A novel digestive complex and its role in Hartnup disorder: trafficking of the neutral amino acid transporter B⁰AT1 by angiotensin converting enzyme 2 (ACE2)

S. Kowalczuk,¹ A. Bröer,¹ N. Tietze,¹ J.M. Vanslambrouck,² J.E.J. Rasko^2,3 and S. Bröer,¹ ¹School of Biochemistry and Molecular Biology, Australian National University, Canberra, ACT 0200, Australia, ²Gene and Stem Cell Therapy Program, Centenary Institute of Cancer Medicine and Cell Biology, University of Sydney, Camperdown, NSW 2050, Australia and ³Cell and Molecular Therapies, Sydney Cancer Centre, Royal Prince Alfred Hospital, Camperdown, NSW 2050, Australia.

In the intestine, proteins are digested by proteases and brush-border peptidases into small peptides and amino acids, which are then absorbed by peptide and amino acid transporters. Neutral amino acids are absorbed by the amino acid transporter B⁰AT1. B⁰AT1 is also expressed in the kidney, where it mediates reabsorption of neutral amino acids from the primary urine. Mutations in B⁰AT1 cause Hartnup disorder, a defect in neutral amino acid transport resulting in neutral aminoaciduria.

In the kidney, B⁰AT1 requires the auxiliary protein collectrin for trafficking to the brush-border membrane. However, collectrin is not expressed in the intestine, suggesting that a different protein facilitates B⁰AT1 trafficking in this tissue. Interestingly, the closest homologue of collectrin is a brush-border carboxypeptidase, angiotensin converting enzyme 2 (ACE2).

Coexpression of B⁰AT1 and ACE2 in Xenopus laevis oocytes* caused a dramatic increase in the surface expression of B⁰AT1. Addition of a peptide containing a carboxyterminal leucine residue to these oocytes resulted in leucine transport by B⁰AT1, demonstrating that B⁰AT1 and ACE2 form a complex that performs two consecutive steps in protein digestion and absorption. The Hartnup disorder associated mutations B⁰AT1 (D173N) and B⁰AT1 (R240Q) showed reduced interaction with both ACE2 and collectrin, thereby explaining how these mutations cause Hartnup disorder.

*Xenopus laevis oocytes were harvested by surgery of anaesthesized frogs (MS-222, 1.5g/l). The procedure was approved by the Animal Experimentation Ethics Committee of the Australian National University.

A novel digestive complex and its role in Hartnup disorder: trafficking of the neutral amino acid transporter B0AT1 by angiotensin converting enzyme 2 (ACE2)

A novel digestive complex and its role in Hartnup disorder: trafficking of the neutral amino acid transporter B⁰AT1 by angiotensin converting enzyme 2 (ACE2)