Calmodulin in bovine rod outer segments.

Abstract

Calmodulin (CaM) has been determined to be present in bovine retinal rod outer segments (ROS) at a level of about 450 ng of CaM per mg of ROS protein by radioimmunoassay (RIA).This was about half the level detected in a membranous retinal inner segment fraction.The ROS membranes contained two populations of high affinity binding sites for 1251-labeled CaM (& = 0.2 X lo-' M and ~3 .0 X M) with approximately equal saturabilities.The ROS and two other membranous retinal fractions contained comparable numbers of lz5I- CaM-binding sites per mg of membrane protein.The binding of lZ5I-CaM to ROS was time and temperature dependent.Binding was also Ca2' dependent and CaM specific since homologs such as troponin C or parvalbumin failed to displace "%CaM from ROS.The number of CaM-binding sites on ROS membranes could account for 64% of the total CaM content of ROS determined by RIA.CaM does not stimulate the ROS