Differential degradation for small heat shock proteins IbpA and IbpB is synchronized in Escherichia coli: implications for their functional cooperation in substrate refolding.

Small heat shock proteins (sHSPs), as a conserved family of ATP-independent molecular chaperones, are known to bind non-native substrate proteins and facilitate the substrate refolding in cooperation with ATP-dependent chaperones (e.g., DnaK and ClpB). However, how different sHSPs function in coordination is poorly understood. Here we report that IbpA and IbpB, the two sHSPs of Escherichia coli, are coordinated by synchronizing their differential in vivo degradation. Whereas the individually expressed IbpA and IbpB are respectively degraded slowly and rapidly in cells cultured under both heat shock and normal conditions, their simultaneous expression leads to a synchronized degradation at a moderate rate. Apparently, such synchronization is linked to their hetero-oligomerization and cooperation in binding substrate proteins. In addition, truncation of the flexible N- and C-terminal tails dramatically suppresses the IbpB degradation, and somehow accelerates the IbpA degradation. In view of these in vivo data, we propose that the synchronized degradation for IbpA and IbpB are crucial for their synergistic promoting effect on DnaK/ClpB-mediated substrate refolding, conceivably via the formation of IbpA-IbpB-substrate complexes. This scenario may be common for different sHSPs that interact with each other in cells.