H. Matsumoto et al., PHOSRESTIN-I UNDERGOES THE EARLIEST LIGHT-INDUCED PHOSPHORYLATION BY A CALCIUM CALMODULIN-DEPENDENT PROTEIN-KINASE IN DROSOPHILA PHOTORECEPTORS/, Neuron, 12(5), 1994, pp. 997-1010
Activation of PI-PLC initiates two independent branches of protein pho
sphorylation cascades catalyzed by either PKC or Ca2+/calmodulin-depen
dent protein kinase (CaMK). We find that phosrestin I (PRI), a Drosoph
ila homolog of vertebrate photoreceptor arrestin, undergoes light-indu
ced phosphorylation on a subsecond time scale which is faster than tha
t of any other protein in vivo. We determine that a CaMK activity is r
esponsible for in vitro PRI phosphorylation at Ser(366) in the C-termi
nal tryptic segment, MetLysSer(P)IleGluGlnHisArg, in which Ser(P) repr
esents phosphoserine(366). We also demonstrate that Ser(366) is the ph
osphorylation site of PRI in vivo by identifying the molecular species
resulting from in-gel tryptic digestion of purified phospho-PRI using
HPLC-electrospray ionization tandem quadrupole mass spectroscopy. Fro
m these data, we conclude that the CaMK pathway, not the PKC pathway,
is responsible for the earliest protein phosphorylation event followin
g activation of PI-PLC in living Drosophila photoreceptors.