Fig. 5. Cytochrome c oxidase (COX) complexes diffuse into 0 mitochondria. Stably transfected 143B-0 cells expressing mtRFP and 143B-+ cells expressing mtGFP were co-plated and analyzed without further treatment (A), or 8 hours (B) or 12 hours (C) after PEG-mediated cell fusion. To inhibit mitochondrial (B) and/or cytosolic (B,C) protein synthesis, fused cells were treated with chloramphenicol (B) and/or cycloheximide (B,C). (A) 143B-0 cells expressing mtRFP display a punctate morphology and are devoid of mitochondrially encoded COX2. 143B-+ cells expressing mtGFP are filamentous and contain mitochondrially encoded COX2. Eight (B) and twelve (C) hours after PEG-mediated cell fusion, polykaryons derived from the fusion of + and 0 cells depict filamentous mitochondria that are positive for mtRFP, mtGFP and COX2. In some polykaryons, the amount of COX2 is lower than that of mtGFP in certain regions of the mitochondrial network (arrowheads), revealing a lower mobility of COX2.