Identification of the mitochondrial glutamate transporter. Bacterial expression, reconstitution, functional characterization, and tissue distribution of two human isoforms.
The mitochondrial carriers are a family of transport proteins in the inner membranes of mitochondria. They shuttle substrates, metabolites, and cofactors through this membrane and connect cytoplasm functions with others in the matrix. Glutamate is co-transported with H(+) (or exchanged for OH(-)), but no protein has ever been associated with this activity. Two human expressed sequence tags encode proteins of 323 and 315 amino acids with 63% identity that are related to the aspartate-glutamate carrier, a member of the carrier family. They have been overexpressed in Escherichia coli and reconstituted into phospholipid vesicles. Their transport properties demonstrate that the two proteins are isoforms of the glutamate/H(+) symporter described in the past in whole mitochondria. Isoform 1 is expressed at higher levels than isoform 2 in all the tissues except in brain, where the two isoforms are expressed at comparable levels. The differences in expression levels and kinetic parameters of the two isoforms suggest that isoform 2 matches the basic requirement of all tissues especially with respect to amino acid degradation, and isoform 1 becomes operative to accommodate higher demands associated with specific metabolic functions such as ureogenesis.
Fiermonte
Giuseppe
G
Department of Pharmaco-Biology, Laboratory of Biochemistry and Molecular Biology, University of Bari, Via E. Orabona 4, 70125 Bari, Italy.
Palmieri
Luigi
L
Todisco
Simona
S
Agrimi
Gennaro
G
Palmieri
Ferdinando
F
Walker
John E
JE
D027322:Amino Acid Transport System X-AG / Q000737:chemistry* / Q000378:metabolism*; D001692:Biological Transport; D001921:Brain / Q000378:metabolism; D003593:Cytoplasm / Q000378:metabolism; D004591:Electrophoresis, Polyacrylamide Gel; D004926:Escherichia coli / Q000378:metabolism; D020224:Expressed Sequence Tags; D006801:Humans; D006859:Hydrogen; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D026901:Membrane Transport Proteins / Q000096:biosynthesis* / Q000737:chemistry*; D008928:Mitochondria / Q000378:metabolism*; D033681:Mitochondrial Membrane Transport Proteins; D024101:Mitochondrial Proteins / Q000096:biosynthesis* / Q000737:chemistry*; D008969:Molecular Sequence Data; D010957:Plasmids / Q000378:metabolism; D016133:Polymerase Chain Reaction; D020033:Protein Isoforms; D013997:Time Factors; D014018:Tissue Distribution
D027322:Amino Acid Transport System X-AG; D026901:Membrane Transport Proteins; D033681:Mitochondrial Membrane Transport Proteins; D024101:Mitochondrial Proteins; D020033:Protein Isoforms; C499216:SLC25A18 protein, human; C499215:SLC25A22 protein, human; D006859:Hydrogen