Cytoplasmic ribosomes primed for protein import cluster on the mitochondrial membrane. (A) Representative plots of the K(r)/KCSR(r) ratio for mitochondrial-associated cytoplasmic ribosomes oriented for protein import within a range of radius (r) values of 27–166 nm. The black line equals to 1. (B) Quantification of the maximum value of K(r)/KCSR(r) for each tomogram at the indicated radius intervals for each ribosome class. Quantification from import-oriented ribosomes n = 87 and non-import-oriented ribosomes n = 89 tomograms are shown. P values from Mann–Whitney U test are indicated. *P < 0.05; ****P < 0.001. (C) Representative membrane surface reconstructions of mitochondria (gray) with ribosomes oriented for import relative to the OMM (blue). Insets show zoomed-in boxed regions of the ribosome models with circle overlays demarking the location of the 3′ mRNA entry (blue), the 5′ mRNA exit sites (orange), the possible pathways of interconnecting mRNA (dashed black line), and the calculated end-to-end distance from 5′ to 3′ of each interconnected mRNA (solid black line). (D) Membrane surface reconstruction of mitochondria (gray) and ER (purple) membranes with corresponding models for co-translating (blue) and non-co-translating ribosomes (pink). (E) Combined histogram of IMM–OMM distances of co-translation-associated and non-co-translation-associated patches in S. cerevisiae treated with CHX. Dashed vertical lines correspond to peak histogram values of pooled data. (F) Histograms of IMM–OMM distances of co-translation-associated and non-co-translation-associated patches in S. cerevisiae treated with vehicle (e.g., no CHX). Dashed vertical lines correspond to peak histogram values of pooled data.
