The Functionality of IbpA from Acholeplasma laidlawii Is Governed by Dynamic Rearrangement of Its Globular-Fibrillar Quaternary Structure.

Small heat shock proteins (sHSPs) represent a first line of stress defense in many bacteria. The primary function of these molecular chaperones involves preventing irreversible protein denaturation and aggregation. In Escherichia coli , fibrillar Ec IbpA binds unfolded proteins and keeps them in a folding-competent state. Further, its structural homologue Ec IbpB induces the transition of Ec IbpA to globules, thereby facilitating the substrate transfer to the HSP70-HSP100 system for refolding. The phytopathogenic Acholeplasma laidlawii possesses only a single sHSP, Al IbpA. Here, we demonstrate non-trivial features of the function and regulation of the chaperone-like activity of Al IbpA according to its interaction with other components of the mycoplasma multi-chaperone network. Our results show that the efficiency of the A. laidlawii multi-chaperone system is driven with the ability of Al IbpA to form both globular and fibrillar structures, thus combining functions of both IbpA and IbpB when transferring the substrate proteins to the HSP70-HSP100 system. In contrast to Ec IbpA and Ec IbpB, Al IbpA appears as an sHSP, in which the competition between the N- and C-terminal domains regulates the shift of the protein quaternary structure between a fibrillar and globular form, thus representing a molecular mechanism of its functional regulation. While the C-terminus of Al IbpA is responsible for fibrils formation and substrate capture, the N-terminus seems to have a similar function to Ec IbpB through facilitating further substrate protein disaggregation using HSP70. Moreover, our results indicate that prior to the final disaggregation process, Al IbpA can directly transfer the substrate to HSP100, thereby representing an alternative mechanism in the HSP interaction network.