SUBUNIT COMPOSITION OF THE PERIPHERIN RDS-ROM-1 DISK RIM COMPLEX FROMROD PHOTORECEPTORS - HYDRODYNAMIC EVIDENCE FOR A TETRAMERIC QUATERNARY STRUCTURE/

Peripherin/rds and rom-1 are homologous integral membrane protein subu nits found as an oligomeric complex at the rim regions of rod and cone photoreceptor outer segment disks. These proteins are essential for t he morphogenesis of normal outer segments and have been linked to a va riety of human retinal degenerative diseases. Previous studies have su ggested that disulfide-linked homodimers of peripherin/rds and rom-1 c an associate noncovalently to form higher order structures. We have ch aracterized the hydrodynamic properties of Triton X-100 solubilized pe ripherin/rds-rom-1 complexes from bovine ROS membranes by,eel exclusio n chromatography on Sepharose C1-6B and velocity sedimentation through H2O- and D2O-based sucrose gradients. A single hydrodynamic species i s observed which has a Stokes radius of 6.2 nm, a sedimentation coeffi cient (S-20,S-w) of 5.8 S, and a partial specific volume of 0.83 mL/g. From these data the molecular mass of the detergent-peripherin/rds-ro m-1 complex is calculated to be 240 kDa. The protein component of this complex is estimated to be 135 kDa, providing direct evidence that th e solubilized peripherin/rds-rom-1 complex is a tetramer. The abundanc e of this complex as measured by competitive ELISA and immunoaffinity purification is approximately 4% of total bovine ROS membrane protein and indicates that peripherin/rds-rom-1 tetramers are present at a rel atively high average surface density (ca. 4100/mu m(2)) at the rim sur faces of rod outer segment disks.