Nephron localization of Na/SO4(2-)-cotransport-related mRNA and protein.

A Na+/SO4(2-)-cotransport system was recently identified from a rat kidney cortex cDNA library by expression cloning (NaSi-1;). In this work the sites of expression of NaSi-1-related mRNA and protein were determined using reverse transcriptase-polymerase chain reaction (RT-PCR) with microdissected nephron segments, and Western blot analysis on isolated tubules or purified membranes using polyclonal antibodies against the C-terminus of NaSi-1. Expression of both NaSi-1-related mRNA and protein was observed in proximal tubules and in papillary collecting ducts. Membrane separation studies indicated that in proximal tubules the NaSi-1-related protein is expressed apically. The observed distribution patterns (together with the earlier functional analysis) of NaSi-1-related mRNA and protein suggest that the NaSi-1 protein is involved in proximal tubular brush-border membrane Na+/SO4(2-)-cotransport. The functional correlation of the observed expression of the NaSi-1-related protein in collecting ducts remains to be determined.