Tissue-Specific Amino Acid Transporter Partners ACE2 and Collectrin Differentially Interact With Hartnup Mutations

Background & Aims: Hartnup amino acid transporter B⁰AT1 (SLC6A19) is the major luminal sodium-dependent neutral amino acid transporter of small intestine and kidney proximal tubule. The expression of B⁰AT1 in kidney was recently shown to depend on its association with collectrin (Tmem27), a protein homologous to the membrane-anchoring domain of angiotensin-converting enzyme (ACE) 2. Methods: Because collectrin is almost absent from small intestine, we tested the hypothesis that it is ACE2 that interacts with B⁰AT1 in enterocytes. Furthermore, because B⁰AT1 expression depends on an associated protein, we tested the hypothesis that Hartnup-causing B⁰AT1 mutations differentially impact on B⁰AT1 interaction with intestinal and kidney accessory proteins. Results: Immunofluorescence, coimmunoprecipitation, and functional experiments using wild-type and ace2-null mice showed that expression of B⁰AT1 in small intestine critically depends on ACE2. Coexpressing new and previously identified Hartnup disorder-causing missense mutations of B⁰AT1 with either collectrin or ACE2 in Xenopus laevis oocytes showed that the high-frequency D173N and the newly identified P265L mutant B⁰AT1 transporters can still be activated by ACE2 but not collectrin coexpression. In contrast, the human A69T and R240Q B⁰AT1 mutants cannot be activated by either of the associated proteins, although they function as wild-type B⁰AT1 when expressed alone. Conclusions: We thus show that ACE2 is necessary for the expression of the Hartnup transporter in intestine and suggest that the differential functional association of mutant B⁰AT1 transporters with ACE2 and collectrin in intestine and kidney, respectively, participates in the phenotypic heterogeneity of human Hartnup disorder.