Cyclic AMP-dependent protein kinase A and protein kinase C phosphorylate α4β2 nicotinic receptor subunits at distinct stages of receptor formation and maturation

Neuronal nicotinic receptor α4 subunits associated with nicotinic α4β2 receptors are phosphorylated by cyclic AMP-dependent protein kinase (PKA) and protein kinase C (PKC), but the stages of receptor formation during which phosphorylation occurs and the functional consequences of kinase activation are unknown. SH-EP1 cells transfected with DNAs coding for human α4 and/or β2 subunits were incubated with 32Pi, and PKA or PKC was activated by forskolin or phorbol 12,13-dibutyrate, respectively. Immunoprecipitation and immunoblotting of proteins from cells expressing α4β2 receptors or only α4 subunits were used to identify free α4 subunits, and α4 subunits present in immature α4β2 complexes and mature α4β2 pentamers containing complex carbohydrates. In the absence of kinase activation, phosphorylation of α4 subunits associated with mature pentamers was three times higher than subunits associated with immature complexes. PKA and PKC activation increased phosphorylation of free α4 subunits on different serine residues; only PKC activation phosphorylated subunits associated with mature α4β2 receptors. Activation of both PKA and PKC increased the density of membrane-associated receptors, but only PKC activation increased peak membrane currents. PKA and PKC activation also phosphorylated β2 subunits associated with mature α4β2 receptors. Results indicate that activation of PKA and PKC leads to the phosphorylation α4β2 receptors at different stages of receptor formation and maturation and has differential effects on the expression and function of human α4β2 receptors.