The heterodimeric complex composed of rBAT (related to b0,+ amino acid transporter), a single-membrane-spanning glycosylated heavy chain, and b0,+AT, a putative 12-membrane-spanning non-glycosylated light chain, is an amino acid transporter that mediates the activity of system b0,+, a major apical transport system for cystine and dibasic amino acids in renal proximal tubule and small intestine. The C-terminus of b0,+AT has been proposed to play an important role in the functional expression of the heterodimeric transporters. In the present study, to reveal the roles of the C-terminus, we analysed b0,+AT mutants whose C-termini were sequentially deleted or replaced by site-directed mutagenesis in polarized MDCKII (Madin–Darby canine kidney II), non-polarized HEK-293 (human embryonic kidney-293) and HeLa cells. Although the deletion of C-terminus of b0,+AT did not affect the formation of a heterodimer with rBAT, it resulted in the loss of apparent transport function, owing to the failure of the plasma-membrane targeting of rBAT–b0,+AT heterodimeric complex associated with incomplete glycosylation of rBAT. A motif-like sequence Val480-Pro481-Pro482 was identified in the C-terminus of b0,+AT to be responsible for the C-terminus action in promoting the trafficking of rBAT–b0,+AT heterodimeric complex from the ER (endoplasmic reticulum) to Golgi apparatus. This is, to our knowledge, the first demonstration of the active contribution of the C-terminus of a light-chain subunit to the intracellular trafficking of heterodimeric transporters. Because the motif-like sequence Val480-Pro481-Pro482 is well conserved among the C-termini of light-chain subunits, common regulatory mechanisms could be proposed among heterodimeric amino acid transporters.
A novel role of the C-terminus of b0,+AT in the ER–Golgi trafficking of the rBAT–b0,+AT heterodimeric amino acid transporter
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Shinichi Sakamoto, Arthit Chairoungdua, Shushi Nagamori, Pattama Wiriyasermkul, Kanyarat Promchan, Hidekazu Tanaka, Toru Kimura, Takeshi Ueda, Masaaki Fujimura, Yasuhiro Shigeta, Yukio Naya, Koichiro Akakura, Haruo Ito, Hitoshi Endou, Tomohiko Ichikawa, Yoshikatsu Kanai; A novel role of the C-terminus of b0,+AT in the ER–Golgi trafficking of the rBAT–b0,+AT heterodimeric amino acid transporter. Biochem J 15 January 2009; 417 (2): 441–448. doi: https://doi.org/10.1042/BJ20081798
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