Cytosolic adaptor protein Dab2 is an intracellular ligand of endocytic receptor gp600/megalin

gp600/megalin, an endocytic receptor, belongs to the low-density lipoprotei n receptor family. It is most abundant in the renal proximal tubular cells, where it is implicated in the reabsorption of a number of molecules filter ed through the glomerulus. The cytoplasmic tail (CT) of gp600/megalin conta ins a number of sequence similarities, which indicate that gp600/megalin mi ght be involved in signal transduction. To find intracellular proteins that would interact with the gp600/megalin CT, a human kidney cDNA library was screened by using the yeast two-hybrid system. The phosphotyrosine interact ion domain (PID) of the Disabled protein 2 (Dab2), a mammalian structural a nalogue of Drosophila Disabled, was found to bind to the gp600/megalin CT i n this system. The interaction between these two proteins was confirmed by a binding assay in vitro and by the co-immunoprecipitation of both proteins from renal cell lysates. The gp600/megalin CT contains three psi XNPXY mot ifs (in which psi represents a hydrophobic residue) that are potentially ab le to interact with PID. Analysis of the CT deletion and point-mutation var iants of gp600/megalin by the two-hybrid system revealed that the third psi XNPXY motif is most probably involved in this interaction. Dab2 is a mitog en-responsive phosphoprotein thought to be an adaptor molecule involved in signal transduction, and a suggested negative regulator of cell growth. Dab 2 is the first intracellular ligand identified for gp600/megalin; gp600/meg alin is the first known transmembrane receptor that interacts with the cyto solic protein Dab2. We speculate that their interaction might involve gp600 /megalin in signal transduction pathways or might mediate the intracellular trafficking of this receptor.