Protein kinase C and cAMP-dependent protein kinase phosphorylate the beta subunit of the purified gamma-aminobutyric acid A receptor.

A number of recent studies have suggested that phosphorylation of the gamma-aminobutyric acid A (GABAA) receptor could modulate receptor function. Activators of protein kinase C and cAMP-dependent protein kinase have been shown to influence GABAA receptor function. In addition, Sweetnam et al. [Sweetnam, P. M., Lloyd, J., Gallombardo, P., Malison, R. T., Gallager, D. W., Tallman, J. F. & Nestler, E. J. (1988) J. Neurochem. 51, 1274-1284] have reported that a kinase associated with a partially purified preparation of the receptor could phosphorylate the alpha subunit of the receptor. Moreover, Kirkness et al. [Kirkness, E. F., Bovenkerk, C. F., Ueda, T. & Turner, A. J. (1989) Biochem. J. 259, 613-616] have recently shown that cAMP-dependent protein kinase could phosphorylate a muscimol binding polypeptide of the GABAA receptor. To explore the issue further, we have examined the ability of specific kinases to catalyze significant phosphorylation of the GABAA receptor that has been purified to near homogeneity. The GABAA receptor was purified as previously described using benzodiazepine affinity chromatography. The purified receptor possessed no detectable kinase activity. Protein kinase C and cAMP-dependent protein kinase catalyzed the phosphorylation of the beta and alpha subunits of the receptor. However, most of the phosphate incorporation was associated with the beta subunit. Two muscimol binding polypeptides designated beta 58 (Mr 58,000) and beta 56 (Mr 56,000) were present in the preparation. The higher molecular weight polypeptide, beta 58, was phosphorylated specifically by cAMP-dependent protein kinase. beta 56 was phosphorylated specifically by protein kinase C. beta 58 and beta 56 gave distinct patterns in a one-dimensional phosphopeptide analysis. The stoichiometry of phosphorylation (mol of phosphate/mol of muscimol binding) catalyzed by cAMP-dependent protein kinase was 0.52 and that catalyzed by protein kinase C was 0.38. Taken together these data confirm that there are two forms of the beta subunit of the GABAA receptor and suggest that these two forms of the beta subunit are phosphorylated by distinct kinases.