Dnaj protein substrates

Hence, although the specific numerical values reported for ATPase rates of stress-70 proteins show some variation, possibly attributable to differences in specific protein preparation and assay procedures utilized by different individuals, the consensus scheme that these data show is that in the absence of substrate peptide, the stress-70 proteins have a low basal ATPase rate, typically found to be 0.01-0.03 (mol ATP/mol stress-70 protein min). This can be enhanced severalfold either by binding of peptides or denatured proteins, or (as demonstrated by the effect of grpE and dnaJ proteins on dnaK) by the action of ancillary proteins. To the extent that the observations on HSC70 can be generalized to other members of the stress-70 protein family, peptide binding appears to relieve the attenuation of ATPase activity and allow it to proceed at the rate characteristic of the ATPase fragment of the protein alone. 

The specific roles of dnaj and grpE in modulating the interaction of dnaK with other protein substrates are not clear. Possible functions include roles as specificity factors and capture factors. Since dnaj is known to bind specific target proteins, such as the bacteriophage PI repA protein and the X P protein, in addition to binding dnaK, it could conceivably function as a specificity factor, targeting dnaK to particular oligomeric... 

The question of how DnaJ proteins interact with substrates and mediate their transfer onto Hsp70 partner proteins is not answered for any of the three classes of DnaJ proteins. Some DnaJ homologs have broad substrate specificity, such as E. coli DnaJ and yeast Ydjl, while others have more restricted substrate spectra. In particular the DnaJ proteins of class III may either bind a restricted number of substrates, such as the clathrin-specific auxilin or the kinesin light chain, or they may not bind substrates themselves but rather are positioned in close proximity to substrates. The latter seems to be the case for Dj 1A in the plasma membrane of E. coli (Clarke et al, 1997 Kelley and Georgopoulos, 1997a), Sec63 at the translocation pore in the ER (Corsi and Schekman, 1996 Rapoport et al., 1996), and cysteine string proteins on the surface of neurosecretory vesicles (Buchner and Bundersen, 1997). 

Szabo A, Korszun R, Hartl FU, Flanagan J (1996) A zinc finger-like domain of the molecular chaperone DnaJ is involved in binding to denatured protein substrates. EMBO J 15 408-417... 

FIGURE 4-30 Chaperones in protein folding. The cyclic pathway by which chaperones bind and release polypeptides is illustrated for the . coli chaperone proteins DnaK and DnaJ, homologs of the eukaryotic chaperones Hsp70 and Hsp40. The chaperones do not actively promote the folding of the substrate protein, but instead prevent aggregation of unfolded peptides. For a population of polypeptides, some... 

Hence, several in vitro activities have demonstrated a participation of stress-70 proteins in the disassembly of macromolecular complexes. The participation of dnaK in initiation of replication for X and PI phages appears to be dependent on ancillary proteins, specifically dnaJ and grpE. This raises the possibility that substrate specificity, in these cases, may be intrinsic to the ancillary proteins rather than residing solely in the dnaK protein. In the case of in vitro dissassembly of clathrin cages, the HSC70 protein can accomplish the reaction without accessory proteins. In all the above cases, ATP hydrolysis is essential for the activities. 

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