In the presence of ClpB these larger aggregates were disaggregated more efficiently even at stoichiometric concentrations of DnaK. Larger aggregates (up to 10-mers) could be solubilized only partially by the DnaK system, which then was necessary in a high molar excess. Small aggregates (2- to 3-mers) of glucose-6-phosphate dehydrogenase were efficiently solubilized and subsequently refolded by the DnaK system alone. coli Hsp70 and Hsp100 homologs, DnaK and ClpB, the requirement for ClpB is alleviated below a threshold size of the aggregates. Protein aggregates can be solubilized by some Hsp70 homologs in cooperation with chaperones of the Hsp100 family (Hsp104/ClpB) and subsequently refolded into the native state. The energy of ATP may be used to induce such conformational changes or alternatively to drive the ATPase cycle in the right direction.
the untangling of a misfolded β-sheet, which helps to overcome kinetic barriers for folding to the native state (‘local unfolding’). In the second mechanism, the binding and release cycles induce local unfolding in the substrate, e.g.
Roles of chaperone proteins in bse free#
Through repetitive substrate binding and release cycles they keep the free concentration of the substrate sufficiently low to prevent aggregation, while allowing free molecules to fold to the native state (‘kinetic partitioning’). In the first mechanism Hsp70s play a rather passive role. There are at least two alternative modes of action. Substrates cycle between chaperone-bound and free states until the ensemble of molecules has reached the native state. Hsp70-dependent protein folding in vitro occurs typically on the time scale of minutes or longer. The mechanism by which Hsp70-chaperones assist the folding of non-native substrates is still unclear. Hsp70 chaperone systems assist non-native folding intermediates to fold to the native state (‘folder’ activity). Only members of the Hsp70 family with general chaperone functions have such general holder activity. Some JDPs such as Escherichia coli DnaJ and Saccharomyces cerevisiae Ydj1 can prevent aggregation by themselves through ATP-independent transient and rapid association with the substrates. Hsp70 proteins together with their co-chaperones of the J-domain protein (JDP) family prevent the aggregation of non-native proteins through association with hydrophobic patches of substrate molecules, which shields them from intermolecular interactions (‘holder’ activity). A more complex folding situation exists for the Hsp70-dependent control of regulatory proteins since several steps in the folding and activation process of these substrates are assisted by multiple chaperones. Mechanistically related but less understood is the role of Hsp70s in the disassembly of protein complexes such as clathrin coats, viral capsids and the nucleoprotein complex, which initiates the replication of bacteriophage λ DNA. In the cellular milieu, Hsp70s exert these activities in the quality control of misfolded proteins and the co- and posttranslational folding of newly synthesized proteins. The role of Hsp70s in the folding of non-native proteins can be divided into three related activities: prevention of aggregation, promotion of folding to the native state, and solubilization and refolding of aggregated proteins. Protein folding processes assisted by Hsp70 Hsp70 proteins with their co-chaperones and cooperating chaperones thus constitute a complex network of folding machines. The broad spectrum of cellular functions of Hsp70 proteins is achieved through (i) the amplification and diversification of hsp70 genes in evolution, which has generated specialized Hsp70 chaperones, (ii) co-chaperones which are selectively recruited by Hsp70 chaperones to fulfill specific cellular functions and (iii) cooperation of Hsp70s with other chaperone systems to broaden their activity spectrum. All of these activities appear to be based on the property of Hsp70 to interact with hydrophobic peptide segments of proteins in an ATP-controlled fashion. Hsp70s have thus housekeeping functions in the cell in which they are built-in components of folding and signal transduction pathways, and quality control functions in which they proofread the structure of proteins and repair misfolded conformers. 70-kDa heat shock proteins (Hsp70s) assist a wide range of folding processes, including the folding and assembly of newly synthesized proteins, refolding of misfolded and aggregated proteins, membrane translocation of organellar and secretory proteins, and control of the activity of regulatory proteins.
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