MUTATIONS OF THE BASIC-AMINO-ACID TRANSPORTER GENE ASSOCIATED WITH CYSTINURIA

To investigate the function of a basic and neutral amino acid transpor ter-like protein (rBAT) which is a candidate gene for cystinuria, we a nalysed the rBAT gene in cystinuric patients. Patient 1 is a compound heterozygote with mutations in the rBAT gene causing a glutamine-to-ly sine transition at amino acid 268, and a threonine-to-alanine transiti on at amino acid 341, who inherited these alleles from his mother (E26 8K) and father (T341A), respectively. Injection of T341A and E268K mut ant cRNAs into oocytes decreased transport activity to 53.9 % and 62.5 % of control (L-cystine transport activity in oocytes injected with w ild-type rBAT cRNA), respectively. Go-injection of E268K and T341A int o oocytes strongly decreased amino acid transport activity to 28 % of control. On the other hand, co-injection of wild-type and mutant rBAT did not decrease transport activity. Furthermore, immunological studie s have demonstrated that the reduction of amino acid transport is not due to a decrease in the amount of rBAT protein expressed in oocyte me mbranes. These results indicate that mutations in the rBAT gene are cr ucial disease-causing lesions in cystinuria. In addition, co-injection experiments suggest that rBAT may function as a transport activator o r regulatory subunit by homo- or hetero-multimer complex formation.