IbpB

IbpB is a small heat shock protein encoded by the ibpB gene in many bacteria, including Escherichia coli. It belongs to the alpha-crystallin domain family of small heat shock proteins and typically consists of about 150 amino acids. In many species, ibpB is coexpressed with its paralog ibpA and both proteins participate in the cellular proteostasis network activated during stress.

Functionally, IbpB acts as a molecular chaperone during stress conditions such as elevated temperature. It binds to partially folded or misfolded proteins to prevent irreversible aggregation, thereby stabilizing client proteins until they can be refolded. IbpB commonly forms hetero-oligomeric complexes with IbpA and can cooperate with the ATP-dependent chaperone system, including DnaK, DnaJ, GrpE, and ClpB, to facilitate substrate refolding or disaggregation when stress subsides.

Expression of ibpB is upregulated in response to heat shock and other environmental stresses as part of the bacterial heat shock response. In model organisms like Escherichia coli, regulation is linked to the heat shock transcription factor sigma32 (RpoH), which coordinates the induction of several chaperones and proteases.

Structurally, IbpB is a soluble cytoplasmic protein containing the conserved alpha-crystallin domain characteristic of sHSPs. It lacks a conventional signal peptide and localizes to the cytoplasm where it can access unfolded proteins.

Biological significance of IbpB includes contributing to thermotolerance and proteome stability under stress. Across bacteria, IbpB and its homologs are widely conserved components of the stress response, reflecting their role in maintaining protein homeostasis during adverse conditions.