GrpE protein

The specific effects of purified dnaj and grpE proteins on the dnaK protein have been characterized (Liberek et al., 1991a). The dnaj and grpE proteins, acting in concert, can stimulate the basal ATPase activity of dnaK 50-fold dnaj appears to enhance the rate of nucleotide hydrolysis by dnaK, but does not substantially affect nucleotide release. The grpE protein appears to enhance the rate of release of nucleotide bound to dnaK. Consequently, neither protein alone exerts a dramatic effect on the overall rate of ATP turnover by dnaK however, the effect of the two proteins combined is to accelerate both the rate of ATP hydrolysis and the rate of nucleotide release, resulting in enhanced ATP turnover. The concentrations of dnaj and grpE required for half-maximal effect are both 0.1-0.2 fiM. 

Bag proteins differ from GrpE proteins by virtue of their ability to associate with ligands other than Hsp70 proteins. Although not shown experimentally, it is generally assumed that Bag proteins use this ability... 

Bohnert M, Pfanner N, van der Laan M (2007) A dynamic machinery for import of mitochondrial precursor proteins. FEBS Lett 581 2802-2810 Bolliger L et al. (1994) A mitochondrial homolog of bacterial GrpE interacts with mitochondrial hsp70 and is essential for viability. EMBO J 13 1998-2006... 

Many of the chaperones double as heat shock-proteins (Hsp). When a cell is put under stress that can cause proteins to denature, such as too high a temperature, it produces heat-shock proteins. Their names are abbreviated to Hsp plus their subunit molecular mass in kDa. Hsp70, for example, is a ubiquitous heat-shock protein in eukaryotes. It is known in E. coli as DnaK for historical reasons because it was first discovered from a supposed role in DNA replication. Hsp70 is also important in protein trafficking and the conveying of proteins across membranes, because the denatured state is important in these processes. In protein biosynthesis, the unfolded state of the nascent polypeptide chain is passed on to DnaK, which maintains it in an extended form. The chain, under the influence of ATP and co-chaperones such as DnaJ and GrpE, is handed over to GroEL. 

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. 

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... 

It is conceivable that the participation of accessory proteins such as dnaj and grpE in some functions of dnaK will have parallels in the functions of other stress-70 proteins. Several dnaJ homologues have been documented in yeast, although as a group they do not show the stringent level of sequence conservation found in the stress-70 proteins. In particular, the primary structure of dnaj appears to be modular, and can be described as (1) an N-terminal region of 80-100 amino acid residues,... 

Liberek, K., Marszalek, J., Ang, D., Georgopoulos, C., and Zylicz, M. (1991a). Escherichia coli DnaJ and GrpE heat shock proteins jointly stimulate ATPase activity of DnaK. Proc. Nad. Acad. Sci. U.S.A. 88, 2874-2878. 

Lipinska, B., King, J., Ang, D., and Georgopoulos, C. (1988). Sequence analysis and transcriptional regulation of the Escherichia coli grpE gene, encoding a heat shock protein. Nucleic Acids Res. 16, 7545-7562. 

Zolkiewski M (1999) ClpB cooperates with DnaK, DnaJ, and GrpE in suppressing protein aggregation. A novel multi-chaperone system from Escherichia coli. J Biol Chem 274 28083-28086... 

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