GLUTX1, a novel mammalian glucose transporter expressed in the central nervous system and insulin-sensitive tissues

Based on homology with GLUT1-5, we have isolated a cDNA for a novel glucose transporter, GLUTX1. This cDNA encodes a protein of 478 amino acids that s hows between 29 and 32% identity with rat GLUT1-5 and 32-36% identity with plant and bacterial hexose transporters. Unlike GLUT1-5, GLUTX1 has a short extracellular loop between transmembrane domain (TM) 1 and TM2 and a long extracellular loop between TM9 and TM10 that contains the only N-glycosylat ion site. When expressed in Xenopus oocytes, GLUTX1 showed strong transport activity only after suppression of a dileucine internalization motif prese nt in the amino-terminal region. Transport activity was inhibited by cytoch alasin B and partly competed by D-fructose and D-galactose. The Michaelis-M enten constant for glucose was approximately 2 mM, When translated in retic ulocytes lysates, GLUTX1 migrates as a 35-kDa protein that becomes glycosyl ated in the presence of microsomal membranes, Western blot analysis of GLUT X1 transiently expressed in HEK293T cells revealed a diffuse band with a mo lecular mass of 37-50 kDa that could be converted to a similar to 35-kDa po lypeptide following enzymatic deglycosylation. Immunofluorescence microscop y detection of GLUTX1 transfected into HEK293T cells showed an intracellula r staining. Mutation of the dileucine internalization motif induced express ion of GLUTX1 at the cell surface, GLUTX1 mRNA was detected in testis, hypo thalamus, cerebellum, brainstem, hippocampus, and adrenal gland, We hypothe size that, in a similar fashion to GLUT4 in vivo cell surface expression of GLUTX1 may be inducible by a hormonal or other stimulus.