HeLa cells were co-transfected with Ub-R-GFP and F1-Myc. Mitochondria were isolated and then treated with or without proteinase K (Pro.K), or proteinase K in combination with digitonin or Triton X-100 (TX-100). Samples were then subjected to Immunoblot analysis. HSP60, TIM23 and TOM20 are representatives of mitochondrial matrix-, mitochondrial inner membrane- and mitochondrial outer membrane-localized proteins, respectively.