Megalin and nonmuscle myosin heavy chain IIA interact with the adaptor protein Disabled-2 in proximal tubule cells

Megalin plays a critical role in the endocytosis of albumin and other filtered low-molecular-weight proteins. Here we studied the interaction between megalin and Disabled-2 (Dab2), an adaptor protein that binds to the cytoplasmic domain of megalin and appears to control its trafficking. We co-immunoprecipitated megalin and Dab2 from cultured proximal tubule cells and identified the proteins by liquid chromatography and tandem mass spectrometry. We found two proteins associated with the megalin/Dab2 complex, nonmuscle myosin heavy chain IIA (NMHC-IIA) and beta-actin. Subcellular fractionation followed by sucrose velocity gradient separation showed that megalin, Dab2, and NMHC-IIA existed as a complex in the same endosomal fractions. In vitro pull-down assays demonstrated that NMHC-IIA was bound to the carboxyl-terminal region of Dab2, but not to megalin's cytoplasmic domain. We then transfected COS-7 cells with plasmids that induced the expression of Dab2, NMHC-IIA, and the megalin minireceptor, a truncated form of megalin. Co-immunoprecipitation studies showed that the minireceptor and NMHC-IIA co-immunoprecipitated only with Dab2. Furthermore, the uptake of (125)I-lactoferrin, an endocytic ligand of megalin, by rat yolk sac-derived megalin-expressing L2 cells was inhibited by blebbistatin, a specific inhibitor of nonmuscle myosin II. Our study shows that NMHC-IIA is functionally linked to megalin by interaction with Dab2 and is likely involved in megalin-mediated endocytosis in proximal tubule cells.