Despite its strong transactivation domain, transcription factor FOXM1c is kept almost inactive by two different inhibitory domains.

FOXM1c (MPP2) is an activating transcription factor with several nuclear localization signals, a forkhead domain for DNA binding, and a very strong acidic transactivation domain. Despite its very strong transactivation domain, FOXM1c is kept almost inactive by two different independent inhibitory domains, the N-terminus and the central domain. The N-terminus as a specific negative-regulatory domain directly binds to and thus inhibits the transactivation domain completely. However, it lacks any transrepression potential. In contrast, the central domain functions as a strong RB-independent transrepression domain and as an RB-recruiting negative-regulatory domain. The N-terminus alone is sufficient to eliminate transactivation, while the central domain alone represses the transactivation domain only partially. This hierarchy of the two inhibitory domains offers the possibility to activate the almost inactive wild type in two steps in vitro: deletion of the N-terminus results in a strong transactivator, while additional deletion of the central domain in a very strong transactivator. We suggest that the very high potential of the transactivation domain has to be tightly controlled by these two inhibitory domains because FOXM1 stimulates proliferation by promoting G1/S transition, as well as G2/M transition, and because deregulation of such potent activators of proliferation can result in tumorigenesis.