GLUT7: a new intestinal facilitated hexose transporter.

The very last member of the SLC2A gene family of facilitated hexose transporters to be cloned was SLC2A7 (hGLUT7). It has been assigned to the class II of the GLUT family on the basis of sequence similarity, and its closest family member is GLUT5, an intestinal fructose transporter. GLUT7 is primarily expressed in the small intestine and colon, although mRNA has been detected in the testis and prostate as well. The protein is expressed in the apical membrane of the small intestine and colon, and it has a high affinity (<0.5 mM) for glucose and fructose. The abundance of the protein in the small intestine does change in parallel with the dietary carbohydrate. However, the distribution of GLUT7 along the small intestine does not entirely match with the availability of glucose and fructose, suggesting that the physiological substrate for this transporter has yet to be identified. Unlike GLUT13, the proton-coupled myoinositol transporter (HMIT), there is no evidence for the coupling of protons to the hexose movement via GLUT7. One area of study in which GLUT7 has provided a useful comparison with GLUT1 has been in the development of the hypothesis that the facilitated hexose transporters may have a selectivity filter at the exofacial opening of the translocation pore, which helps to determine which hexoses can be transported. If substantiated, the elucidation of this mechanism may prove useful in the design of hexose analogs for use in cancer imaging and therapeutics.