SmpB

SmpB is a small bacterial protein that plays a crucial role in trans-translation, a ribosome rescue mechanism that freeses stalled ribosomes and prevents production of aberrant proteins. It functions together with tmRNA, also known as SsrA, to recognize and resolve ribosomes stuck on damaged or truncated mRNAs and to tag the resulting polypeptides for degradation.

In the trans-translation process, SmpB binds to tmRNA and aids its delivery to the A site of a ribosome halted during translation. The tmRNA then acts as both a tRNA and an mRNA: its tRNA-like domain donates an amino acid to the nascent chain, and its short open reading frame provides a degradation tag that is appended to the incomplete polypeptide as translation resumes on the tmRNA message. SmpB thus helps ensure correct entry and frame selection, allowing the ribosome to switch from the problematic mRNA to the tmRNA template.

SmpB is typically about 140–160 amino acids in length and is a basic, single-domain protein. It has a conserved structure that supports simultaneous binding to tmRNA and the ribosome. The C-terminal region of SmpB is involved in interactions with the ribosome’s mRNA channel and influences the accommodation of tmRNA, acting as a gate that facilitates proper rescue.

Genetically, smpB is commonly found in bacterial genomes, often in proximity to the ssrA (tmRNA) gene, reflecting their functional partnership. Disruption of smpB or tmRNA components impairs trans-translation, leading to accumulation of stalled ribosomes and reduced fitness, especially under stress conditions. SmpB remains a focus of research due to its central role in ribosome rescue and potential as a target for antimicrobial strategies.