Characterization of rat TOM40, a central component of the preprotein translocase of the mitochondrial outer membrane

We cloned a 38-kDa rat mitochondrial outer membrane protein (OM38) with structural homology to the central component of preprotein translocase of the fungal mitochondrial outer membrane, Tom40. Although it has no predictable alpha-helical transmembrane segments, OM38 is resistant to alkaline carbonate extraction and is inaccessible to proteases and polyclonal antibodies added from outside the mitochondria, suggesting that it is embedded in the membrane, probably in a beta-barrel structure, as has been similarly speculated for fungal Tom40. Immunoprecipitation demonstrated that OM38 is associated with the major import receptors rTOM20 and rTOM22, and several other unidentified components with molecular masses of 5-10 kDa in digitonin-solubilized membrane: OM10, OM7.5, and OM5. Blue native polyacrylamide gel electrophoresis revealed that OM38 is a component of a approximately 400-kDa complex, firmly associating with rTOM22 and loosely associating with rTOM20. The preprotein in transit to the matrix interacted with the TOM complex containing OM38, and immunodepletion of OM38 resulted in the loss of preprotein import activity of the detergent-solubilized and reconstituted outer membrane vesicles. Taken together, these results indicate that OM38 is a structural and functional homolog of fungal Tom40 and functions as a component of the preprotein import machinery of the rat mitochondrial outer membrane.