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Articles in PresS, published online ahead of print February 12, 2002
Am J Physiol Renal Physiol, 10.1152/ajprenal.00212.2001
Submitted on July 6, 2001
Accepted on February 6, 2002
1 Institute of Physiology, University of Zurich, Zurich, Switzerland
* To whom correspondence should be addressed. E-mail: verrey{at}physiol.unizh.ch.
The luminal uptake of L-cystine and cationic amino acids by (re)absorptive epithelia, as found in the small intestine and the proximal kidney tubule, is mediated by the transport system b0,+, which is defective in cystinuria. Expression studies in Xenopus oocytes and other non-epithelial cells as well as genetic studies on cystinuria patients have demonstrated that two gene products, the glycoprotein rBAT and the multitransmembrane-domain protein b0,+AT, are required for system b0,+ function. To study in an epithelial context the biosynthesis, surface expression, polarity and function of this heterodimer, we have established stable MDCK cell lines expressing rBAT and/or b0,+AT. Confocal immunofluorescence microscopy shows that both subunits depend on each other for apical surface expression. Immunoprecipitation of biosynthetically labeled proteins indicates that b0,+AT is stable in the absence of rBAT whereas rBAT is rapidly degraded in the absence of b0,+AT. When both are coexpressed, they associate covalently and rBAT becomes fully glycosylated and more stable. Functional experiments show that the expressed transport is of high affinity b0,+-type, and restricted to the apical side of the epithelia. In conclusion, coexpression experiments in MDCK epithelia strongly suggest that the intracellular association of rBAT and b0,+AT is required for the surface expression of either subunit, which together form a functional heterocomplex at the apical cell membrane.
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