Abstract
Analysis of the secondary structure of human and rat ornithine carbamyltransferase's targeting sequence revealed the presence of a highly homologous domain with the following key features: an hydrophobic patch opposite to an hydrophilic surface characterized by the disposition of basic residues at potentially strategic positions. The functional role of this domain was established using a synthetic peptide corresponding to amino acids 1-19 of the rat ornithine carbamyltransferase precursor (pOCT 1-19). When added to an in vitro import assay system, pOCT (1-19) blocked the import of pOCT specifically: it did not impede the entry and processing of the precursor to subunit 2 of the F1-ATPase (p beta). This finding suggests that at least two distinct precursor(s)-specific pathways are required for the import of mitochondrial inner membrane and matrix proteins.
Highlights
Analysis of the secondary structure of humanand that arises from the absence of homology between these raotrnithincearbamyltransferase’tsargeting se- presequences, shown to be necessary and sufficient for directquence revealed the presence of a highly homologous ing proteins to mitochondria [11,12,13,14,15], and the fact that the domain with the following key features: an hydropho- number of mitochondrial proteins to be imported far exceeds bic patch opposite to an hydrophilic surface charactetrh-at of potential “receptors” on the mitochondrial surface
Depicting well this issue is the case of ornithine carbamyltransferase (OCT),’ a tissue-specific nuclear-coded enzyme of the urea cycle localized exclusivelyto the mitochondrial matrix of hepatocytes and epithelial cells of the intestinal mucosa [16]
This analysis showed that the homology of the mature enzyme portion is 93%, whereas that of their presequences is only 69%, suggesting either 1)
Summary
Since all known selective transport systems are mediated Analysis of the Secondary Structureof pOCT TargetingSequenceby specific gene products, it is generally believed that translocation of nuclear-coded mitochondrial proteins into mitochondria proceeds via a mechanism similar to thaotf receptormediated transport systems. In support of this hypothesis, recent reports have indicated that segregation and/or trans-. The primary structure of the rat anhduman pOCT presequences (1719)were fedinto an IBM XT microcomputer,and secondarystructure predictions were performed by a program (Protein Information Analyzer, PRINAS) obtained from Mitsui Knowledge Industry Co., Tokyo, Japan This program includes the algorithm of Garnier and Robson for the prediction of secondary structures [21,22].
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