A family of mitochondrial proteins involved in bioenergetics and biogenesis

Abstract

THE respiratory chain complexes of mitochondria consist of many different subunits, of which only a few partake directly in electron transport. The functions of the subunits that do not contain prosthetic groups are largely unknown¹. The cytochrome reductase complex of Neurospora crassa , for example, consists of nine different subunits², of which the peripheral membrane proteins I and II (ref. 3) that are located on the matrix side of the mitochondrial inner membrane⁴ are the largest subunits devoid of redox centres. Significantly, a cytochrome reductase fraction lacking these two subunits was inactive in electron transfer⁵, and in yeast mutants with defective genes for either of the two subunits, assembly of the reductase is disrupted^6,7. Most mitochondrial proteins are imported into the mitochondrion as precursor proteins, and two proteins are necessary for cleaving their presequences⁸, namely the matrix processing peptidase (MPP) and the processing enhancing protein (PEP), the latter strongly stimulating the activity of the former⁹. Temperature-sensitive yeast mutants, which are affected in PEP or MPP, accumulate precursors at the non-permissive temperature^{10 –12}. We report here that subunit I of the cytochrome reductase complex of N. crassa. is identical to PEP and that the protein is therefore bifunctional, participating in both electron transport and protein processing. The processing proteins and subunits I and II of cytochrome reductase can be grouped as members of the same protein family.