Submitted on August 4, 2008
Revised on February 5, 2009
Accepted on February 6, 2009
Membrane targeting and intracellular trafficking of the
human sodium-dependent multivitamin transporter (hSMVT)
in polarized epithelial cells
Veedamali S Subramanian1, Jonathan S Marchant2, Micheal J Boulware3, Thomas Y Ma3, and Hamid M. Said4*
1 University of California
2 University of Minnesota
3 University of New Mexico
4 UCI/VA Medical Center
* To whom correspondence should be addressed. E-mail: hmsaid{at}uci.edu.
The human sodium-dependent multivitamin transporter (hSMVT) mediates sodium-dependent uptake of biotin in renal and intestinal epithelia. To date, however, there is nothing known about the structure-function relationship or targeting sequences in the hSMVT polypeptide that control its polarized expression within epithelia. Here, we focused on the role of the carboxy (COOH)-terminal tail of hSMVT in the targeting and functionality of this transporter. A full-length hSMVT-GFP fusion protein was functional and expressed at the apical membrane in renal and intestinal cell lines. Microtubule disrupting agents disrupted the mobility of trafficking vesicles and impaired cell surface delivery of hSMVT, which was also prevented in cells treated with dynamitin (p50), brefeldin or monensin. Progressive truncation of the COOH terminal tail impaired the functionality and targeting of the transporter. First, biotin transport decreased by ~20-30% on deletion of up to 15 COOH-terminal amino acids of hSMVT, a decrease mimicked solely by deletion of the terminal PDZ motif (TSL). Second, deletions into the COOH-terminal tail (between residues 584-612, containing a region of predicted high surface accessibility), resulted in a further drop in hSMVT transport (to ~40% of wild-type). Third, apical targeting was lost on deletion of a helical-prone region between amino acids 570-584. We conclude that the COOH-tail of hSMVT contains several determinants important for polarized targeting and biotin transport.
