The eye has a remarkable property in that it can function efficiently over a very wide range of illuminations from single photon to bright sun. The rod cells which have photosensitive rhodopsin are more sensitive to dim light and dark adapts to increase their sensitivity. However, the rod cells cease their sensitive phototransduction in bright light, the cone cells are in contrast operative in bright light. Rhodopsin, transducin, PDE, rhodopsin kinase and S-antigen have been known to be associated with the phototransduction cascade, our focus of interest is on rhodopsin kinase and S-antigen. In order to further understand this light dependent modulatory mechanism in rod outer segments, we have characterized S-antigen, rhodopsin kinase, calmodulin and 24K ROS specific proteins using recombinant DNA technologies. S-antigen had local regions of sequence homology with alpha-transducin including the putative rhodopsin binding and phosphoryl binding sites. Rhodopsin kinase is a family of proteins which have conserved features and similar catalytic domains among themselves. Also, calmodulin is a family of Ca++ binding proteins and it had conserved domains too. The 24k ROS specific protein did not have any sequence similarity with other known proteins. Thus, the amino acid sequences of these proteins further substantiated the functional roles of these proteins in the phototransduction cascade.
