Identification of a Novel Human Subfamily of Mitochondrial Carriers with Calcium-binding Domains

Abstract

Aralar1 and citrin were identified as calcium binding aspartate/glutamate carriers (AGC) in mitochondria. The presence of calcium binding motifs facing the extramitochondrial space allows the regulation of the transport activity of these carriers by cytosolic calcium and provides a new mechanism to transduce calcium signals in mitochondria without the requirement of calcium entry in the organelle. We now report the complete characterization of a second subfamily of human calcium binding mitochondrial carriers named SCaMC (short calcium-binding mitochondrial carriers). We have identified three SCaMC genes in the human genome. All code for highly conserved proteins (about 70-80% identity), of about 500 amino acids with a characteristic mitochondrial carrier domain at the C terminus, and an N-terminal extension harboring four EF-hand binding motifs with high similarity to calmodulin. All SCaMC proteins were found to be located exclusively in mitochondria, and their N-terminal extensions were dispensable for the correct mitochondrial targeting of the polypeptides. SCaMC-1 is the human orthologue of the rabbit Efinal protein, which was reported to be located in peroxisomes, and SCaMC-2 is the human orthologue of the rat MCSC protein, described as up-regulated by dexamethasone in AR42J cells. One of the SCaMC genes, SCaMC-2, has four variants generated by alternative splicing, resulting in proteins with a common C terminus but with variations in their N-terminal halves, including the loss of one to three EF-hand motifs. These results make SCaMC one of most complex subfamilies of mitochondrial carriers and suggest that the large number of isoforms and splice variants may confer different calcium sensitivity to the transport activity of these carriers.

The calcium-dependent ATP-Mg/Pi mitochondrial carrier is a target of glucose-induced calcium signalling in Saccharomyces cerevisiae

“…It was identified during the sequencing of the genome of S. cerevisiae, and it was found to be non-essential [12,13]. Since the sequences of human SCaMCs and Sal1p (the protein encoded by YNL083w, renamed SAL1 by Chen [14]) are very similar, with 34-37 % identity at the amino acid level along the 300-amino-acid long C-terminus [5], we have attempted to study the function and calcium regulation of Sal1p as a first step to study that of the various human SCaMC isoforms.…”

“…This function is performed thanks to the transport of calcium from the cytosol to mitochondria across the calcium uniporter, followed by activation of calcium-sensitive dehydrogenases [1,2]. A second mechanism to transduce calcium signals to mitochondria is through CaMCs (calcium-binding mitochondrial carriers), the AGCs (aspartate/glutamate carriers) aralar and citrin [3,4], and the SCaMC (short CaMC) [5] that correspond to ATP-Mg/P i carriers [6]. CaMCs possess N-terminal extensions with EF-hand motifs (EF does not stand for elongation factor, but corresponds to α-helices E and F of parvalbumin, the archetypal calciumbinding protein).…”

Glucagon Regulation of Oxidative Phosphorylation Requires an Increase in Matrix Adenine Nucleotide Content through Ca2+ Activation of the Mitochondrial ATP-Mg/Pi Carrier SCaMC-3

“…Samples were resolved in SDS-PAGE using 10% gels, transferred to nitrocellulose membranes, and incubated with the indicated primary antibodies. Rabbit polyclonal antibodies against SCaMC-1, -2, and -3 were used at a dilution of 1:5,000, as previously described (9). Mouse monoclonal Hsp60 antibody (Sigma) and rabbit polyclonal ␤-ATPase antibody (a kind gift from Dr. J. M. Cuezva, CBMSO) were used at a dilution of 1:10,000.…”

“…In the first, entry of cytosolic Ca 2ϩ into the mitochondria would inhibit the matrix pyrophosphatase, leading to an increase in pyrophosphate that would then be exchanged by cytosolic AdNs through the ATP/ADP translocase (ANT) (6). The second mechanism, proposed by Aprille and co-workers (7,8), involves the ATP-Mg/P i carrier, a Ca 2ϩ -dependent mitochondrial carrier that has been recently identified at the molecular level (9,10).…”

“…To date, five different paralogs, termed SCaMC (short calcium-binding mitochondrial carrier) 1-3, -1-like and -3-like, have been described in mammals (9,10,(12)(13)(14), of which two, SCaMC-1-like and SCaMC-3-like, are only expressed in testis. In addition, several splicing isoforms have been described (9), making it the most complex subgroup of mitochondrial carriers (15). The transport activity catalyzed by the SCaMCs is the electroneutral and reversible exchange of ATP-Mg 2Ϫ or HADP 2Ϫ with HPO 4 2Ϫ (16).…”