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

Abstract

Ribosomes 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 RRFmt and EF-G2mt. In addition, we have captured the structure of a short-lived intermediate state of the 55S•RRFmt•EF-G2mt complex. These structures clarify the role of a mitochondria-specific segment of RRFmt in mitoribosome recycling, identify the structural distinctions between the two isoforms of EF-Gmt that 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-G2mt to the antibiotic fusidic acid reveals that the molecular mechanism of antibiotic resistance for EF-G2mt is 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.

Structural basis of translation termination, rescue, and recycling in mammalian mitochondria

“…Besides this well-resolved hydrophobic pocket, the remaining part of the NTE remains invisible probably because of structural flexibility. Recently, it has been proposed that the N-terminal extension contains a small subdomain deposited between SSU 12S rRNA h44, LSU 16S rRNA H69, and mtRRF domain 1 to stabilize a rotated conformation of the mitoribosome that promotes mtEFG2 binding to the recycling complex (Koripella et al, 2020a). Interestingly, this subdomain appears to encompass residues 1-11 of the NTE, which are consistently attributed by prediction algorithms to be part of the cleavable mitochondrial import (targeting) sequence (MTS) of mtRRF.…”