SUBUNIT-VII OF UBIQUINOL-CYTOCHROME-C OXIDOREDUCTASE FROM NEUROSPORA-CRASSA IS FUNCTIONAL IN YEAST AND HAS AN N-TERMINAL EXTENSION THAT IS NOT ESSENTIAL FOR MITOCHONDRIAL TARGETING
G. Lobohajdu et al., SUBUNIT-VII OF UBIQUINOL-CYTOCHROME-C OXIDOREDUCTASE FROM NEUROSPORA-CRASSA IS FUNCTIONAL IN YEAST AND HAS AN N-TERMINAL EXTENSION THAT IS NOT ESSENTIAL FOR MITOCHONDRIAL TARGETING, Biochemical journal, 320, 1996, pp. 769-775
cDNA clones encoding subunit VII of the Neurospora crassa bc(1) comple
x (ubiquinol: cytochrome-c oxidoreductase), which is homologous with s
ubunit VIII of the complex from yeast (encoded by QCR8), were identifi
ed on the basis of functional complementation of a yeast QCR8 deletion
strain. The clones contain an open reading frame encoding a protein w
ith a calculated molecular mass of 11.8 kDa. The N-terminal eight resi
dues of the amino acid sequence deduced from the cDNA clones are absen
t from the mature protein, as revealed by direct sequencing of the iso
lated protein. To investigate the potential role of the N-terminal oct
apeptide in mitochondrial targeting, constructs were made encoding the
precursor and the mature form of subunit VII from Neurospora. Incubat
ion of isolated mitochondria with the two proteins revealed that the N
-terminal extension of the precursor is removed on import, However, th
e presequence does not encode information for targeting, as the protei
ns encoded by both constructs can be imported into isolated mitochondr
ia with equal efficiency. In contrast, the octapeptide seems to have f
unctional importance: the defect in the yeast qcr8-null mutant is not
complemented on transformation with the construct encoding mature subu
nit VII from N. crassa in a single-copy plasmid. We therefore speculat
e that the N-terminal extension plays a role in intramitochondrial sor
ting of N. crassa subunit VII. This is supported by the fact that the
subunit VII precursor is processed by a protease other than the genera
l mitochondrial processing peptidase. Interestingly, the presequence o
f N. crassa subunit VII has an amino acid composition similar to the o
ctapeptides cleaved off by the mitochondrial intermediate peptidase.