Crosslinking of ROS-GC1 and GCAP-1. A suspension of purified bovine rod outer segments was incubated with 1 mM DSS, a homobifunctional crosslinker (Pierce), in the presence of 1 mM Ca²⁺ or 1 mM EGTA . Control incubations were done without addition of crosslinker (lanes C) and without crosslinker but with either Ca²⁺ or EGTA. Incubations were stopped by the addition of 50 mM Tris-HCl pH 8.0 and samples were analyzed by electrophoresis (7.5% acrylamide gel). The gel was blotted and the blot was probed by anti-ROS-GC1 (left side) and anti-GCAP-1 (right side) specific antibodies. The anti-ROS-GC1 antibody recognized a monomer, a dimer and an oligomer of ROS-GC1.²⁵ The anti-GCAP-1 antibody recognized GCAP-1,³¹ which cannot be seen on a 7.5% gel, and three bands above the ROS-GC1 monomer.