The imported preprotein of the proteolipid subunit of the mitochondrial ATP synthase from Neurospora crassa. Molecular cloning and sequencing of the mRNA.

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Abstract
The proteolipid subunit of the mitochondrial ATP synthase from Neurospora crassa is an extremely hydrophobic protein of 81 amino acid residues, which is imported into mitochondria as a precursor of mol. wt. 15 000. The primary structure of the imported form has now been determined by isolating and analyzing cDNA clones of the preproteolipid mRNA. An initial cDNA clone was identified by hybridizing total polyadenylated RNA to pooled cDNA recombinant plasmids from an ordered clone bank and subsequent cell‐free translation of hybridization‐selected mRNA. Further preproteolipid clones were identified at a frequency of 0.2% by colony filter hybridization. One isolated cDNA represented the major part of the preproteolipid mRNA. The nucleotide sequence showed 243 bases corresponding to the mature proteolipid and, in addition, 178 bases coding for an amino‐terminal presequence . Non‐coding sequences of 48 bases at the 5′ end and of 358 bases at the 3′ end plus a poly(A) tail were determined. The long presequence of 66 amino acids is very polar, in contrast to the lipophilic mature proteolipid, and includes 12 basic and no acidic side chains. It is suggested that the presequence is specifically designed to solubilize the proteolipid for post‐translational import into the mitochondria.