MOLECULAR-BIOLOGY APPROACHES TO COMPARATIVE-STUDY OF NA+-GLUCOSE COTRANSPORT

The rabbit intestinal Na+-glucose cotransporter has been cloned and sequenced. The cDNA encoding the cotransporter has been used in two general lines of research related to comparative aspects of Na+-glucose cotransport that are reviewed here. First, defined regions of the predicted amino acid sequence were used to raise antibodies, and the species distribution of epitopes recognized by those antibodies was investigated. Intestinal brush-border membranes from mammals, birds, an amphibian, and a reptile were all found to contain proteins that were recognized by the antibodies in Western analysis. The cDNA encoding the rabbit intestinal Na+-glucose cotransporter was also used directly to examine the species distribution of related mRNA in Northern studies and to isolate new cDNAs encoding other Na+-glucose cotransporters. Northern blots revealed the presence of related mRNAs in intestines of mammals and a fish, as well as rabbit kidney and gills of marine mussels. The cDNAs encoding mammalian Na+-glucose cotransporters and bacterial Na+-dependent cotransporters for proline and pantothenate share sequence homology. There has been evolutionary conservation of the structure and function of the Na+-glucose cotransporter, and there appears to be a gene family that codes for the Na+-coupled cotransporters.