FIGURE 2.
Does-dependent effect of the cytoplasmic proteins on dephosphorylation in Opsin-P. Shown is dephosphorylation in Opsin-P in rod membranes (A) and in cone membranes (B) in the presence of their cytoplasmic proteins at various concentrations indicated (mg/ml). The level of phosphorylation is expressed as the number of phosphates in an opsin molecule (Pi/opsin). Each data point in A and B is shown as the mean ± S.D. (n = 3). C, dose-dependent effect of the cytoplasmic proteins on the initial dephosphorylation rate constant (see “Experimental Procedures”) in rod and cone membranes. Initial rates of dephosphorylation (Pi/opsin) obtained in A and B are plotted against the protein concentration of the cytoplasmic proteins in rod (open circles) and cone (filled triangles) membranes. Each data point is shown as the mean ± S.D. (n = 3), except that obtained at 150 mg/ml protein concentration in cone membranes; it was examined only one time because of limitation of availability of cone material. The results were fitted to the Michaelis-Menten equation for rod (dotted curve) and cone (solid curve) membranes. The fitted Km and Vmax values in rod membranes were 2.7 mg/ml and 0.16 Pi released from an opsin molecule per min (Pi/opsin/min), respectively, and those in cone membranes were 17 mg/ml and 0.47 Pi/opsin/min. Data at 0.4 and 10 mg/ml in rod membranes and those at 0.4 mg/ml in cone membranes are not shown in A and B, respectively, to avoid crowding.