Structures of the human mitochondrial ribosome recycling complexes reveal distinct mechanisms of recycling and antibiotic resistance

Abstract

AbstractRibosomes are recycled for a new round of translation initiation by dissociation of ribosomal subunits, messenger RNA and transfer RNA from their translational post-termination complex. Mitochondrial ribosome recycling factor (RRFmt) and a recycling-specific homolog of elongation factor G (EF-G2mt) are two proteins with mitochondria-specific additional sequences that catalyze the recycling step in human mitochondria. We have determined high-resolution cryo-EM structures of the human 55S mitochondrial ribosome (mitoribosome) in complex with RRFmt, and the mitoribosomal large 39S subunit in complex with both RRFmtand EF-G2mt. In addition, we have captured the structure of a short-lived intermediate state of the 55S•RRFmt•EF-G2mtcomplex. These structures clarify the role of a mitochondria-specific segment of RRFmtin mitoribosome recycling, identify the structural distinctions between the two isoforms of EF-Gmtthat confer their functional specificity, capture recycling-specific conformational changes in the L7/L12 stalk-base region, and suggest a distinct mechanistic sequence of events in mitoribosome recycling. Furthermore, biochemical and structural assessments of the sensitivity of EF-G2mtto the antibiotic fusidic acid reveals that the molecular mechanism of antibiotic resistance for EF-G2mtis markedly different from that exhibited by mitochondrial elongation factor EF-G1mt, suggesting that these two homologous mitochondrial proteins have evolved diversely to negate the effect of a bacterial antibiotics.