Recognition of preproteins by the isolated TOM complex of mitochondria

A multisubunit complex in the mitochondrial outer membrane, the TOM complex , mediates targeting and membrane translocation of nuclear-encoded preprote ins. We have isolated the TOM hole complex, containing the preprotein recep tor components Tom70 and Tom20, and the TOM core complex, which lacks these receptors, The interaction of recombinant mitochondrial preproteins with b oth types of soluble TOM complex was analyzed. Preproteins bound efficientl y in a specific manner to the isolated complexes in the absence of chaperon es and lipids in a bilayer structure. Using fluorescence correlation spectr oscopy, a dissociation constant in the nanomolar range was determined. The affinity was lower when the preprotein was stabilized in its folded conform ation. Following the initial binding, the presequence was transferred into the translocation pore in a step that required unfolding of the mature part of the preprotein. This translocation step was also mediated by protease-t reated TOM hole complex, which contains almost exclusively Tom40, Thus, the TOM core complex, consisting of Tom40, Tom22, Tom6 and Tom7, is a molecula r machine that can recognize and partially translocate mitochondrial precur sor proteins.