Purified inner membrane protease I of yeast mitochondria is a heterodimer

Inner membrane protease I is bound to the outer face of the yeast mitochondrial inner membrane and mediates the proteolytic maturation of cytochrome b2 and cytochrome oxidase subunit II. We have previously shown that one of its subunits is a 21.4-kDa integral membrane protein encoded by the nuclear IMP1 gene. We have now purified the active protease approximately 300-fold from yeast mitochondria. It has an apparent molecular weight of 35,000 and contains not only Imp1p but also a previously unrecognized 19-kDa subunit. Partial amino acid sequencing identifies this subunit as the product of the recently described IMP2 gene (Nunnari, J., Fox, T., and Walter, P. (1993) Science 262, 1997-2004).