A signal sequence domain essential for processing, but not import, of mitochondrial pre-ornithine carbamyl transferase
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Studies using deletion mutagenesis indicate that a processing recognition site lies proximal to the normal cleavage position between gln32 and ser33 of pre-ornithine carbamyl transferase (pOCT). pOCT cDNA was manipulated to delete codons specifying amino acids 22-30 of the signal sequence. The mutant precursor, designated pOCT delta 22-30, was imported to the matrix compartment by purified mitochondria, but remained largely unprocessed; the low level of processing that was observed did not involve the normal cleavage site. Several manipulations performed downstream of the normal pOCT processing site (deletion, substitution, and hybrid protein constructions) affected neither import nor correct processing. Our data suggest that domains specifying import and processing site recognition may be functionally segregated within the signal peptide; that processing is not requisite for import of pOCT; and that a proximal region, not involving the normal signal peptide cleavage site, is required for processing site recognition.
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