Translation initiation in human mitochondria relies upon specialized mitoribosomes and initiation factors, mtIF2 and mtIF3, which have diverged from their bacterial counterparts. Here we report two ...Translation initiation in human mitochondria relies upon specialized mitoribosomes and initiation factors, mtIF2 and mtIF3, which have diverged from their bacterial counterparts. Here we report two distinct mitochondrial pre-initiation assembly steps involving those factors. Single-particle cryo-EM revealed that in the first step, interactions between mitochondria-specific protein mS37 and mtIF3 keep the small mitoribosomal subunit in a conformation favorable for a subsequent accommodation of mtIF2 in the second step. Combination with fluorescence cross-correlation spectroscopy analyses suggests that mtIF3 promotes complex assembly without mRNA or initiator tRNA binding, where exclusion is achieved by the N-terminal and C-terminal domains of mtIF3. Finally, the association of large mitoribosomal subunit is required for initiator tRNA and leaderless mRNA recruitment to form a stable initiation complex. These data reveal fundamental aspects of mammalian protein synthesis that are specific to mitochondria.
EMDB-10021, PDB-6rw4: Structure of human mitochondrial 28S ribosome in complex with mitochondrial IF3 Method: EM (single particle) / Resolution: 2.97 Å
EMDB-10022, PDB-6rw5: Structure of human mitochondrial 28S ribosome in complex with mitochondrial IF2 and IF3 Method: EM (single particle) / Resolution: 3.14 Å
EMDB-10023: Mitochondrial 28S ribosome with IF3 (masked refined the body core) Method: EM (single particle) / Resolution: 2.87 Å
EMDB-10024: Mitochondrial 28S ribosome with IF3 (masked refined the head core) Method: EM (single particle) / Resolution: 2.85 Å
EMDB-10025: Mitochondrial 28S ribosome with IF3 (masked refined the tail) Method: EM (single particle) / Resolution: 3.11 Å
EMDB-10026: Mitochondrial 28S ribosome with IF3 (masked refined mS39) Method: EM (single particle) / Resolution: 3.04 Å
EMDB-10027: Mitochondrial 28S ribosome with IF3 (masked refined IF3) Method: EM (single particle) / Resolution: 2.85 Å
EMDB-10028: Mitochondrial 28S ribosome with IF2 and IF3 (masked refined the body core) Method: EM (single particle) / Resolution: 3.02 Å
EMDB-10029: Mitochondrial 28S ribosome with IF2 and IF3 (masked refined the head core) Method: EM (single particle) / Resolution: 3.02 Å
EMDB-10030: Mitochondrial 28S ribosome with IF2 and IF3 (masked refined the tail) Method: EM (single particle) / Resolution: 3.38 Å
EMDB-10031: Mitochondrial 28S ribosome with IF2 and IF3 (masked refined mS39) Method: EM (single particle) / Resolution: 3.38 Å
EMDB-10032: Mitochondrial 28S ribosome with IF2 and IF3 (masked refined IF2 and IF3-CTD) Method: EM (single particle) / Resolution: 3.0 Å
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