Central component of the receptor complex responsible for the recognition and translocation of cytosolically synthesized mitochondrial preproteins. Together with TOM22 functions as the transit peptide receptor at the surface of the mitochondrion outer membrane and facilitates the movement of preproteins into the translocation pore (By similarity). Forms part of the preprotein translocase complex of the outer mitochondrial membrane (TOM complex). Belongs to the Tom20 family.