Progressive C-terminal deletions of the renal cystine transporter, NBAT, reveal a novel bimodal pattern of functional expression

Nearly identical proteins (denoted NAA-Tr, rBAT, D2, NEAT), cloned from mam malian kidneys, induce a largely sodium-independent high-affinity transport system for cystine, basic amino acids, and some neutral amino acids in Xen opus oocytes (system b(0,+)-like). Mutations in the human NEAT gene have be en found in several type I cystinurics. In kidney, NEAT is associated with a second, smaller protein (approximately 45 kDa), and this heterodimer has been proposed to be the minimal functional unit of the renal cystine transp orter (Wang, Y., and Tate, S, S. (1995) FEBS Lett, 368, 389-392), To deline ate regions minimally required for functional expression in oocytes, we con structed a series of C-terminal truncated mutants of rat kidney NEAT (mild- type (WT), 683 amino acids). Expression of these mutants in oocytes yielded an unusual bimodal pattern for the induction of amino acid transport activ ity. Thus, initial C-terminal truncations aborted elicitation of transport activity. The next mutant in the series, Delta 588-683, exhibited most of t he transport-inducing potential inherent in the WT/NBAT. Further deletions again attenuated transport activity. Although both the WT/NBAT and the trun cated mutant, Delta 588-683, induce qualitatively similar transport systems , the two forms of the protein exhibit contrasting sensitivities toward a p oint mutation in which the cysteine residue at position 111 was mutated to serine, This mutation did not greatly affect induction of transport by the WT/NBAT; however, the Delta 588-683 mutant was inactivated by this mutation . Our data further suggest that cysteine 111 is probably the site of disulf ide Linkage with an approximately 45-kDa oocyte protein producing a complex equivalent to that seen in kidney membranes.