Background and aims:

Hartnup amino acid transporter B⁰AT1 (SLC6A19) is the major luminal sodium-dependent neutral amino acid transporter of kidney proximal tubule and small intestine. The expression of B⁰AT1 in kidney proximal tubule was recently shown to depend on its association with Collectrin (Tmem27), a homologous of angiotensin converting enzyme 2 (ACE2). Since Collectrin is almost absent from small intestine, we tested the hypothesis that its homolog ACE2 interacts with B⁰AT1 in enterocytes. Furthermore, since B⁰AT1 expression depends on the presence of an associated protein, we tested the differential impact of Hartnup-causing mutations on B⁰AT1 interaction with kidney and intestinal accessory proteins.

Results: 

Immunofluorescence and western blot experiments in collectrin and ace2 null mice demonstrate that expression of B⁰AT1 in small intestine critically depends on ACE2. In contrast ACE2 is not required for the expression of B⁰AT1 in kidney where Collectrin is the accessory protein. Co-expression of four newly and five previously identified Hartnup disorder-causing missense mutations of B⁰AT1 with Collectrin or ACE2 in Xenopus laevis oocytes reveals differential interactions. For instance, we show that the function of the high frequency D173N and of the newly identified P265L mutant B⁰AT1 transporters is increased in the presence of ACE2 but not of Collectrin.

Conclusion: 

Our data show that ACE2 and Collectrin are tissue-specific partner proteins of the amino acid transporter B⁰AT1 in intestine and kidney, respectively. Differential functional association of mutant B⁰AT1 transporters with tissue-specific proteins is suggested to participate to the phenotypic heterogeneity of human Hartnup disorder.