CLONING AND EXPRESSION OF CDNA FOR A NA/PI COTRANSPORT SYSTEM OF KIDNEY CORTEX

A cDNA library from rabbit kidney cortex was screened for expression of Na-dependent transport of phosphate (P(i)) using Xenopus laevis oocytes as an expression system. A single clone was eventually isolated (designated NaP(i)-1) that stimulated expression of Na/P(i) cotransport almost-equal-to 700-fold compared to total mRNA. The predicted sequence of the Na/P(i) cotransporter consists of 465 amino acids (relative molecular mass, 51,797); hydropathy profile predictions suggest six (possibly eight) membrane-spanning segments. In vitro translation of NaPi-1/complementary RNA in the presence of pancreatic microsomes indicated NaPi-1 to be a glycosylated protein; four potential N-glycosylation sites are present in the amino acid sequence. Northern blot analysis demonstrated the presence of NaPi-1/mRNA in kidney cortex and liver; no hybridization signal was obtained with mRNA from other tissues (including small intestine). Kinetic analysis of Na/P(i) cotransport expressed by NaPi-1/complementary RNA demonstrated characteristics (sodium interaction) similar to those observed in cortical apical membranes. The alignment of 5 amino acid residues (Gly342/Ala381XaaXaaXaaXaaLeu386XaaXaaXaaPro390Arg391) is consistent with a motif proposed for Na-dependent transport systems. We conclude that we have cloned a cDNA for a Na/P(i) cotransport system present in rabbit kidney cortex.