Protein kinase C in rod outer segments: Effects of phosphorylation of the phosphodiesterase inhibitory subunit

The inhibitory subunit (PDEγ) of the cGMP phosphodiesterase (PDEαβγ₂) in rod outer segments (ROS) realizes its regulatory role in phototransduction by inhibition of PDEαβ catalytic activity. The photoreceptor G-protein, transducin, serves as a transducer from the receptor (rhodopsin) to the effector (PDE) and eliminates the inhibitory effect of PDEγ by direct interaction with PDEy. Our previous study [Udovichenko, Cunnick, Gonzalez and Takemoto (1994) J. Biol. Chem. 269, 9850-9856] has shown that PDEy is a substrate for protein kinase C (PKC) from ROS and that phosphorylation by PKC increases the ability of PDEγ to inhibit PDEαβ catalytic activity. Here we report that transducin is less effective in activation of PDEαβ(γ_p)₂ (a complex of PDEαβ with phosphorylated PDEγ, PDEγ_p) than PDEαβγ₂. PDEγ_p also increases the rate constant of GTP hydrolysis of transducin (from 0.16 s^-1 for non-phosphorylated PDEy to 0.21 s^-1 for PDEγ_p). These data suggest that phosphorylation of the inhibitory subunit of PDE by PKC may regulate the visual transduction cascade by decreasing the photoresponse.