Molecular Equilibrist: The Small Heat Shock Protein IbpA from Mycoplasma.

Small heat shock proteins (sHSPs) serve as "first aid" stress-response proteins in both eukaryotes and prokaryotes. Their holdase activity enables binding to partially denatured proteins, maintaining them in a folding-competent state under stress. The sHSP IbpA from the mycoplasma Acholeplasma laidlawii is a unique member of its family, combining the functions of two Escherichia coli sHSPs that typically act in tandem. In this study, we demonstrate for the first time that IbpA forms distinct supramolecular structures under contrasting temperature stresses in crowded environments without any artificial truncations or mutations at the protein termini. Upon cooling, IbpA in vitro forms long fibril bundles, whereas heating induces the formation of large, rounded agglomerates. At the temperature optimal for culture growth, the protein exists as a mixture of short fibrils and small globules, with the latter predominating. IbpA's cellular localization mirrors in vitro properties, with an increased proportion of surface-associated proteins among the sHSP partners during cold shock. We also report, for the first time, a rapid and reversible transition of IbpA to a fibrillar form in response to cold. We propose hypotheses regarding potential roles of IbpA in the mycoplasma cell. IbpA from A. laidlawii appears to act as a "molecular equilibrist," protecting the cell against damage under opposing stresses, though the precise mechanism of its action during cold shock remains to be elucidated.