The ethylene-inducible PK12 kinase mediates the phosphorylation of SR splicing factors

The tobacco PK12 is induced by the plant hormone ethylene and is a member of the LAMMER family of protein kinases. Members of this family contain in their C-terminus a unique 'EHLAMMERI/VLGPLP' motif of unknown function, and are related to cyclin- and mitogen-activated protein (MAP)-dependent kinases. The animal members of this class play a role in differentiation. They phosphorylate and physically interact with serine/arginine-rich (SR) splicing factors in vivo to alter their activity and the splicing of target mRNAs. SR proteins have been recently described in plants. The capability of PK12 LAMMER kinase to bind and phosphorylate SR proteins was tested in vitro by kinase and binding assays. The tobacco PK12 phosphorylated both animal and plant SR proteins and specifically interacted with the plant splicing factor atSRp34/SR1. In addition, by site-directed mutagenesis, the LAMMER motif was found to be required for PK12 kinase activity but was not necessary for substrate binding. Consistent with a role in phosphorylation of splicing factors, PK12 was found to localize to the nucleus when transiently over-expressed in suspension cells.