PURIFIED INNER MEMBRANE PROTEASE-I OF YEAST MITOCHONDRIA IS A HETERODIMER

Inner membrane protease I is bound to the outer face of the yeast mito chondrial inner membrane and mediates the proteolytic maturation of cy tochrome b(2) and cytochrome oxidase subunit II. We have previously sh own that one of its subunits is a 21.4-kDa integral membrane protein e ncoded by the nuclear IMP1 gene. We have now purified the active prote ase approximately 300-fold from yeast mitochondria. It has an apparent molecular weight of 35,000 and contains not only Imp1p but also a pre viously unrecognized 19-kDa subunit. Partial amino acid sequencing ide ntifies this subunit as the product of the recently described IMP2 gen e (Nunnari, J., Fox, T., and Walter, P. (1993) Science 262, 1997-2004) .