PDZK1: II. An anchoring site for the PKA-binding protein D-AKAP2 in renal proximal tubular cells

Abstract

PDZK1, a multiple PDZ protein, was recently found to interact with the type IIa Na/Pi cotransporter (NaPi-IIa) in renal proximal tubular cells. In a preceding study, yeast two-hybrid screens using single PDZ domains of PDZK1 as baits were performed. Among the identified proteins, a C-terminal fragment of the dual-specific A-kinase anchoring protein 2 (D-AKAP2) was obtained by screening PDZ domain 4. After its molecular cloning by means of RACE, the renal expression of D-AKAP2 was analyzed by real-time polymerase chain reaction (PCR) and immunohistochemistry. Protein interactions were characterized by overlays, pull-downs, and immunoprecipitations from transfected opossum kidney (OK) cells. Based on 5'-RACE and PDZK1 overlays of mouse kidney cortex separated by two-dimensional electrophoresis, it was suggested that the renal isoform of D-AKAP2 in mouse comprises 372 amino acids and exists as a protein of >40 kD. Immunohistochemistry and real-time PCR localized D-AKAP2 only to the subapical pole of proximal tubular cells in mouse kidney. In pull-down experiments, D-AKAP2 tightly bound PDZK1 as well as N+/H+ exchanger regulator factor (NHERF-1), but the latter with an apparent fourfold lower affinity. Similarly, His-tagged D-AKAP2 specifically retained PDZK1 from mouse kidney cortex homogenate. In addition, myc-tagged D-AKAP2 and HA-tagged PDZK1 co-immunoprecipitated from transfected OK cells. We conclude that D-AKAP2 anchors protein kinase A (PKA) to PDZK1 and to a lesser extent to NHERF-1. Since PDZK1 and NHERF-1 both sequester NaPi-IIa to the apical membrane, D-AKAP2 may play an important role in the parathyroid hormone (PTH)-mediated regulation of NaPi-IIa by compartmentalization of PKA.