The cleaved presequence is not required for import of subunit 6 of the cytochrome bc(1) complex into yeast mitochondria or assembly into the complex

Subunit 6 of the yeast cytochrome bc(1) complex contains a 25 amino acid pr esequence that is not present in the mature form of the protein in the bc(1 ) complex, The presequence of subunit 6 is atypical of presequences respons ible for targeting proteins to mitochondria, Whereas mitochondrial targetin g sequences rarely contain acidic residues and typically contain basic resi dues that can potentially form an amphiphilic structure, the presequence of subunit 6 contains only one basic amino acid and is enriched in acidic ami no acids. If the 25 amino acid presequence is deleted, subunit 6 is importe d into mitochondria and assembled into the cytochrome bc(1) complex and the activity of the bc(1) complex is identical to that from a mild-type yeast strain. However, if the C-terminal 45 amino acids are truncated from the pr otein, subunit 6 is not present in the mitochondria and the activity of the bc(1) complex is diminished by half, identical to that of the bc(1) comple x from a yeast strain in which the QCR6 gene is deleted. These results indi cate that the presequence of subunit 6 is not required for targeting to mit ochondria or assembly of the subunit into the bc(1) complex and that inform ation necessary for targeting and import into mitochondria may be present i n the C-terminus of the protein. (C) 1999 Federation of European Biochemica l Societies.