Protein phosphorylation in mitochondria from human placenta

The aim of this study was to investigate whether mitochondria from human placenta contain phosphorylated proteins and kinases. Interestingly, the placenta contains two types of mitochondria with different sizes. These are ‘heavy’ mitochondria which sediment at a much lower g force than ‘light’ mitochondria. Mitochondria were incubated with [γ32]P-ATP and labelled proteins analysed by electrophoresis and autoradiography. A major protein band of 20 kDa was detected with minor bands at 22, 38 and 85 kDa. The 20 kDa band was attenuated by 83 per cent by the co-incubation of mitochondria with Herbimycin, a tyrosine kinase inhibitor. A 20 kDa protein was also identified using an anti-tyrosine phosphate antibody and detection of this protein was significantly higher in heavy mitochondria as opposed to light mitochondria. Protein kinase A enzyme activity was also detected in mitochondria at a level not significantly different than that found in whole non-fractionated cells. These data indicate that mitochondria from human placenta contains kinase activity and phosphoproteins. These molecules may have functions in signalling systems in this organelle. Phosphoprotein signalling systems may be differentially modulated in heavy mitochondria as compared with light mitochondria.