| 研究実績の概要 |
Mutations on the C-terminus of amino acid transporter b0,+AT cause a type of cystinuria frequently found in Japanese patients, so-called Japanese-type cystinuria. In physiological conditions, b0,+AT forms heterodimer with the glycoprotein rBAT to function as cystine and dibasic amino acid transporter at the plasma membrane of the renal proximal tissue. In order to understand the pathogenic mechanism of cystinuria mutations, we applied multiple strategies including the examination of protein structure of rBAT-b0,+AT. Continuing from the last year when we successfully solved the rBAT-b0,+AT structure with high resolution, we intensively analyzed all residues, especially the pathogenic mutations. We found that C-terminus of b0,+AT forms van-der-Waals interaction with the C-terminus of rBAT. This finding raises a clue of interaction alteration in the pathogenic mutants.
Another achievement extending from the structural information is the discovery of the role of calcium in rBAT-b0,+AT biogenesis. We identified calcium at the ectodomain of rBAT in the structure of two rBAT-b0,+AT dimers, so called super-dimer. By biochemical assays, we found that calcium binding on the super-dimer is the key process to stabilizing the complex and completing the biogenesis process at the endoplasmic reticulum. Such the process is obligatory for the protein trafficking and transport function at the plasma membrane. This finding clearly explains the pathogenic mechanism of type I cystinuria, a disease type found worldwide.
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