AQP2 is a substrate for endogenous PP2B activity within an inner medullaryAKAP-signaling complex

We have demonstrated that inner medullary collecting duct (IMCD) heavy endo somes purified from rat kidney IMCD contain the type II protein kinase A (P KA) regulatory subunit (RII), protein phosphatase (PP)2B, PKC zeta, and an RII-binding protein (relative molecular mass similar to 90 kDa) representin g a putative A kinase anchoring protein (AKAP). Affinity chromatography of detergent-solubilized endosomes on cAMP-agarose permits recovery of a prote in complex consisting of the 90-kDa AKAP, RII, PP2B, and PKC zeta. With the use of small-particle flow cytometry, RII and PKC zeta were localized to a n identical population of endosomes, suggesting that these proteins are com ponents of an endosomal multi-protein complex. P-32-labeled aquaporin-2 (AQ P2) present in these PKA-phosphorylated endosomes was dephosphorylated in v itro by either addition of exogenous PP2B or by an endogenous endosomal pho sphatase that was inhibited by the PP2B inhibitors EDTA and the cyclophilin -cyclosporin A complex. We conclude that IMCD heavy endosomes possess an AK AP multiprotein-signaling complex similar to that described previously in h ippocampal neurons. This signaling complex potentially mediates the phospho rylation of AQP2 to regulate its trafficking into the IMCD apical membrane. In addition, the PP2B component of the AKAP-signaling complex could also d ephosphorylate AQP2 in vivo.