Abstract
Normal and hybrid proteins were synthesized following transcription of pSP64 recombinant plasmids with SP6 polymerase and translation of resultant mRNAs in a rabbit reticulocyte lysate. The precursor to the rat liver mitochondrial matrix enzyme, ornithine carbamyltransferase (Mr = approximately 40,000), was efficiently imported by rat heart mitochondria in vitro and processed to mature protein. Import of the precursor to a second matrix enzyme, carbamyl-phosphate synthetase I (Mr = approximately 165,000), could not be demonstrated. However, a 33-kDa hybrid protein, bearing the precarbamyl-phosphate synthetase I signal sequence (38 amino acids) and 55 amino acids from the amino terminus of the mature enzyme, followed by the carboxyl-terminal 209 amino acids of ornithine carbamyltransferase, was imported and processed under identical conditions. The topogenic function of the preornithine carbamyltransferase signal sequence (32 amino acids) was confirmed by constructing a hybrid protein bearing the signal sequence and five amino acids of mature enzyme, followed by 250 amino acids of the cytosolic enzyme of Escherichia coli, asparagine synthetase; the hybrid was transported to the matrix compartment of heart mitochondria where processing, albeit incorrect, took place. A hybrid asparagine synthetase bearing the pre-ornithine carbamyltransferase signal sequence plus 28 amino acids of mature ornithine carbamyltransferase, however, was imported and processed with apparent fidelity.
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