Citrin and aralar1 are Ca super(2+)-stimulated aspartate/glutamate transporters in mitochondria

• Published in: The EMBO journal Vol. 20; no. 18; pp. 5060 - 5069
• Main Authors: Palmieri, L, Pardo, B, Lasorsa, F M, Arco, Ad, Kobayashi, K, Iijima, M, Runswick, MJ, Walker, JE, Saheki, T, Satrustegui, J, Palmieri, F
• Format: Journal Article
• Language: English
• Published: 17.09.2001
• Subjects: Aralar1; aspartic acid transporter; Citrin; glutamic acid transporter; NADH
• ISSN: 0261-4189

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 super(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 super(+). 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 super(2+) on the external side of the inner mitochondrial membrane, where the Ca super(2+)-binding domains of these proteins are localized. These results show that the aspartate/glutamate carrier is regulated by Ca super(2+) through a mechanism independent of Ca super(2+) entry into mitochondria, and suggest a novel mechanism of Ca super(2+) regulation of the aspartate/malate shuttle.