From inventory to functional mechanisms: regulation of the mitochondrial protein import machinery by phosphorylation

For decades, the pyruvate dehydrogenase complex in the mitochondrial matrix was considered as a rare example of how protein kinases and phosphatases can regulate important functions within this organelle. During the last decade, several proteomic studies revealed that a large fraction of mitochondrial proteins are indeed phosphorylated. A surprisingly high number of phosphorylation sites was found at the preprotein import machinery, TOM, in the outer membrane that provides the central protein import gate for most mitochondrial precursors synthesized in the cytosol. This review describes current knowledge of the mitochondrial phosphoproteome and introduces the first regulatory mechanisms of protein import dynamics by reversible phosphorylation, which have been uncovered mainly in the model organism Saccharomyces cerevisiae.