Effects of phosphorylation site mutant Drp1 on mitochondrial morphology. (A) Epifluorescence micrographs of immunofluorescently labelled mitochondria from CV1 fibroblasts in which endogenous Drp1 was replaced with wild-type (WT), Ser656Ala and Ser656Asp GFP–Drp1 (transfected cells are outlined). (B) Quantification of the mitochondrial area in CV1 cells substituted with WT, Ser656Ala (A) and Ser656Asp (D) GFP–Drp1 by computer-aided image analysis (mean±s.e.m. of six experiments with 60–80 cells per condition each, normalized to WT). *P<0.01, **P<5 × 10−4 by Student's t-test. Drp1, dynamin-related protein 1; GFP, green fluorescent protein.