Members of this protein family are the bacterial ATP-dependent chaperone ClpB. This protein belongs to the AAA family of ATPases associated with various cellular activities (). This molecular chaperone does not act as a protease, but rather serves to disaggregate misfolded and aggregated proteins [, ].
This entry represents IbpB, which belongs to the small heat shock protein (HSP20) family. IbpB associates with aggregated proteins, together with IbpA, to stabilise and protect them from irreversible denaturation and extensive proteolysis during heat shock and oxidative stress. Aggregated proteins bound to the IbpAB complex are more efficiently refolded and reactivated by the ATP-dependent chaperone systems ClpB and DnaK/DnaJ/GrpE. Its activity is ATP-independent.
The type VI secretion system (T6SS) is a supra-molecular bacterial complex that resembles phage tails. It is a toxin delivery systems which fires toxins into target cells upon contraction of its TssBC sheath []. Thirteen essential core proteins are conserved in all T6SSs: the membrane associated complex TssJ-TssL-TssM, the baseplate proteins TssE, TssF, TssG, and TssK, the bacteriophage-related puncturing complex composed of the tube (Hcp), the tip/puncturing device VgrG, and the contractile sheath structure (TssB and TssC). Finally, the starfish-shaped dodecameric protein, TssA, limits contractile sheath polymerization at its distal part when TagA captures TssA []. ClpV is an AAA(+) ATPase that disassembles the type VI secretion system contracted sheath, which resets the systems for reassembly of an extended sheath that is ready to fire again []. The ClpV proteins are most similar to ClpB proteins within the Hsp100/Clp family, but cluster in a separate phylogenetic tree with a remarkable distance to ClpB []. However, this entry also includes ClpB from Yersinia enterocolitica [].
Small heat shock proteins IbpA associate with aggregated proteins, together with IbpB, to stabilise and protect them from irreversible denaturation and extensive proteolysis during heat shock and oxidative stress. Aggregated proteins bound to the IbpAB complex are more efficiently refolded and reactivated by the ATP-dependent chaperone systems ClpB and DnaK/DnaJ/GrpE. Its activity is ATP-independent[], [].These proteins form homomultimers of about 100-150 subunits at optimal growth temperatures. The conformation changes to monomers at high temperatures or high ionic concentrations. Proteins in this entry belong to the small heat shock protein (HSP20) family.
The ClpA/B family of ATP-binding proteins includes the regulatory subunit of the ATP-dependent protease Clp, ClpA; heat shock proteins ClpB, 104 and 78; and chloroplast proteins CD4a (ClpC) and CD4b [, ]. The proteins are thought to protect cells from stress by controlling the aggregation and denaturation of vital cellular structures. They vary in size, but share two conserved regions of about 200 amino acids that each contains an ATP-binding site [].This entry represents a conserved site found in the second conserved region.Proteins containing this site are listed below:Escherichia coli clpA, which acts as the regulatory subunit of the ATP- dependent protease clp.Rhodopseudomonas blastica clpA homolog.Escherichia coli heat shock protein clpB and homologues in other bacteria.Bacillus subtilis protein mecB.Yeast heat shock protein 104 (gene HSP104), which is vital for tolerance to heat, ethanol and other stresses.Neurospora heat shock protein hsp98.Yeast mitochondrial heat shock protein 78 (gene HSP78) [].CD4A and CD4b, two highly related tomato proteins that seem to be located in the chloroplast.Trypanosoma brucei protein clp.Porphyra purpurea chloroplast encoded clpC.
This family belongs to the AAA+ (ATPase associated with diverse cellular activities) superfamily. Most of these proteins of this family (ClpA, ClpC, ClpD, ClpE, ClpX, HslU) are involved in proteolysis and associate with a separate proteolytic subunit (ClpP, HslV) to form the active protease. ClpB is not involved in proteolysis but rather acts in collaboration with the DnaK (Hsp70) chaperone system to disassemble and refold large protein aggregates.A group of ATP-binding proteins that includes the regulatory subunit of the ATP-dependent protease clpA; heat shock proteins clpB, 104 and 78; and chloroplast proteins CD4a (ClpC) and CD4b belong to this family [, ]. The proteins are thought to protect cells from stress by controlling the aggregation and denaturation of vital cellular structures. They vary in size, but share a domainwhich contains an ATP-binding site.These signatures which span the ATP binding region also identify the bacterial DNA polymerase III subunit tau (), ATP-dependent protease La () and the mitochondrial lon protease homologue (), both of which belong to MEROPS peptidase family S16.
