A 40-kDa polypeptide from papain digestion of the rabbit intestinal Na+/phosphate cotransporter retains Na+ and
1Department of Physiology and Biophysics, University of Texas Medical Branch, Galveston 77555-0641, USA. bpeerce@utmb.edu
Abstract:
The rabbit intestinal brush border membrane Na+/phosphate cotransporter was digested with a variety of proteolytic enzymes. Limited papain digestion generated a 40-kDa polypeptide (P40) which retained putative substrate site markers, fluorescein isothiocyanatophenyl glyoxal and eosin n-acetyl imidazole. P40 retained Na+- and phosphate-selective tryptophan fluorescence quenching, pH sensitivity of ion-induced conformational changes, and tight Na+ and H(2)PO(4)(-) binding. Reconstituted into proteoliposomes, P40 catalyzed Na+-dependent phosphate uptake. The N-terminus of P40 was blocked. An internal sequence of P40 demonstrated that it was derived from NaPi II b. These results suggest that P40 may be a useful model system for studies of the molecular mechanism of Na+-dependent phosphate cotransport and a starting point for structural studies.
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