title: Weak mitochondrial targeting sequence determines tissue-specific subcellular localization of glutamine synthetase in liver and brain cells.
abstract: Evolution of the uricotelic system for ammonia detoxification required a mechanism for tissue-specific subcellular localization of glutamine synthetase (GS). In uricotelic vertebrates, GS is mitochondrial in liver cells and cytoplasmic in brain. Because these species contain a single copy of the GS gene, it is not clear how tissue-specific subcellular localization is achieved. Here we show that in chicken, which utilizes the uricotelic system, the GS transcripts of liver and brain cells are identical and, consistently, there is no difference in the amino acid sequence of the protein. The N-terminus of GS, which constitutes a 'weak' mitochondrial targeting signal (MTS), is sufficient to direct a chimeric protein to the mitochondria in hepatocytes and to the cytoplasm in astrocytes. Considering that a weak MTS is dependent on a highly negative mitochondrial membrane potential (DeltaPsi) for import, we examined the magnitude of DeltaPsi in hepatocytes and astrocytes. Our results unexpectedly revealed that DeltaPsi in hepatocytes is considerably more negative than that of astrocytes and that converting the targeting signal into 'strong' MTS abolished the capability to confer tissue-specific subcellular localization. We suggest that evolutional selection of weak MTS provided a tool for differential targeting of an identical protein by taking advantage of tissue-specific differences in DeltaPsi.
authors.affiliation: Department of Biochemistry, George S. Wise Faculty of Life Sciences, Tel Aviv University, 69978 Tel Aviv, Israel.
authors.forename: Gideon D
authors.initials: GD
authors.lastname: Matthews
authors.forename: Noa
authors.initials: N
authors.lastname: Gur
authors.forename: Werner J H
authors.initials: WJ
authors.lastname: Koopman
authors.forename: Ophry
authors.initials: O
authors.lastname: Pines
authors.forename: Lily
authors.initials: L
authors.lastname: Vardimon
mesh_terms: D017398:Alternative Splicing; D000595:Amino Acid Sequence; D000818:Animals; D001921:Brain / Q000378:metabolism* / Q000648:ultrastructure; D002478:Cells, Cultured; D002645:Chickens; D005974:Glutamate-Ammonia Ligase / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D047468:Immunoprecipitation; D008099:Liver / Q000378:metabolism* / Q000648:ultrastructure; D013058:Mass Spectrometry; D053078:Membrane Potential, Mitochondrial / Q000502:physiology; D046529:Microscopy, Electron, Transmission; D008928:Mitochondria / Q000378:metabolism* / Q000648:ultrastructure; D008969:Molecular Sequence Data; D020133:Reverse Transcriptase Polymerase Chain Reaction; D017386:Sequence Homology, Amino Acid
chemical_list: D005974:Glutamate-Ammonia Ligase