The alpha4 subunit of rat alpha4beta2 nicotinic receptors is phosphorylated in vivo

The intracellular domains of the alpha4 and beta2 neuronal nicotinic subunits between transmembrane segments 3 and 4 contain a number of predicted phosphorylation sites but there is no direct evidence that any of these sites are actually phosphorylated in vivo. We expressed rat alpha4beta2 nicotinic receptors in Xenopus oocytes, labeled them by an overnight incubation in [32P]orthophosphate, and analyzed the immunoprecipitated receptors by autoradiography and Western blotting. Our results show that the oocytes contained three kinds of alpha4 subunits with apparent weights of 69, 79, and 89 kDa. The 89 kDa alpha4 subunit was the most heavily phosphorylated.