**4.11. GLUT11** *(SLC2A11)*

The human GLUT11 was cloned by PCR on the basis of sequence information obtained from ESTs and a genomic sequence. Three variants of GLUT11 (GLUT11-A, GLUT11-B, and GLUT11-C) have been identified that only differ in their N-terminal sequences. Since for each of the variants the corresponding 5' sequences upstream of exon 1 exhibit promoter activity, transcriptional regulation is assumed to occur by alternative promoter usage. The three isoforms are expressed in a tissue specific manner: GLUT11-A is present in heart, skeletal muscle and kidney; GLUT11-B is expressed in placental, adipose, and kidney tissue; while GLUT11-C is found in adipose, heart, skeletal muscle and pancreatic tissue. Glu‐ cose transport activity for GLUT11 was detected in liposomes reconstituted with GLUT11 containing membranes. The transporter shows affinity for glucose with a Km of 0.16 mM when measured in *Xenopus laevis* oocytes. GLUT11 also transports fructose but not galactose and shows a rather low affinity for cytochalasin B. Mutational analysis of the DSV motif in GLUT11 that corresponds to the "fructose" transporter motif "NAI" also showed that also in GLUT11 this particular region of helix 7 determines substrate selectivity of the transporter. Endogenous GLUT11 protein was localized in heart and skeletal muscle tissue with an antibody raised against the C-terminus of GLUT11 that does not distinguish between the different variants. Human GLUT11 is expressed exclusively in slow-twitch muscle fibres and is unaffected by physiological and pathophysiological conditions except in primary myopathy. Surprisingly, the human SLC2A11 gene has not orthologue in the rat and mouse genome.
