AFC1, a LAMMER kinase from Arabidopsis thaliana, activates STE12-dependent processes in yeast

In the yeast Saccharomyces cerevisiae a kinase cascade activates the transcription factor STE12 leading to mating in haploid cells and pseudohyphal growth in diploid cells. To investigate related signal transduction pathways in higher plants, we have isolated a putative protein kinase gene from Arabidopsis thaliana that restores STE12-dependent functions to yeast with mutations in this signal transduction pathway. This Arabidopsis gene, AFC1, induces three STE12-dependent processes even in signal transduction-defective yeast strains: mating-specific gene expression in haploid yeast, mating of haploid yeast to yield diploids, and pseudohyphal growth in diploid yeast. AFC1 has no effect on transcription of the STE12 gene and, instead, is likely to activate the STE12 protein. However, AFC1 has only limited homology to FUS3 and KSS1, the endogenous yeast kinase regulators of STE12. AFC1 is a member of a recently described CDC2-related kinase subfamily, the LAMMER kinases. A close AFC1 homolog, AFC2, lacks STE12 activation phenotypes, indicating the specificity of AFC1. The phenotypes of AFC1 in yeast provide us with tools to elucidate the role of this kinase in Arabidopsis.