FAR1 AND FUS3 LINK THE MATING PHEROMONE SIGNAL-TRANSDUCTION PATHWAY TO 3 G1-PHASE CDC28 KINASE COMPLEXES

In the yeast Saccharomyces cerevisiae, the Cdc28 protein kinase contro ls commitment to cell division at Start, but no biologically relevant G1-phase substrates have been identified. We have studied the kinase c omplexes formed between Cdc28 and each of the G1 cyclins Cln1, Cln2, a nd Cln3. Each complex has a specific array of coprecipitated in vitro substrates. We identify one of these as Far1, a protein required for p heromone-induced arrest at Start. Treatment with alpha-factor induces a preferential association and/or phosphorylation of Far1 by the Cln1, Cln2, and Cln3 kinase complexes. This induced interaction depends upo n the Fus3 protein kinase, a mitogen-activated protein kinase homolog that functions near the bottom of the alpha-factor signal transduction pathway. Thus, we trace a path through which a mitogen-activated prot ein kinase regulates a Cdc2 kinase.