CHARACTERIZATION OF A MURINE TYPE-II SODIUM-PHOSPHATE COTRANSPORTER EXPRESSED IN MAMMALIAN SMALL-INTESTINE

An isoform of the mammalian renal type II Na/P-i-cotransporter is desc ribed. Homology of this isoform to described mammalian and nonmammalia n type II cotransporters is between 57 and 75%. Based on major diversi ties at the C terminus, the new isoform is designed as type IIb Na/P-i -cotransporter, Na/P-i-cotransport mediated by the type IIb cotranspor ter was studied in oocytes of Xenopus laevis. The results indicate tha t type IIb Na/P-i-cotransport is electrogenic and in contrast to the r enal type II isoform of opposite pH dependence. Expression of type IIb mRNA was detected in various tissues, including small intestine. The type IIb protein was detected as a 108-kDa protein by Western blots us ing isolated small intestinal brush border membranes and by immunohist ochemistry was localized at the luminal membrane of mouse enterocytes. Expression of the type IIb, protein in the brush borders of enterocyt es and transport characteristics suggest that the described type IIb N a/P-i-cotransporter represents a candidate for small intestinal apical IIB Na/P-i-cotransport.