ACETYLCHOLINE-RECEPTOR ASSEMBLY IS STIMULATED BY PHOSPHORYLATION OF ITS GAMMA SUBUNIT

Different combinations of Torpedo acetylcholine receptor (AChR) subunits stably expressed in mouse fibroblasts were used to establish a role for phosphorylation in AChR biogenesis. When cell lines expressing fully functional AChR complexes (alpha-2-beta-gamma-delta) were labeled with P-32, only gamma and delta-subunits were phosphorylated. Forskolin, which causes a 2- to 3-fold increase in AChR expression by stimulating subunit assembly, increased unassembled gamma-phosphorylation, but had little effect on unassembled-delta. The forskolin effect on subunit phosphorylation was rapid, significantly preceding its effect on expression. The pivotal role of the gamma-subunit was established by treating alpha-beta-gamma and alpha-beta-delta-cell lines with forskolin and observing increased expression of only alpha-beta-gamma-complexes. This effect was also observed in alpha-gamma, but not alpha-delta-cells. We conclude that the cAMP-induced increase in expression of cell surface AChRs is due to phosphorylation of unassembled gamma-subunits, which leads to increased efficiency of assembly of all four subunits.