Endothelial nitric-oxide synthase (type III) is activated and becomes calcium independent upon phosphorylation by cyclic nucleotide-dependent proteinkinases
E. Butt et al., Endothelial nitric-oxide synthase (type III) is activated and becomes calcium independent upon phosphorylation by cyclic nucleotide-dependent proteinkinases, J BIOL CHEM, 275(7), 2000, pp. 5179-5187
Endothelial nitric-oxide synthase (NOS-III) is defined as being strictly de
pendent on Ca2+/calmodulin (CaM) for activity, although NO release from end
othelial cells has been reported to also occur at intracellular free Ca2+ l
evels that are substimulatory for the purified enzyme. We demonstrate here
that NOS-III, but neither NOS-I nor -II, is rapidly and strongly activated
and phosphorylated on both Ser and Thr in the presence of cGMP-dependent pr
otein kinase II (cGK II) and the catalytic subunit of cAMP-dependent protei
n kinase (cAK) in vitro. Phosphopeptide analysis by mass spectrometry ident
ified Ser(1177), as well as Ser(633) which is situated in a recently define
d CaM autoinhibitory domain within the flavin-binding region of human NOS-I
II, Phosphoamino acid analysis identified a putative phosphorylation site a
t Thr(495) in the CaM-binding domain. Importantly, both cAK and cGK phospho
rylation of NOS-III in vitro caused a highly reproducible partial (10-20%)
NOS-III activation which was independent of Ca2+/CaM, and as much as a 4-fo
ld increase in V-max in the presence of Ca2+/CaM. cAK stimulation in intact
endothelial cells also increased both Ca2+/CaM-independent and -dependent
activation of NOS-III. These data collectively provide new evidence for cAK
and cGK stimulation of both Ca2+/CaM-independent and -dependent NOS-III ac
tivity, and suggest possible cross-talk between the NO and prostaglandin I,
pathways and a positive feedback mechanism for NO/cGMP signaling.