The TOM core complex: The general protein import pore of the outer membrane of mitochondria

Translocation of nuclear-encoded preproteins across the outer membrane of m itochondria is mediated by the multicomponent transmembrane TOM complex. We have isolated the TOM core complex of Neurospora crassa by removing the re ceptors Tom70 and Tom20 from the isolated TOM holo complex by treatment wit h the detergent dodecyl maltoside. It consists of Tom40, Tom22, and the sma ll Tom components, Tom6 and Tom7. This core complex was also purified direc tly from mitochondria after solubilization with dodecyl maltoside. The TOM core complex has the characteristics of the general insertion pore; it cont ains high-conductance channels and binds preprotein in a targeting sequence -dependent manner. It forms a double ring structure that, in contrast to th e hole complex, lacks the third density seen in the latter particles. Three -dimensional reconstruction by electron tomography exhibits two open pores traversing the complex with a diameter of similar to 2.1 nm and a height of similar to 7 nm. Tom40 is the key structural element of the TOM core compl ex.