Citrin and aralar1 are Ca(2+)-stimulated aspartate/glutamate transporters in mitochondria.
The mitochondrial aspartate/glutamate carrier catalyzes an important step in both the urea cycle and the aspartate/malate NADH shuttle. Citrin and aralar1 are homologous proteins belonging to the mitochondrial carrier family with EF-hand Ca(2+)-binding motifs in their N-terminal domains. Both proteins and their C-terminal domains were overexpressed in Escherichia coli, reconstituted into liposomes and shown to catalyze the electrogenic exchange of aspartate for glutamate and a H(+). Overexpression of the carriers in transfected human cells increased the activity of the malate/aspartate NADH shuttle. These results demonstrate that citrin and aralar1 are isoforms of the hitherto unidentified aspartate/glutamate carrier and explain why mutations in citrin cause type II citrullinemia in humans. The activity of citrin and aralar1 as aspartate/glutamate exchangers was stimulated by Ca(2+) on the external side of the inner mitochondrial membrane, where the Ca(2+)-binding domains of these proteins are localized. These results show that the aspartate/glutamate carrier is regulated by Ca(2+) through a mechanism independent of Ca(2+) entry into mitochondria, and suggest a novel mechanism of Ca(2+) regulation of the aspartate/malate shuttle.
Palmieri
L
L
Department of Pharmaco-Biology, University of Bari, Via Orabona 4, 70125 Bari, Italy.
Pardo
B
B
Lasorsa
F M
FM
del Arco
A
A
Kobayashi
K
K
Iijima
M
M
Runswick
M J
MJ
Walker
J E
JE
Saheki
T
T
Satrústegui
J
J
Palmieri
F
F
D026922:Amino Acid Transport Systems, Acidic; D017920:Antiporters; D002118:Calcium / Q000494:pharmacology*; D002135:Calcium-Binding Proteins / Q000235:genetics / Q000502:physiology*; D002352:Carrier Proteins / Q000235:genetics / Q000502:physiology*; D002460:Cell Line; D020159:Citrullinemia / Q000209:etiology*; D004926:Escherichia coli / Q000235:genetics; D006801:Humans; D007700:Kinetics; D026901:Membrane Transport Proteins; D008928:Mitochondria / Q000378:metabolism*; D033681:Mitochondrial Membrane Transport Proteins; D024101:Mitochondrial Proteins; D008956:Models, Chemical; D027361:Organic Anion Transporters; D011510:Proteolipids / Q000378:metabolism; D014162:Transfection
D026922:Amino Acid Transport Systems, Acidic; D017920:Antiporters; D002135:Calcium-Binding Proteins; D002352:Carrier Proteins; D026901:Membrane Transport Proteins; D033681:Mitochondrial Membrane Transport Proteins; D024101:Mitochondrial Proteins; D027361:Organic Anion Transporters; D011510:Proteolipids; C114606:SLC25A12 protein, human; C051046:aspartate-glutamate carrier; C020485:proteoliposomes; C008822:citrin; D002118:Calcium