Quantitative EM analysis of ER–mitochondria contacts in HT-1080 and COS-7 cells. (A) Representative EM images of HT-1080 and COS-7 cells. Insets show rough ER–mitochondria contacts (RER-mito) in HT-1080 cells (green arrowheads) and smooth ER–mitochondria contacts (SER-mito) in HT-1080 and COS-7 cells (red arrowheads). (B) Quantification of contact width, contact length, contact length relative to mitochondria perimeter, and number of contacts per mitochondria profile are shown for SER-mito and RER-mito contacts in HT1080 and COS-7 cells. (C) The relative ratio of SER-mito and RER-mito contacts in HT-1080 and COS-7 cells based on the number of contacts per mitochondria or length of contacts. (D) The number of ribosomes per RER-mito contact is plotted versus the length of the contact in nm for HT-1080 and COS-7 cells. RER-mito contacts with five or less ribosomes are shown in red; those with more than five ribosomes are specific to HT-1080 cells and are shown in blue and defined as riboMERCs. n = 27 images from two independent biological replicates; ±SEM; ***P < 0.001; ****P < 0.0001; B: one-way ANOVA; C: Chi2 test. Bar = 500 nm; inset: 200 nm.