PTH-Induced Downregulation of the Type IIa Na/Pi-Cotransporter Is Independent of Known Endocytic Motifs

Abstract

Abstract. Parathyroid hormone (PTH)-induced inhibition of renal proximal tubular Na/Pi cotransport involves two consecutive steps: endocytosis followed by lysosomal degradation of the type IIa Na/Pi cotransporter. Tyrosine-, dileucine-, and diacidic-based motifs are suggested to be involved in endocytosis and/or lysosomal targeting of different plasma membrane proteins. The rat type IIa cotransporter (NaPi2) contains two cytoplasmic tyrosine residues (Y) within sequences highly homologous to tyrosine-based motifs (GY402FAM and Y509RWF), three cytoplasmic dileucine (LL101, LL374, and LI591) and two cytoplasmic diacidic motifs (EE81 and EE616). We studied the role of these motifs on the PTH-induced retrieval and lysosomal degradation of the NaPi2 cotransporter. To follow its trafficking in vivo, the NaPi2 protein was fused to the carboxyl-terminal end of the enhanced green fluorescence protein. This fusion did not impair the apical targeting or the PTH-induced endocytosis of the wild-type cotransporter when transfected in opossum kidney cells. Single and multiple Y and LL mutants retained the apical targeting and the PTH-induced degradation. Mutations of the diacidic motifs were also without effect. These data suggest that the above three motifs are not required for the PTH-induced internalization and/or degradation of the cotransporter.