A novel role of the C-terminus of b0,+AT in the ER–Golgi trafficking of the rBAT–b0,+AT heterodimeric amino acid transporter

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.