The outer membrane of plant mitochondria contains a calcium-dependent protein kinase and multiple phosphoproteins

Highly purified mitochondria from potato (Solanum tuberosum L. cv. Bintje) tubers were subfractionated into a matrix fraction, an inner membrane fraction and an outer membrane fraction with minimal cross-contamination. When the matrix and inner membrane fractions were incubated with [gamma-32P]ATP only one and three prominent phosphoproteins were detected after SDS-PAGE and autoradiography, respectively. In contrast, more than 20 phosphoproteins could be labelled in the outer membrane fraction, the main ones at 12, 18, 26, 43, 58, 60, 65, 74 and 110 kDa. Only one band, at 18 kDa, was detectable when the labelling was done in the presence of EGTA. We conclude that the outer membrane of plant mitochondria contains at least one Ca(2+)-dependent protein kinase and more than 20 endogenous substrates.