The cDNAs of mammalian
amino acid transporters already identified could be grouped into four families. One of these
protein families is composed of the
protein rBAT and the heavy chain of the
cell surface antigen 4F2 (4F2hc). The cRNAs of rBAT and 4F2hc induce
amino acid transport activity via systems b(0,+) -like and y(+)L -like inXenopus oocytes respectively. Surprisingly, neither rBAT nor 4F2hc is very hydrophobic, and they seem to be unable to form a pore in the plasma membrane. This prompted the hypothesis that rBAT and 4F2hc are subunits or modulators of the corresponding
amino acid transporters. The association of rBAT with a light subunit of ~40kDa has been suggested, and such an association has been demonstrated for 4F2hc.The b(0,+)-like system expressed in oocytes by rBAT
cRNA transports
L-cystine, L-dibasic and L-
neutral amino acids with high-affinity. This transport system shows exchange of
amino acids through the plasma membrane ofXenopus oocytes, suggesting a tertiary active transport mechanism. The rBAT gene is mainly expressed in the outer stripe of the outer medulla of the kidney and in the mucosa of the small intestine. The
protein localizes to the microvilli of the proximal straight tubules (S3 segment) of the nephron and the mucosa of the small intestine. All this suggested the participation of rBAT in a high-affinity reabsorption system of
cystine and
dibasic amino acids in kidney and intestine, and indicated rBAT (named SLC3A1 in Gene Data Bank) as a good candidate gene for
cystinuria. This is an inherited aminoaciduria due to defective renal and intestinal reabsorption of
cystine and
dibasic amino acids. The poor solubility of
cystine causes the formation of renal
cystine calculi. Mutational analysis of the rBAT gene of patients with
cystinuria is revealing a growing number (~20) of
cystinuria-specific mutations, including missense, nonsense, deletions and insertions. Mutations M467T (substitution of
methionine 467 residue for
threonine) and R270X (stop
codon at
arginine residue 270) represent approximately half of the cystinuric chromosomes where mutations have been found. Mutation M467T reduces transport activity of rBAT in oocytes. All this demonstrates that mutations in the rBAT gene cause
cystinuria.Three types of
cystinuria (types, I, II and III) have been described on the basis of the genetic, biochemical and clinical manifestations of the disease. Type I
cystinuria has a complete recessive inheritance; type I heterozygotes are totally silent. In contrast, type II and III heterozygotes show, respectively, high or moderate hyperaminoaciduria of
cystine and
dibasic amino acids. Type III homozygotes show moderate, if any, alteration of intestinal absorption of
cystine and
dibasic amino acids; type II homozygotes clearly show defective intestinal absorption of these
amino acids. To date, all the rBAT
cystinuria-specific mutations we have found are associated with type I
cystinuria (~70% of the chromosomes studied) but not to types II or III. This strongly suggests genetic heterogeneity for
cystinuria. Genetic linkage analysis with markers of the genomic region of rBAT in chromosome 2 (G band 2p16.3) and intragenic markers of rBAT have demonstrated genetic heterogeneity for
cystinuria; the rBAT gene is linked to type I
cystinuria, but not to type III. Biochemical, genetic and clinical studies are needed to identify the additional
cystinuria genes; a low-affinity
cystine reabsortion system and the putative light subunit of rBAT are additional candidate genes for
cystinuria.