YBEY is required for normal mitochondrial morphology and respiration. (A) MTT assay for cell proliferation reveals a slower growth of YBEY KO cells in a glucose-containing medium and nearly no growth when galactose is used as carbon source. Means ± SEM for n = 12 are shown; P-values, two-tailed Mann–Whitney test. (B) Transmission electron microscopy of HEK293T-REx cells shows enlarged mitochondria with disorganized cristae systems in YBEY KO cells. A group of mitochondria (left) and individual organelles (right) are shown. Scale bars, 500 nm. (C) Quantitative analysis of mitochondrial morphology from images like in (B). Medians, interquartile ranges, 10th and 90th percentiles are shown. For YBEY+ and YBEY KO cells, n = 116 and 88 mitochondria were analysed, respectively; P-values, two-tailed Mann-Whitney test. (D) Respiration phenotypes of YBEY KO cells, as assessed by oxygen consumption rate measurement with the Seahorse Mito Stress test. Means ± SEM for n = 8 (YBEY+) and n = 7 (YBEY KO) are shown; P-values, two-tailed Welch's test. (E) YBEY loss results in significantly decreased activities of respiratory complexes I and IV. Means ± SEM (n = 3) of citrate synthase (CS)-normalized activities are shown; P-values, two-tailed Welch's test.