A molecular chaperone, ClpA, functions like DnaK and DnaJ.

Abstract

The two major molecular chaperone families that mediate ATP-dependent protein folding and refolding are the heat shock proteins Hsp6Os (GroEL) and Hsp7Os (DnaK). Cip proteins, like chaperones, are highly conserved, present in all organisms, and contain ATP and polypeptide binding sites. We discovered that CipA, the ATPase component of the ATP-dependent ClpAP protease, is a molecular chaperone. CipA performs the ATP-dependent chaperone function of DnaK and DnaJ in the in vitro activation of the plasmid P1 RepA replication initiator protein. RepA is activated by the conversion of dimers to monomers. We show that CipA targets RepA for degradation by ClpP, demonstrating a direct link between the protein unfolding function of chaperones and proteolysis. In another chaperone assay, ClpA protects luciferase from irreversible heat inactivation but is unable to reactivate luciferase.Molecular chaperones interact with other proteins to mediate ATP-dependent protein folding, refolding, assembly, and disassembly of proteins. The Hsp7O chaperone system of Escherichia coli consists of the DnaK, DnaJ, and GrpE heat shock proteins. In vivo these three heat shock proteins function together in many cellular processes, as demonstrated by the observations that mutants in dnaK, dnaJ, and grpE have similar effects on DNA replication of E. coli, plasmids P1 and F and phage A, RNA synthesis, cell division, protein transport, regulation of the heat shock response, protection of enzymes from misfolding or aggregation during heat shock, and degradation of abnormal proteins (reviewed in ref.

C‐terminal domain mutations in ClpX uncouple substrate binding from an engagement step required for unfolding

“…ClpX and related members of the Clp/Hsp100 protein family are AAA + ATPases (Neuwald et al ., 1999) which serve as specialized energy-dependent molecular chaperones to unfold specific target proteins, disassemble multimeric complexes and solubilize protein aggregates (Wickner et al ., 1994;Levchenko et al ., 1995;Wawrzynow et al ., 1995;Weber-Ban et al ., 1999;Zolkiewski, 1999;Kim et al ., 2000;Seonga et al ., 2000;Konieczny and Liberek, 2002). The minimal AAA + unit contains an ATPase domain with a mixed ab structure and a Cterminal domain that is largely a -helical, but most family members also contain additional structural domains (Schirmer et al ., 1996;Neuwald et al ., 1999;Bochtler et al ., 2000;Maurizi and Li, 2001).…”

MecA, an adaptor protein necessary for ClpC chaperone activity

“…For some HSP100͞Clp proteins, like ClpA and ClpX, it was shown that they act as chaperones by promoting the ATP-dependent unfolding (12) and translocation of substrate proteins into the catalytically active centers of the associated ClpP peptidase ring (13,14). However, these and other HSP100͞Clp proteins also have chaperone activities independent of their role in the protease complex (12,15,16). The HSP100͞Clp proteins cooperate with the other chaperone systems in a combined network that cannot only prevent aggregation and refold misfolded proteins but also disaggregate and refold or degrade previously aggregated proteins (9,17,18).…”

The role of protein degradation in mitochondrial function and biogenesis

“…Degradation of complete mitochondrial translation products in an Afg3p-dependent manner indicates that proteolysis can occur post-translationally (Guélin et al 1996). Thus, Afg3p/Rca1p may also have a role in the removal of denatured or otherwise damaged proteins from the membrane can combine with ClpP subunits to form an ATP-dependent protease (Gottesman and Maurizi 1992;, can function independently as chaperones (Wickner et al 1994;Levchenko et al 1995;Wawrzynow et al 1995). Yeast mitochondrial Hsp78p, another member of the Clp-family, can partly substitute for mt-Hsp70 (Schmitt et al 1995) and/or stabilize mutant forms of the latter (Moczko et al 1995).…”

“…Another is what the proposed chaperone function of the FtsH-subfamily proteins is in molecular terms. It must be noted in this respect that while 'assembly' or 'folding' are commonly used when referring to the proposed (second) function of these proteins, ClpA and ClpX promote disaggregation or monomerization (Wickner et al 1994;Levchenko et al 1995;Wawrzynow et al 1995).…”