Synergistic interaction of tubby and tubby-like protein 1 (Tulp1).

Mutations in either tubby or tubby-like protein 1 (Tulp1) cause retinal degeneration with undefined mechanisms. We recently identified both proteins with unconventional secretion as novel MerTK-specific phagocytosis ligands for retinal pigment epithelium (RPE) cells. Using our newly-developed open reading frame (ORF) phage display as a technology for protein-protein interactions, we identified Tulp1 as a Tubby-binding protein. The interaction of tubby and Tulp1 was verified by yeast two-hybrid and protein pull-down assays. Tubby and Tulp1 form heterodimer or heterooligomer and their interaction was functionally revealed by their synergistic stimulation of RPE phagocytosis. Tubby and Tulp1 mediated phagocytosis through MerTK-dependent signaling with non-muscle myosin II redistribution leading to colocalization of phagocytosed vesicles with rearranged NMMIIA.