Cyclin A-dependent phosphorylation of the ETS-related protein, MEF, restricts its activity to the G(1) phase of the cell cycle

MEF, a recently identified member of the E74 family of ETS-related transcri ption factors, is a strong transcriptional activator of cytokine gene expre ssion. Using a green fluorescent protein gene reporter plasmid regulated by an MEF-responsive promoter, we determined that the transcriptional activit y of MEF is largely restricted to the G(1) phase of the cell cycle. MEF-dep endent transcription was suppressed by the expression of cyclin A but not b y cyclin D or cyclin E. This effect was due to the kinase activity generate d by cyclin A expression, as co-expression of the cyclin-dependent kinase i nhibitors p21 or p27, or a dominant negative form of CDK2 (DNK2), abrogated the reduction of MEF transcriptional activity by cyclin A. Cyclin A-CDK2 p hosphorylated MEF protein in vitro more efficiently than cyclin D-CDK4 or c yclin E-CDK2, and phosphorylation of MEF by cyclin A-CDK2 reduced its abili ty to bind DNA. We determined one site of phosphorylation by cyclin A-CDK2 at the C terminus of MEF, using mass-spectrometry; mutation of three serine or threonine residues in this region significantly reduced phosphorylation of MEF by cyclin A and reduced cyclin A-mediated suppression of its transa ctivating activity. These amino acid substitutions also reduced the restric tion of MEF activity to G(1). Phosphorylation of MEF by the cyclin A-CDK2 c omplex controls its transcriptional activity during the cell cycle, establi shing a novel link between the ETS family of proteins and the cell cycle ma chinery.