Identification of mammalian TOM22 as a subunit of the preprotein translocase of the mitochondrial outer membrane

A mitochondrial outer membrane protein of similar to 22 kDa (1C9-2) was pur ified from Vero cells assessing immunoreactivity with a monoclonal antibody , and the cDNA was cloned based on the partial amino acid sequence of the t rypsin-digested fragments. 1C9-2 had 19-20% sequence identity to fungal Tom 22, a component of the preprotein translocase of the outer membrane (the TO M complex) with receptor and organizer functions. Despite such a low sequen ce identity, both shared a remarkable structural similarity in the hydropho bicity profile, membrane topology in the Ncyt-Cin orientation through a tra nsmembrane domain in the middle of the molecule, and the abundant acidic am ino acid residues in the N-terminal domain. The antibodies against 1C9-2 in hibited the import of a matrix-targeted preprotein into isolated mitochondr ia. Blue native polyacrylamide gel electrophoresis of digitonin-solubilized outer membranes revealed that 1C9-2 is firmly associated with TOM40 in the similar to 400-kDa complex, with a size and composition similar to those o f the fungal TOM core complex. Furthermore, 1C9-2 complemented the defects of growth and mitochondrial protein import in Delta tom22 yeast cells. Take n together, these results demonstrate that 1C9-2 is a functional homologue of fungal Tom22 and functions as a component of the TOM complex.