Bm. Willardson et al., REGULATION OF PHOSDUCIN PHOSPHORYLATION IN RETINAL RODS BY CA2+ CALMODULIN-DEPENDENT ADENYLYLCYCLASE/, Proceedings of the National Academy of Sciences of the United Statesof America, 93(4), 1996, pp. 1475-1479
The phosphoprotein phosducin (Pd) regulates many guanine nucleotide bi
nding protein (G protein)-linked signaling pathways. In visual signal
transduction, unphosphorylated Pd blocks the interaction of light-acti
vated rhodopsin with its G protein (G(t)) by binding to the beta gamma
subunits of G(t) and preventing their association with the G(t) alpha
subunit. When Pd is phosphorylated by cAMP-dependent protein kinase,
it no longer inhibits G(t) subunit interactions. Thus, factors that de
termine the phosphorylation state of Pd in rod outer segments are impo
rtant in controlling the number of G(t)s available for activation by r
hodopsin. The cyclic nucleotide dependencies of the rate of Pd phospho
rylation by endogenous cAMP-dependent protein kinase suggest that cAMP
, and not cGMP, controls Pd phosphorylation. The synthesis of cAMP by
adenylyl cyclase in rod outer segment preparations was found to be dep
endent on Ca2+ and calmodulin. The Ca2+ dependence was within the phys
iological range of Ca2+ concentrations in rods (K-1/2 = 230 +/- 9 nM)
and was highly cooperative (n(app) = 3.6 +/- 0.5). Through its effect
on adenylyl cyclase and cAMP-dependent protein kinase, physiologically
high Ca2+ (1100 nM) was found to increase the rate of Pd phosphorylat
ion 3-fold compared to the rate of phosphorylation at physiologically
low Ca2+ (8 nM). No evidence for Pd phosphorylation by other Ca2+-depe
ndent kinases was found. These results suggest that Ca2+ can regulate
the light response at the level of G(t) activation through its effect
on the phosphorylation state of Pd.