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1 Medicine, UCI/VA Medical centers, Long Beach, California, United States
2 Pharmacology, Univ of Minnesota Med School, Minneapolis, Minnesota, United States
3 Medical Research, UCI/VA Medical Center, Long Beach, California, United States; Medicine & Physiology/ Biophysics, Univerisity of California, Medical Scinces I, C-354, Irvine, California, 92697, United States
* To whom correspondence should be addressed. E-mail: hmsaid{at}uci.edu.
The human proton coupled folate transporter (hPCFT) is a recently discovered intestinal transporter involved in folate uptake in epithelia (and possibly other cells). Little is currently known about the structure-function relationship of the different domains of this transporter, in particular which regions are important for substrate transport as well as targeting of the transporter to the apical cell surface of polarized cells. Here we have investigated the role of the COOH-terminal domain and a well conserved sequence separating transmembrane (TM) domains TM2 and TM3 (DXXGRR; amino acids 109-114) speculated by others to be important for transport function. Using live cell imaging approaches, we demonstrate that (i) a hPCFT-YFP construct is functionally expressed at the apical membrane domain, and is localized differentially to the human reduced folate carrier (hRFC), (ii) the predicted cytoplasmic COOH-terminal region of hPCFT is not essential for apical targeting or transporter functionality, (iii) mutations that ablate a consensus b-turn sequence separating predicted TM2 and TM3 abolished apical 3H-folic acid uptake as a consequence of endoplasmic reticulum retention of mutant, likely misfolded, transporters, and (iv) cell surface delivery of hPCFT is disrupted by microtubule depolymerization, or by overexpression of the dynactin complex dynamitin (p50). For the first time, our data presents information regarding structure-function and membrane targeting of the hPCFT polypeptide, as well as the mechanisms that control its steady state expression in polarized cells.
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