Signaling at the gate: phosphorylation of the mitochondrial protein import machinery

Protein import into mitochondria is an essential process in every eukaryotic organism. While most of the components of the import machinery have been identified and are mechanistically quite well understood, regulation of this process had been a largely neglected area of research in the past. Recently, we demonstrated for the first time that the translocase of the outer mitochondrial membrane (TOM) is phosphorylated and regulated by several cytosolic protein kinases. Among these, casein kinase 2 (CK2) governs the assembly of TOM complexes, while protein kinase A (PKA) controls translocase function. Here, we outline the current model of protein import regulation, together with additional mitochondrial functions of CK2 and PKA. We also reflect the data on mitochondria-associated protein kinases and phosphatases in the model organism baker's yeast.