Phosphorylation of bovine rod photoreceptor cyclic GMP phosphodiesterase.

The cyclic GMP phosphodiesterase (PDE) of retinal rods plays a key role in phototransduction and consists of two catalytic subunits (PDE alpha and PDE beta) and two identical inhibitory subunits (PDE gamma). Here we report that PDE alpha and PDE gamma are phosphorylated by protein kinase(s) C (PKC) from brain and rod outer segments (ROS). These same two types of PKC also phosphorylate PDE alpha in trypsin-activated PDE (without PDE gamma). In contrast, cyclic-AMP-dependent protein kinase catalytic subunit phosphorylates both PDE alpha and PDE beta, but not PDE gamma. This kinase does not phosphorylate trypsin-activated PDE. The synthetic peptides AKVISNLLGPREAAV (PDE alpha 30-44) and KQRQTRQFKSKPPKK (PDE gamma 31-45) inhibited phosphorylation of PDE by PKC from ROS. These data suggest that sites (at least one for each subunit) for phosphorylation of PDE by PKC are localized in these corresponding regions of PDE alpha and PDE gamma. Isoenzyme-specific PKC antibodies against peptides unique to the alpha, beta, gamma, delta, epsilon and zeta isoforms of protein kinase C were used to show that a major form of PKC in ROS is PKC alpha. However, other minor forms were also present.