Complementation between Mitochondrial Processing Peptidase (MPP) Subunits from Different Species

Mitochondrial processing peptidase (MPP), a dimer of nonidentical subunits, is the primary peptidase responsible for the removal of leader peptides from nuclearly encoded mitochondrial proteins. Alignments of the α and β subunits of MPP (α- and β-MPP) from different species show strong protein sequence similarity in certain regions, including a highly negatively charged region as well as a domain containing a putative metal ion binding site. In this report, we describe experiments in which we combine the subunits of MPP from yeast, rat, and Neurospora crassa, both in vivo and in vitro and mesure the resultant processing activity. For in vivo complementation, we used the temperature sensitive mif1 and mif2 yeast mutants, which lack MPP activity at the nonpermissive temperature (37°C). We found that the defective α-MPP of mif2 cannot be substituted for by the α-MPP from rat or Neurospora. On the other hand, the β-MPP from rat and Neurospora can fully substitute for the defective β-MPP in the mif1 mutant. These results were confirmed in in vitro experiments in which individually expressed subunits were combined. Only combinations of the α-MPP from yeast with the β-MPP from rat or Neurospora produced active MPP.