The Glycogen Synthase Kinase-3 Phosphoproteome: GSK-3 Phosphorylates Multiple Splicing Factors and Regulates Alternative Splicing

Glycogen Synthase Kinase-3 (GSK-3) is a constitutively active, ubiquitously expressed protein kinase that regulates multiple signaling pathways. Over 100 putative GSK-3 substrates have been reported in diverse cell types based on in vitro kinase assays or genetic and pharmacological manipulation of GSK-3. Many more have been predicted based on a recurrent GSK-3 consensus motif, but this prediction has not been tested by analyzing the GSK-3 phosphoproteome. We used stable isotope labeling of amino acids in culture (SILAC) and mass spectrometry to analyze the repertoire of GSK-3 dependent substrates in mouse embryonic stem cells (ESCs). A comparison of wild-type and Gsk3a;Gsk3b knockout (DKO) ESCs revealed prominent GSK-3-dependent phosphorylation of multiple splicing factors and regulators of RNA biosynthesis, as well as proteins that regulate transcription, translation, and cell division. We demonstrate direct, GSK-3-dependent phosphorylation of the splicing factors RBM8A and PSF as well as the nucleolar protein NPM1. RNA sequencing to compare the transcriptomes of wild-type and Gsk3 DKO ESCs identified more than 210 genes that are alternatively spliced in a GSK-3-dependent manner, supporting a broad role for GSK-3 in regulating alternative splicing. Overall, this study provides the first unbiased analysis of the GSK-3 phosphoproteome and strong evidence for GSK-3 as a regulator of alternative splicing. Overall design: 2 biological replicates of Gsk3a+/+;Gsk3b+/+ (Wild Type) and Gsk3a-/-;Gsk3b-/- (DKO) E15 mouse embryonic stem cells were analyzed by RNA-Seq