Retina cognin does not bind to itself during membrane interaction in vitro

Retina cognin (R-cognin) is an intrinsic membrane protein of vertebrate retinal cells which supports tissue-specific cell adhesion and mediates cell type-specific associations during development. As a first step in understanding how R-cognin mediates specific adhesion of retinal cell membranes, we asked if cognin bound to another cognin molecule or to a different macromolecule, a possible cognin-binding protein. To do this, we constructed an affinity column with retinal cell membrane proteins (enriched for cognin) bound to the matrix. Proteins in a detergent extract of retinal cell membranes were exposed to this matrix and those which bound specifically eluted and identified by immunoelectrophoresis. Most prominent among these was a protein with an apparent mass of 64 kDa. The binding of this material to the column was blocked by cognin antibody. To eliminate possible artifacts of molecular interactions in vitro, we sought independent confirmation that 64 kDa protein actually bound R-cognin. Using a modified retina membrane vesicle system, we asked what proteins could be photoaffinity cross-linked to cognin during vesicle aggregation. Cross-linking produced a 114 kDa complex on gels which could be resolved into a 50 kDa (cognin) and a 64 kDa band under reducing conditions. Identification of a 64 kDa protein by independent techniques suggests that cognin promotes association of embryonic chick neural retina cells by binding to this macromolecule or these molecules. Identification of a second component in the mechanism should allow elucidation of cognin's role in mediating cell-cell interactions in developing neural retina.