Nt. Price et al., CLONING AND SEQUENCING OF 4 NEW MAMMALIAN MONOCARBOXYLATE TRANSPORTER(MCT) HOMOLOGS CONFIRMS THE EXISTENCE OF A TRANSPORTER FAMILY WITH ANANCIENT PAST, Biochemical journal, 329, 1998, pp. 321-328
Measurement of monocarboxylate transport kinetics in a range of cell t
ypes has provided strong circumstantial evidence for a family of monoc
arboxylate transporters (MCTs). Two mammalian MCT isoforms (MCT1 and M
CT2) and a chicken isoform (REMP or MCT3) have already been cloned, se
quenced and expressed, and another MCT-like sequence (XPCT) has been i
dentified. Here we report the identification of new human MCT homologu
es in the database of expression sequence tags and the cloning and seq
uencing of four new full-length MCT-like sequences from human cDNA lib
raries, which we have denoted MCT3, MCT4, MCT5 and MCT6. Northern blot
ting revealed a unique tissue distribution for the expression of mRNA
for each of the seven putative MCT isoforms (MCT1-MCT6 and XPCT). All
sequences were predicted to have 12 transmembrane (TM) helical domains
with a large intracellular loop between TM6 and TM7. Multiple sequenc
e alignments showed identities ranging from 20% to 55%, with the great
est conservation in the predicted TM regions and more variation in the
C-terminal than the N-terminal region. Searching of additional sequen
ce databases identified candidate MCT homologues from the yeast Saccha
romyces cerevisiae, the nematode worm Caenorhabditis elegans and the a
rchaebacterium Sulfolobus solfataricus. Together these sequences const
itute a new family of transporters with some strongly conserved sequen
ce motifs, the possible functions of which are discussed.