Self-regulatory function of bacterial small heat shock protein IbpA through mRNA binding is conferred by a conserved arginine

Bacterial small heat shock proteins, IbpA and IbpB, co-aggregate with denatured proteins and recruit other chaperones for the processing of aggregates, thereby assisting in their refolding. In addition, as a recently revealed uncommon feature, Escherichia coli IbpA self-represses its own translation through interaction with the 5 untranslated region (UTR) of the ibpA mRNA, enabling IbpA to act as a mediator of negative feedback regulation. Although IbpA also suppresses the expression of IbpB, IbpB does not have the self-repression activity despite the two Ibps being highly homologous. This study demonstrates that the self-repression function of IbpA is conserved in other bacterial IbpAs. Moreover, a cationic residue-rich region in the -crystallin domain (ACD) of IbpA, which is not conserved in IbpB, is critical for the self-suppression activity. Notably, arginine 93 (R93) located within the ACD is an essential residue that cannot be replaced by the other 19 amino acids, including lysine. IbpA-R93 mutants completely lost the interaction with the 5 UTR of the ibpA mRNA but retained almost all of the chaperone activity to sequester denatured proteins. Taken together, the conserved Arg93-mediated translational control of IbpA through RNA binding would be beneficial for a rapid and massive supply of the chaperone on demand.