Sequences distal to the mitochondrial targeting sequences are necessary for the maturation of the F1-ATPase beta-subunit precursor in mitochondria.

The beta-subunit of the mitochondrial F1-ATPase is synthesized as a precursor in the cytoplasm which is delivered through two bilayers bounding the mitochondria prior to its assembly with other proteins into a functional complex. In order to determine the role of the amino-terminal 50 residues of the precursor on its localization, maturation, and assembly, a set of deletions within this region of the ATP2 gene encoding the beta-subunit has been analyzed. These studies reveal that deletions between residue 10 of the F1 beta-presequence and residue 36 can still direct in vivo mitochondrial import and assembly of the mutant subunit into a functional complex. Deletions within ATP2 which contain less than the first 10 residues of the precursor are not imported. Thus, the extreme amino terminus (about half of the transient presequence) of the F1 beta-subunit can direct its mitochondrial import. The wild-type F1 beta-subunit precursor is matured by the matrix-located metalloprotease at Lys19-Gln20; however, small in-frame deletions up to 17 residues distal to this site fail to be matured either in vitro or in vivo. This nonmatured F1 beta-subunit is also assembled into a functional enzyme and supports growth of its host on a nonfermentable carbon source. These data indicate that maturation of the F1 beta-subunit precursor is dependent on a protein sequence located distal to the proteolytic maturation site which is distinct from the mitochondrial targeting sequence.