Ser727-dependent recruitment of MCM5 by Stat1 alpha in IFN-gamma-induced transcriptional activation

Stat1 alpha is a latent cytoplasmic transcription factor activated in respo nse to interferon-gamma (IFN-gamma). The C-terminal 38 amino acids of Stat1 alpha are required to trigger transcription and therefore may possibly ser ve as a transcription activation domain (TAD), Here we show that the C-term inus of Stat1 alpha is an independent TAD which can interact with a specifi c group of nuclear proteins. Mutation of the Stat1 Ser727 and Leu724 decrea ses its transcriptional activity and affinity for the nuclear proteins. One of the interacting proteins was identified as MCMS, a member of the minich romosome maintenance (MCM) family involved in DNA replication, Both in vitr o and in vivo interaction of Stat1 alpha and MCMS were demonstrated. Furthe rmore, the in vitro interaction required Ser727 and was enhanced by its pho sphorylation, transient overexpression of MCMS enhanced transcriptional act ivation by Stat1 alpha in a Ser727-dependent manner. Finally, changes in th e level of nuclear localized MCMS during the cell cycle correlated with the changes in transcriptional response to IFN-gamma lacting through Stat1 alp ha. These results strongly suggest that MCM5 is recruited through interacti on with Stat1 alpha in a Ser727- and Leu724-dependent manner to play a role in optimal transcriptional activation.