expression of the fos, fra-1, and jun protooncogenes. In addition, ligation of the IL-2R leads to rapid increases in myc expression and more delayed increases in the expression of the cdc2 and cdk2 kinases and the cyclins through a tyrosine phosphorylation independent pathway. Whether other biochemical processes initiated by IL-2R ligation, including activation of the MAP2, p70S6 and p90RSK serine threonine kinases, activation of NF-kappa B, and increased expression of Raf-1, Pim-1, bcl-2, IL-2R alpha and IL-2R beta, are consequences of the IL-2-induced tyrosine kinase cascade remains to be determined.(ABSTRACT TRUNCATED AT 400 WORDS)     

Molecular cloning and functional expression of equine interleukin-1 receptor antagonist

Equine interleukin-1 receptor antagonist (IL-1ra) was molecularly cloned to establish a basis for cytokine therapy of acute and chronic inflammatory diseases in the horse. cDNA clones encoding the whole coding sequence of equine IL-1ra were isolated from equine peripheral blood mononuclear cells (PBMC) that had been stimulated with lipopolysaccharide (LPS). The equine IL-1ra cDNA obtained in this study contained an open reading frame encoding 177 amino acid residues. The predicted amino acid sequence of equine IL-1ra shared 75.7, 75.3 and 76.3% similarity with sequences of human, murine and rabbit IL-1ras, respectively. An N-glycosylation site and five cysteine residues conserved in human, murine and rabbit IL-1ras were also found at the corresponding positions in equine IL-1ra. Recombinant glutathione S-transferase (GST)-equine IL-1ra fusion protein produced by Escherichia coli was purified. This protein was shown to inhibit the cytostatic or cytotoxic activity of IL-1 on A375S2 cells, indicating that the equine IL-1ra cDNA obtained in this study encodes biologically active equine IL-1ra.     

Multiple regions of human Fc gamma RII (CD32) contribute to the binding of IgG

The low affinity receptor for IgG, Fc gamma RII (CD32), has a wide distribution on hematopoietic cells where it is responsible for a diverse range of cellular responses crucial for immune regulation and resistance to infection. Fc gamma RII is a member of the immunoglobulin superfamily, containing an extracellular region of two Ig-like domains. The IgG binding site of human Fc gamma RII has been localized to an 8-amino acid segment of the second extracellular domain, Asn154-Ser161. In this study, evidence is presented to suggest that domain 1 and two additional regions of domain 2 also contribute to the binding of IgG by Fc gamma RII. Chimeric receptors generated by exchanging the extracellular domains and segments of domain 2 between Fc gamma RII and the structurally related Fc epsilon RI alpha chain were used to demonstrate that substitution of domain 1 in its entirety or the domain 2 regions encompassing residues Ser109-Val116 and Ser130-Thr135 resulted in a loss of the ability of these receptors to bind hIgG1 in dimeric form. Site-directed mutagenesis performed on individual residues within and flanking the Ser109-Val116 and Ser130-Thr135 domain 2 segments indicated that substitution of Lys113, Pro114, Leu115, Val116, Phe129, and His131 profoundly decreased the binding of hIgG1, whereas substitution of Asp133 and Pro134 increased binding. These findings suggest that not only is domain 1 contributing to the affinity of IgG binding by Fc gamma RII but, importantly, that the domain 2 regions Ser109-Val116 and Phe129-Thr135 also play key roles in the binding of hIgG1. The location of these binding regions on a molecular model of the entire extracellular region of Fc gamma RII indicates that they comprise loops that are juxtaposed in domain 2 at the interface with domain 1, with the putative crucial binding residues forming a hydrophobic pocket surrounded by a wall of predominantly aromatic and basic residues.     

Molecular characterization of an anchor protein (AKAPCE) that binds the RI subunit (RCE) of type I protein kinase A from Caenorhabditis elegans

Classical A kinase anchor proteins (AKAPs) preferentially tether type II protein kinase A (PKAII) isoforms to sites in the cytoskeleton and organelles. It is not known if distinct proteins selectively sequester regulatory (R) subunits of type I PKAs, thereby diversifying functions of these critical enzymes. In Caenorhabditis elegans, a single type I PKA mediates all aspects of cAMP signaling. We have discovered a cDNA that encodes a binding protein (AKAPCE) for the regulatory subunit (RCE) of C. elegans PKAICE. AKAPCE is a novel, highly acidic RING finger protein composed of 1,280 amino acids. It binds RI-like RCE with high affinity and neither RIIalpha nor RIIbeta competitively inhibits formation of AKAPCE.RCE complexes. The RCE-binding site was mapped to a segment of 20 amino acids in an N-terminal region of AKAPCE. Several hydrophobic residues in the binding site align with essential Leu and Ile residues in the RII-selective tethering domain of prototypic mammalian AKAPs. However, the RCE-binding region in AKAPCE diverges sharply from consensus RII-binding sites by inclusion of three aromatic amino acids, exclusion of a highly conserved Leu or Ile at position 8 and replacement of C-terminal hydrophobic amino acids with basic residues. AKAPCE.RCE complexes accumulate in intact cells.     

The receptor revolution--multiplicity of G-protein-coupled receptors

The superfamily of G-protein-coupled receptors (GPCR) is probably the largest protein-encoding gene family in our genome. It is already known to include hundreds of members and many more are expected to emerge as the molecular cloning revolution proceeds. By definition the GPCR respond to ligands by interacting with intracellular G-proteins and thereby transduce external signals to the interior of the cell. A large body of evidence suggests that the GPCR are organized in the cell membrane like bacteriorhodopsin (BR). All GPCR possess seven hydrophobic membrane-spanning segments which seem to form a characteristic BR-like barrel structure. Thus, the three-dimensional structure of BR may be used as a framework for computer-aided structural modelling of GPCR. The ligands which activate the various members of the GPCR family include an enormous variety of molecules such as amines, amino acids and peptides as well as several small hydrophobic molecules. Many ligands bind to multiple distinct GPCR, e.g. neuropeptide Y (NPY). We have isolated molecular clones encoding a human NPY receptor whose binding properties conform to those of the Y1 subtype. This clone will be a useful tool in our efforts to unravel the molecular mechanisms of the many physiological functions of neuropeptide Y.     

rab3 mediates cortical granule exocytosis in the sea urchin egg

Egg activation at fertilization in the sea urchin results in the exocytosis of approximately 15,000 cortical granules that are docked at the plasma membrane. Previously, we reported that several integral membrane proteins modeled in the SNARE hypothesis, synaptotagmin, VAMP, and syntaxin, in addition to a small GTPase of the ras superfamily, rab3, were present on cortical granules (Conner, S., Leaf, D., and Wessel, G., Mol. Reprod. Dev. 48, 1-13, 1997). Here we report that rab3 is associated with cortical granules throughout oogenesis, during cortical granule translocation, and while docked at the egg plasma membrane. Following cortical granule exocytosis, however, rab3 reassociates with a different population of vesicles, at least some of which are of endocytic origin. Because of its selective association with cortical granules in eggs and oocytes, we hypothesize that rab3 functions in cortical granule exocytosis. To test this hypothesis, we used a strategy of interfering with rab3 function by peptide competition with its effector domain, a conserved region within specific rab types. We first identified the effector domain sequence in Lytechinus variegatus eggs and find the sequence 94% identical to the effector domain of rab3 in Stronglocentrotus purpuratus. Then, with synthetic peptides to different regions of the rab3 protein, we find that cortical granule exocytosis is inhibited in eggs injected with effector domain peptides, but not with peptides from the hypervariable region or with a scrambled effector peptide. Additionally, effector-peptide-injected eggs injected with IP3 are blocked in their ability to exocytose cortical granules, suggesting that the inhibition is directly on the membrane fusion event and not the result of interference with the signal transduction mechanism leading to calcium release. We interpret these results to mean that rab3 functions in the regulation of cortical granule exocytosis following vesicle docking.     

Molecular genetic and protein chemical characterization of the cytochrome ba3 from Thermus thermophilus HB8

Thermus thermophilus HB8 cells grown under reduced dioxygen tensions contain a substantially increased amount of heme A, much of which appears to be due to the presence of the terminal oxidase, cytochrome ba3. We describe a purification procedure for this enzyme that yields approximately 100 mg of pure protein from 2 kg of wet mass of cells grown in < or = 50 microM O2. Examination of the protein by SDS-polyacrylamide gel electrophoresis followed by staining with Coomassie Blue reveals one strongly staining band at approximately 35 kDa and one very weakly staining band at approximately 18 kDa as reported earlier (Zimmermann, B.H., Nitsche, C.I., Fee, J. A., Rusnak, F., and Münck, E. (1988) Proc. Natl. Acad. Sci. U.S.A. 85, 5779-5783). By contrast, treatment of the gels with AgNO3 reveals that the larger polypeptide stains quite weakly while the smaller polypeptide stains very strongly. These results suggested the presence of two polypeptides in this protein. Using partial amino acid sequences from both proteins to obtain DNA sequence information, we isolated and sequenced a portion of the Thermus chromosome containing the genes encoding the larger protein, subunit I (cbaA), and the smaller protein, subunit II (cbaB). The two polypeptides were isolated using reversed phase liquid chromatography, and their mole percent amino acid compositions are consistent with the proposed translation of their respective genes. The two genes appear to be part of a larger operon, but we have not extended the sequencing to identify initiation and termination sequences. The deduced amino acid sequence of subunit I includes the six canonical histidine residues involved in binding the low spin heme B and the binuclear center Cu(B)/heme A. These and other conserved amino acids are placed along the polypeptide among alternating hydrophobic and hydrophilic segments in a pattern that shows clear homology to other members of the heme- and copper-requiring terminal oxidases. The deduced amino acid sequence of the subunit II contains the CuA binding motif, including two cysteines, two histidines, and a methionine, but, in contrast to most other subunits II, it has only one region of hydrophobic sequence near its N terminus. Alignment of these two polypeptides with other cytochrome c and quinol oxidases, combined with secondary structure analysis and previous spectral studies, clearly establish cytochrome ba3 as a bona fide member of the superfamily of heme- and copper-requiring oxidases. The alignments further indicate that cytochrome ba3 is phylogenetically distant from other cytochrome c and quinol oxidases, and they substantially decrease the number of conserved amino acid residues.     

Site-directed mutagenesis of a human brain ecto-apyrase: evidence that the E-type ATPases are related to the actin/heat shock 70/sugar kinase superfamily

On the basis of sequence homologies observed between members of the E-type ATPases and the phosphate binding motifs of the actin/heat shock protein 70/sugar kinase superfamily, a human ecto-apyrase was analyzed by site-directed mutagenesis of conserved amino acids in apyrase conserved regions (ACR) I and IV. The expressed proteins were analyzed to assess the significance of these amino acids. A conserved aspartic acid residue in ACR IV was mutated to alanine, asparagine, and glutamic acid, and the relative activity and Km for ATP and ADP were determined. Mutation of this Asp 219 to Ala or Asn yielded an enzyme severely reduced in ATP hydrolyzing activity (>90%) and completely devoid of ADPase activity, along with a similar extent of inhibition of hydrolysis of other nucleoside di- and triphosphates. Interestingly, mutation of Asp 219 to Glu completely restored the ability of the enzyme to hydrolyze nucleoside triphosphates at levels above that of the wild-type enzyme, while the ability to hydrolyze nucleoside diphosphates was slightly reduced. Mutation of a second conserved aspartic acid in ACR I (Asp 62) and two invariant glycine residues in both ACR I (Gly 64) and ACR IV (Gly 221) also severely disrupted nucleotidase activity. These results demonstrate that the E-type ATPases contain the nucleoside phosphate binding domains present in the actin/heat shock protein/sugar kinase superfamily. Together with analysis of computer-predicted secondary structures, the results suggest that the ecto-ATPases and ecto-apyrases are part of, or closely related to, the actin superfamily of proteins.     

Mutagenesis and chemical rescue indicate residues involved in beta-aspartyl-AMP formation by Escherichia coli asparagine synthetase B

Site-directed mutagenesis and kinetic studies have been employed to identify amino acid residues involved in aspartate binding and transition state stabilization during the formation of beta-aspartyl-AMP in the reaction mechanism of Escherichia coli asparagine synthetase B (AS-B). Three conserved amino acids in the segment defined by residues 317-330 appear particularly crucial for enzymatic activity. For example, when Arg-325 is replaced by alanine or lysine, the resulting mutant enzymes possess no detectable asparagine synthetase activity. The catalytic activity of the R325A AS-B mutant can, however, be restored to about 1/6 of that of wild-type AS-B by the addition of guanidinium HCl (GdmHCl). Detailed kinetic analysis of the rescued activity suggests that Arg-325 is involved in stabilization of a pentacovalent intermediate leading to the formation beta-aspartyl-AMP. This rescue experiment is the second example in which the function of a critical arginine residue that has been substituted by mutagenesis is restored by GdmHCl. Mutation of Thr-322 and Thr-323 also produces enzymes with altered kinetic properties, suggesting that these threonines are involved in aspartate binding and/or stabilization of intermediates en route to beta-aspartyl-AMP. These experiments are the first to identify residues outside of the N-terminal glutamine amide transfer domain that have any functional role in asparagine synthesis.     

Ras effector-homologue region on Rac regulates protein associations in the neutrophil respiratory burst oxidase complex

Rac, a small molecular weight GTPase in the Ras superfamily, participates in the activation of the multicomponent superoxide-generating NADPH oxidase of human neutrophils. Rac is 30% identical to Ras overall, but is 75% identical within the sequence corresponding to the effector region of Ras, which regulates mitogenesis through interactions with the protein kinase Raf1. We investigated the role of this region in Rac1 using site-directed mutagenesis. In a cell-free semirecombinant NADPH oxidase system, mutants in the 26, 33, 38, and 45 amino acids showed 20-110-fold reduced binding to the oxidase complex as judged by EC50 values and reduced (44-80%) maximal activities in superoxide generation. Only the GTP gamma S-bound form associated, since the GDP-bound form of Rac neither activated alone nor competed with GTP gamma S-Rac. EC50 values for neither p47-phox nor p67-phox were affected when mutant Racs were used in place of Rac. Data indicate direct binding of the Rac effector region to one or more components of the respiratory burst oxidase. Results indicate a general role for conserved effector-equivalent regions in small GTPases in the regulation of protein-protein interactions.     

Overexpression and functional analysis of a mitogen-inducible nuclear GTPase activating protein, Spa-1

The two Ras-related GTPases called Rap1 and Rsr1, which share 50% sequence identity with Ras GTPases are known to be activated by two distinct mammalian GAPs, i.e. cytosolic GAP3c of 55 kDa and membrane-bound GAP3m of 85 kDa. Recently we have cloned a gene encoding a 68 kDa (p68) protein product, which is associated with chromosomes during interphase. The N-terminal 190 amino acids share 43% sequence identity with the second half of the GTPase activating domain (residues 210-397) of GAP3m. The N-terminal fragment of 209 amino acids of Spa-1 (called Span-N) was overproduced in E. coli as a glutathione S-transferase (GST) fusion protein and affinity purified. Rap1 and Rsr1 GTPase stimulatory activity of Spa-1 was tested and compared with GAP3m. Spa-1 preferentially stimulates Rsr1 GTPase rather than Rap1 GTPase, while GAP3m has a preference for Rap1 GTPase. This suggests that although Spa-1 and GAP3m stimulate GTPase of Rap1 family members, they differ in affinity for them. By mutational analysis it was also found that amino acid residues 10-183 are enough for Rap GAP activity of Spa-1.     

A conserved motif in the tail domain of vinculin mediates association with and insertion into acidic phospholipid bilayers

The tail domain of vinculin (Vt) contains a salt-insensitive binding site for acidic phospholipids which is masked by the intramolecular head-tail interaction in native vinculin [Johnson, R. P., and Craig, S. W. (1995) Biochem. Biophys. Res. Commun. 210, 159-164]. To characterize further this phospholipid binding site, we have used hydrophobic photolabeling with a photoactivatable phosphatidylcholine analogue to detect insertion of protein into the lipid bilayer. We show here that, although the properties of binding to acidic phospholipid vesicles and spontaneous insertion into the bilayer are cryptic and inactive in vinculin at physiologic ionic strength, these activities of the purified tail domain can be activated by physical and chemical disruption of the intramolecular interaction between the head and tail domains. By analyzing the lipid binding and insertion activity of a series of GST-Vt fusion proteins, we defined 55 amino acids, comprising vinculin residues 916-970, that mimic the lipid-binding and insertion activity of Vt. Predictions of secondary structure suggest that these 55 amino acids form a basic, amphipathic helical hairpin. This prediction is supported by circular dichroism analysis, which indicates that at least 80% of the residues in residues 916-970 are in a helical conformation. This predicted helical hairpin motif, which is conserved in all vinculins and is present in an acidic phospholipid-binding region of alpha-catenin, is distinct from C2 and PH domains, and likely represents a third type of acidic phospholipid-binding structure.     

Dimerization specificity of P22 and 434 repressors is determined by multiple polypeptide segments

The repressor protein of bacteriophage P22 binds to DNA as a homodimer. This dimerization is absolutely required for DNA binding. Dimerization is mediated by interactions between amino acids in the carboxyl (C)-terminal domain. We have constructed a plasmid, p22CT-1, which directs the overproduction of just the C-terminal domain of the P22 repressor (P22CT-1). Addition of P22CT-1 to DNA-bound P22 repressor causes the dissociation of the complex. Cross-linking experiments show that P22CT-1 forms specific heterodimers with the intact P22 repressor protein, indicating that inhibition of P22 repressor DNA binding by P22CT-1 is mediated by the formation of DNA binding-inactive P22 repressor:P22CT-1 heterodimers. We have taken advantage of the highly conserved amino acid sequences within the C-terminal domains of the P22 and 434 repressors and have created chimeric proteins to help identify amino acid regions required for dimerization specificity. Our results indicate that the dimerization specificity region of these proteins is concentrated in three segments of amino acid sequence that are spread across the C-terminal domain of each of the two phage repressors. We also show that the set of amino acids that forms the cooperativity interface of the P22 repressor may be distinct from those that form its dimer interface. Furthermore, cooperativity studies of the wild-type and chimeric proteins suggest that the location of cooperativity interface in the 434 repressor may also be distinct from that of its dimerization interface. Interestingly, changes in the dimer interface decreases the ability of the 434 repressor to discriminate between its wild-type binding sites, O(R)1, O(R)2, and O(R)3. Since 434 repressor discrimination between these sites depends in large part on the ability of this protein to recognize sequence-specific differences in DNA structure and flexibility, this result indicates that the C-terminal domain is intimately involved in the recognition of sequence-dependent differences in DNA structure and flexibility.