SmpB, a unique RNA-binding protein essential for the peptide-tagging activity of SsrA (tmRNA).
In bacteria, SsrA RNA recognizes ribosomes stalled on defective messages and acts as a tRNA and mRNA to mediate the addition of a short peptide tag to the C-terminus of the partially synthesized nascent polypeptide chain. The SsrA-tagged protein is then degraded by C-terminal-specific proteases. SmpB, a unique RNA-binding protein that is conserved throughout the bacterial kingdom, is shown here to be an essential component of the SsrA quality-control system. Deletion of the smpB gene in Escherichia coli results in the same phenotypes observed in ssrA-defective cells, including a variety of phage development defects and the failure to tag proteins translated from defective mRNAs. Purified SmpB binds specifically and with high affinity to SsrA RNA and is required for stable association of SsrA with ribosomes in vivo. Formation of an SmpB-SsrA complex appears to be critical in mediating SsrA activity after aminoacylation with alanine but prior to the transpeptidation reaction that couples this alanine to the nascent chain. SsrA RNA is present at wild-type levels in the smpB mutant arguing against a model of SsrA action that involves direct competition for transcription factors.
Department of Biology, Massachusetts Institute of Technology, Cambridge, MA 02139, USA.
A W
AW
Karzai
M M
MM
Susskind
R T
RT
Sauer
D000409:Alanine / Q000378:metabolism; D000412:Alanine-tRNA Ligase / Q000378:metabolism; D000595:Amino Acid Sequence; D001426:Bacterial Proteins / Q000235:genetics / Q000302:isolation & purification / Q000378:metabolism*; D010582:Bacteriophage lambda / Q000235:genetics / Q000254:growth & development; D010583:Bacteriophage mu / Q000254:growth & development; D018388:Codon, Terminator / Q000235:genetics; D004926:Escherichia coli / Q000235:genetics* / Q000254:growth & development; D017353:Gene Deletion; D007700:Kinetics; D008565:Membrane Proteins / Q000235:genetics / Q000302:isolation & purification / Q000378:metabolism*; D010641:Phenotype; D011485:Protein Binding; D014176:Protein Biosynthesis / Q000235:genetics; D021382:Protein Sorting Signals / Q000235:genetics / Q000502:physiology; D012329:RNA, Bacterial / Q000096:biosynthesis / Q000235:genetics / Q000502:physiology*; D012333:RNA, Messenger / Q000235:genetics / Q000378:metabolism; D016601:RNA-Binding Proteins / Q000235:genetics / Q000378:metabolism*; D011993:Recombinant Fusion Proteins / Q000096:biosynthesis / Q000737:chemistry / Q000302:isolation & purification; D012270:Ribosomes / Q000235:genetics / Q000378:metabolism
D001426:Bacterial Proteins; D018388:Codon, Terminator; D008565:Membrane Proteins; D021382:Protein Sorting Signals; D012329:RNA, Bacterial; D012333:RNA, Messenger; D016601:RNA-Binding Proteins; D011993:Recombinant Fusion Proteins; C092408:smpB protein, Staphylococcus hominis; C096222:tmRNA; D000412:Alanine-tRNA Ligase; D000409:Alanine