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pubmed:abstractText |
The dual signal approach, i.e. a mitochondrial signal at the N-terminus and an ER (endoplasmic reticulum) or a peroxisomal signal at the C-terminus of EGFP (enhanced green fluorescent protein), was employed in transfected HeLa cells to test for a co-translational import model. The signal peptide from OTC (ornithine transcarbamylase) or arginase II was fused to the N-terminus of EGFP, and an ER or peroxisomal signal was fused to its C-terminus. The rationale was that if the free preprotein remained in the cytosol, it could be distributed between the two organelles by using a post-translational pathway. The resulting fusion proteins were imported exclusively into mitochondria, suggesting that co-translational import occurred. Native preALDH (precursor of rat liver mitochondrial aldehyde dehydrogenase), preOTC and rhodanese, each with the addition of a C-terminal ER or peroxisomal signal, were also translocated only to the mitochondria, again showing that a co-translational import pathway exists for these native proteins. Import of preALDH(sp)-DHFR, a fusion protein consisting of the leader sequence (signal peptide) of preALDH fused to DHFR (dihydrofolate reductase), was studied in the presence of methotrexate, a substrate analogue for DHFR. It was found that 70% of the preALDH(sp)-DHFR was imported into mitochondria in the presence of methotrexate, implying that 70% of the protein utilized the co-translational import pathway and 30% used the post-translational import pathway. Thus it appears that co-translational import is a major pathway for mitochondrial protein import. A model is proposed to explain how competition between binding factors could influence whether or not a cytosolic carrier protein, such as DHFR, uses the co- or post-translational import pathway.
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pubmed:affiliation |
Department of Biochemistry, Purdue University, 175 S. University Street, West Lafayette, IN 47907-2063, USA.
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