Required for rescue of stalled ribosomes mediated by trans-translation. Binds to tmRNA (also known as SsrA and 10Sa), required for stable association of tmRNA with ribosomes, probably by bridging tmRNA to 50S subunit. tmRNA and SmpB together mimic tRNA shape, replacing the anticodon stem-loop with SmpB (PubMed:22422985, PubMed:17488812). tmRNA is encoded by the ssrA gene; the 2 termini fold to resemble tRNA(Ala) and it encodes a 'tag peptide', a short internal open reading frame. During trans-translation Ala-aminoacylated tmRNA acts like a tRNA, entering the A-site of stalled ribosomes, displacing the stalled mRNA. The ribosome then switches to translate the ORF on the tmRNA; the nascent peptide is terminated with the 'tag peptide' encoded by the tmRNA and targeted for degradation. The ribosome is freed to recommence translation, which seems to be the essential function of trans-translation. Binds tmRNA (PubMed:12677067, PubMed:20953161, PubMed:22422985, PubMed:17488812, PubMed:20940705, PubMed:22622583). Binds to both the 50S and 30S ribosomal subunits via rRNA contacts, bridges tmRNA binding to stalled ribosomes. The tmRNA-SmpB complex associates with stalled 70S ribosomes (PubMed:20953161, PubMed:22422985, PubMed:20940705, PubMed:22622583). Athough the Valle et al., electron microscopy paper indicates this protein came from T.thermophilus, its sequence maps to A.aeolicus (PubMed:12677067). The Fu et al., paper maps the coordinates of T.thermophilus to E.coli (PubMed:20940705). Belongs to the SmpB family.