Figure 1: Comparative membrane topologies of Class I, II and III glucose transporters and the vitamin C transporter, SVCT2. Class I, II and III glucose transporters as well as SVCT2 have 12 putative transmembrane domains (TM; numbers 1-12) base on prediction algorithms for transmembrane topology. A,B. The signature sequences conserved between classes I and II (A) and class III (B) glucose transporters are shown in boxes. Differences between the position of the large extracellular loop containing the N-glycosylation site, the proline-containing motif between TM6 and TM7, and the presence of a dileucine motif in the amino-terminal tail of class III glucose transporters (except for GLUT10) have been noted. C. The consensus sites for N-glycosylation and protein kinase C phosphorylation are shown. Important sequences for SVCT2 function and sorting include five histidine residues (H109, H203, H206, H269 and H413), the signature motif ((Q/E/P)NXGXXXXT(R/ K/G)), a basolateral targeting sequence (LMAI) and a C-termini sequence for cell surface targeting, membrane incorporation, and retention (SYLPISPTFVGYTWKGLR) [105-107].