Signal sequence region of mitochondrial precursor proteins binds to mitochondrial import receptor.

An integral mitochondrial membrane protein (p32) of yeast has previously been molecularly cloned and sequenced and suggested to function as a mitochondrial import receptor. However, this protein has also been proposed to function as phosphate translocator [Guérin, B., Bukusoglu, C., Rakotomanana, F. & Wohlrab, H. (1990) J. Biol. Chem. 265, 19736-19741; Phelps, A., Schobert, C.T. & Wohlrab, H. (1991) Biochemistry 30, 248-252]. Here we have purified p32 after expression of its gene in Escherichia coli and assayed its ability to bind to various preproteins containing signal sequences for protein translocation into mitochondria, chloroplasts, or the endoplasmic reticulum. Our data suggest that p32 contains a binding site specific for the signal sequence region of mitochondrial preproteins. These data are consistent with the previous assignment of p32 as an import receptor and are discussed with regard to the apparently conflicting assignment of this protein as phosphate translocator.