The type IIa Na+-dependent inorganic phosphate (Na/P-i) cotransporter is lo
calized in the apical membrane of proximal tubular cells and is regulated b
y an endocytotic pathway. Because molecular processes such as apical sortin
g, internalization, or subsequent degradation might be assisted by associat
ed proteins, a yeast two-hybrid screen against the C-terminal, cytosolic ta
il of type IIa cotransporter was designed. Most of the potential proteins f
ound belonged to proteins with multiple PDZ modules and were either identic
al/related to PDZK1 or identical to NHERF-1. Yeast trap truncation assays c
onfined the peptide-protein association to the C-terminal amino acid residu
es TRL of type IIa cotransporter and to single PDZ domains of each identifi
ed protein, respectively. The specificity of these interactions were confir
med in yeast by testing other apical localized transmembraneous proteins. M
oreover, the type IIa protein was recovered in vitro by glutathione S-trans
ferase-fused PDZ proteins from isolated renal brush border membranes or fro
m type IIa-expressing oocytes. Further, these PDZ proteins are immunohistoc
hemically detected either in the microvilli or in the subapical compartment
of proximal tubular cells. Our results suggest that the type IIa Na/P-i co
transporter interacts with various PDZ proteins that might be responsible f
or the apical sorting, parathyroid hormone controlled endocytosis or the ly
sosomal sorting of internalized type IIa cotransporter.