cAMP-Dependent Phosphorylation of the Nicotinic Acetylcholine Receptor: Characterization of the Protein Kinase in Torpedo Electric Organ, Lack of Correlation in Myotubes between Increased Intracellular Levels of cAMP and Influx of Monovalent Ions, Block of Influx by Forskolin Independently of cAMP Levels

Abstract

The nicotinic acetylcholine receptor (nAChR) from Torpedo electroplax is phosphorylated by at least three different protein kinases (PK) (review: Heilbronn et al, 1985), one of which is cAMP-dependent (Huganir & Greengard, 1983; Zavoiko et al, 1984; Eriksson et al, 1985). Experiments performed in vitro with exogenous catalytic PK subunit (Huganir & Greengard, 1983) or with endogenous PK and receptor (Eriksson et al, 1986) show that cAMP-dependent PK mainly phosphorylates the and the subunits of the receptor. From the known primary structure of Torpedo nAChR (Noda et al, 1983) it can be deduced that the serine residues suggested to face the inside of the electrocyte may be the target of this phosphorylation (Vandlen et al, 1979; Huganir et al, 1983). Preliminary sequence studies on the phosphorylated S subunit of the electric organ receptor (Liljeqvist et al, unpublished) as well as work with a synthetic peptide (Safran, personal communication) suggest that only one of the serines is phosphorylated.