Phosphate deprivation induces overexpression of two proteins related to the rat renal phosphate cotransporter NaPi-2

Polyclonal antibodies were raised in rabbits against the C-terminal portion of the rat renal brush-border membrane sodium/phosphate cotransporter NaPi-2. Antibody specificity and molecular sizes of proteins related to NaPi-2 were assayed by Western blot analysis. Proteins of 40 and 70-75 kDa (p40 and p70) were immunodetected in rat and mouse brush-border membranes and proteins of 72 and 82 kDa were detected in rabbit. The absence or presence of beta-EtSH in the samples before electrophoresis greatly influenced the immunodetection profile of the rat proteins. Since the 40 kDa protein (p40) can only be detected under reducing conditions, it probably originates from reduction of disulfide bonds in p70. Tryptic cleavage of p40 and p70 revealed identical protein fragments showing the close structural identity of those proteins. Both proteins were more abundant in the outer cortex portion of the rat kidney than in the juxtamedullary portion. Furthermore, rats fed a low-phosphate diet for 24 h showed a 20- and 14-fold increase in the amount of p40 and p70, respectively, compared to control rats, showing that the adaptation to P-i deprivation by increasing renal phosphate reabsorption is not only the result of overproduction of p70, as previously shown, but is also due to the novel p40 which most probably derives from p70.