Mammalian cytosolic DnaJ homologues affect the hsp70 chaperone-substrate reaction cycle, but do not interact directly with nascent or newly synthesized proteins

Members of the hsp70 family of molecular chaperones interact with and stabilize nascent polypeptides during synthesis and/or translocation into organelles. The bacterial hsp70 homologue DnaK requires the DnaJ cofactor for its reaction cycle with polypeptide substrates. DnaJ stimulates the ATPase activity of the DnaK chaperone and thereby is thought to regulate the affinity of DnaK for its protein target. Herein we have analyzed some of the biochemical properties of two mammalian cytosolic DnaJ homologues, the hdj-1 and hdj-2 proteins. We were particularly interested in examining the proposal that DnaJ homologues are the first molecular chaperones to interact directly with nascent polypeptides. Nascent/newly synthesized proteins, nascent polypeptides released from the ribosome by puromycin, or polypeptides misfolded as a result of incorporation of an amino acid analogue were not found in complexes with either of the two HeLa cell DnaJ homologues. We still were unable to demonstrate any interactions between hdj-1p and nascent/newly synthesized proteins even after chemical cross-linking. We did find that hdj-1p, like bacterial DnaJ, stimulated the ATPase activity of hsp70. Stable complex formation between hsp70 and an unfolded polypeptide substrate in vitro was found to be reduced in the presence of hdj-1p and ATP. Thus, while hdj-1p likely does function as a cofactor for the hsp70 chaperone, having effects on hsp70's ATPase activity and conformation/oligomeric structure and the stability of hsp70-substrate complexes, it was not observed to interact directly with nascent/newly synthesized proteins. Rather, hdj-1p likely serves a regulatory role, governing the reaction cycle of hsp70 with polypeptide substrates.     

The major histocompatibility complex region marked by HSP70-1 and HSP70-2 variants is associated with clozapine-induced agranulocytosis in two different ethnic groups

Genes of the major histocompatibility complex (MHC) have been associated with susceptibility to drug-induced adverse reactions. We previously found that clozapine-induced agranulocytosis (CA) is associated with the HLA-DRB1*0402, DRB4*0101, DQB1*0302, DQA1*0301 haplotype in Ashkenazi Jewish patients and with the HLA-DRB1*1601, DRB5*02, DQB1*0502, DQA1*0102 haplotype in non-Jewish patients. In the present study, we tested the hypothesis that the variants of the heat-shock protein 70 (HSP-70) encoded by the HSP-70 loci located within the MHC region and known to be involved in apoptosis and regulation of cell proliferation could play an important role in molecular mechanisms of CA. First, we analyzed HSP70-2 polymorphism in risk-associated haplotypes from HLA homozygous cells and normal individuals and confirmed that the HSP70-2 9-kb variant was associated invariably with DR4 (HLA-DRB1*0402, DQB1*0302) and DR2 (HLA-DRB1*01601, DQB1*0502, DQA1*0102 and HLA-DRB1*1501, DQB1*0602) haplotypes, which were the haplotypes found increased in Jewish and non-Jewish patients with CA, respectively. The 9.0-kb variant was also found to be associated with HLA-B44, DRB1*0401 and HLA-B44, DRB1*07 haplotypes. Second, in patients with CA (12 Ashkenazi Jewish and 20 non-Jewish patients), HSP70-1 A and HSP70-2 9.0-kb variants were associated with the MHC haplotypes found by us to be markers of susceptibility to CA. The clozapine-treated control group had an excess number of HSP70-1 C and HSP70-2 8.5-kb variants, consistent with genetic resistance to CA associated with those variants. This finding supports our hypothesis that a dominant gene within the MHC region (marked by HSP70-1 and HSP70-2), but not necessarily HLA, is associated with CA in two different ethnic groups.     

Dhh1p, a putative RNA helicase, associates with the general transcription factors Pop2p and Ccr4p from Saccharomyces cerevisiae

It is generally believed that the neuronal form of nitric oxide synthase (nNOS) is constitutively expressed and that regulation of this enzyme's activity is mediated solely by changes in cytosolic calcium concentration. Serendipitously, however, we observed that pretreatment of Chinese hamster ovary (CHO) cells, which coexpress muscarinic M1 receptors and nNOS, with 3.3 microM or 1 mM carbachol (CCh) for 48 h resulted in marked enhancement of maximal muscarinic receptor-stimulated nNOS activity as determined by L-[3H]citrulline and cyclic [3H]GMP production. This was accompanied by a decrease in the potency of CCh. Muscarinic receptor density was reduced in the agonist-pretreated cells, as determined by specific [N-methyl-3H]scopolamine methyl chloride binding, whereas competition binding studies revealed no changes in agonist affinity. Both receptor-stimulated inositol phosphate formation and elevation of intracellular calcium concentrations were found to be desensitized in agonist-pretreated cells in a manner dependent on CCh pretreatment concentration. It is interesting that ionomycin-stimulated nNOS activity was greater in CCh-pretreated cells. Also, western analysis revealed increased nNOS immunoreactivity in pretreated cells. A similar increase in nNOS immunoreactivity following agonist treatment was demonstrated in N1E-115 neuroblastoma cells, which endogenously express nNOS and muscarinic M1 receptors. Thus, the enhancement of maximal receptor-stimulated nNOS activity following agonist pretreatment can be attributed to up-regulation of nNOS. It is interesting that this augmentation of the response takes place in spite of receptor down-regulation and desensitization of multiple steps involved in nNOS activation.     

Identification and cloning of prs a 1, a 32-kDa endochitinase and major allergen of avocado, and its expression in the yeast Pichia pastoris

Avocado, the fruit of the tropical tree Persea americana, is a source of allergens that can elicit diverse IgE-mediated reactions including anaphylaxis in sensitized individuals. We characterized a 32-kDa major avocado allergen, Prs a 1, which is recognized by 15 out of 20 avocado- and/or latex-allergic patients. Natural Prs a 1 was purified, and its N-terminal and two tryptic peptide sequences were determined. We isolated the Prs a 1 encoding cDNA by PCR using degenerate primers and 5'-rapid amplification of cDNA ends. The Prs a 1 cDNA coded for an endochitinase of 326 amino acids with a leader peptide of 25 amino acids. We expressed Prs a 1 in the yeast Pichia pastoris at 50 mg/liter of culture medium. The recombinant Prs a 1 showed endochitinase activity, inhibited growth and branching of Fusarium oxysporum hyphae, and possessed IgE binding capacity. IgE cross-reactivity with latex proteins including a 20-kDa allergen, most likely prohevein, was demonstrated, providing an explanation for the commonly observed cross-sensitization between avocado and latex proteins. Sequence comparison showed that Prs a 1 and prohevein had 70% similarity in their chitin-binding domains. Characterization of chitinases as allergens has implications for engineering transgenic crops with increased levels of chitinases.     

Involvement of heat shock elements and basal transcription elements in the differential induction of the 70-kDa heat shock protein and its cognate by cadmium chloride in 9L rat brain tumor cells

The EphA3 receptor tyrosine kinase has been implicated in guiding the axons of retinal ganglion cells as they extend in the optic tectum. A repulsive mechanism involving opposing gradients of the EphA3 receptor on retinal axons and its ligands, ephrin-A2 and ephrin-A5, in the tectum influences topographic mapping of the retinotectal projection. To investigate the overall role of the Eph family in patterning of the visual system, we have used in situ hybridization to localize nine Eph receptors in the chicken retina and optic tectum at Embryonic Day 8. Three of the receptors examined correspond to the novel chicken homologs of EphA2, EphA6, and EphA7. Unexpectedly, we found that many Eph receptors are expressed not only in retinal ganglion cells, but also in tectal cells, In particular, EphA3 mRNA is prominently expressed in the anterior tectum, with a pattern reciprocal to that of ephrin-A2 and ephrin-A5. Similarly, ephrin-A5 is expressed not only in tectal cells but also in the nasal retina, with a pattern reciprocal to that of its receptor EphA3 and partially overlapping with that of its other receptor EphA4. Consistent with the even distribution of EphA4 and the polarized distribution of EphA4 ligands in the retina, probing EphA4 immunoprecipitates from different sectors of the retina with anti-phosphotyrosine antibodies revealed spatial differences in receptor phosphorylation. These complex patterns of expression and tyrosine phosphorylation suggest that Eph receptors and ephrins contribute to establishing topography of retinal axons through multiple mechanisms, in addition to playing a role in intraretinal and intratectal organization.