Fig. 3: Mitochondrial GTPase MTG2 interacts with the functional regions of the immature human mitoribosomal large subunit.

a Interaction of the MTG2 (red cartoon) with the 16S rRNA and ribosomal proteins in state C. The N-terminal Obg domain of MTG2 contacts the peptidyl transferase center (PTC) region of the 16S rRNA (light cyan, PTC helices depicted individually, P loop in pink), whereas the G domain associates with the ribosomal GTPase-associated center (GAC) components uL11m (green) and the sarcin-ricin loop (SRL). b Detailed view of G domain interactions with the GAC rotated by 180° relative to panel (a). The Switch loops I and II are schematically indicated as dashed lines. The color key is the same as in panel (a). c The 16S rRNA PTC loop (light yellow) stacking with the P loop (pink) as a result of an immature rRNA arrangement in state A. d Specific interactions of the NSUN4 N-terminal tail (teal) and the MTG2 Obg domain (red) with the P loop (pink). Amino acid residues involved in coordinating the P loop are highlighted. e Effects of N-terminal deletion of residues 26–36 in NSUN4 on its association with mitoribosomal fractions and 55S monosome formation were determined by immunoblotting. A continuous 10–30% sucrose gradient was used to separate mitochondrial lysates from HEK 293T cells expressing the MTERF4 and NSUN4 mutant to determine their distribution and co-migration with mitochondrial ribosomal fractions. The small and large ribosomal subunit and polysomes in mitochondria isolated from NSUN4 ΔN 26-36 transfected cells were followed by immunoblotting for mitochondrial ribosomal protein markers of the small (bS16m) and large (bL12m) ribosomal subunits. The input mitochondrial lysate was used as a positive control. The blots are representative of results obtained with at least three independent biological experiments. Source data are provided as a Source Data file. f Cross-section of the mitochondrial large subunit state C assembly intermediate. The spatial arrangement of assembly factors NSUN4 (teal), MTERF4 (light green), and MTG2 (red) ensure probing of the P loop (pink) and the entrance to the nascent polypeptide tunnel in the mitochondrial large subunit assembly intermediate (gray). The mL45 (blue) N-terminal tail occupies the exit tunnel, contributing to an inactive state of the subunit.