The core binding factor (CBF) alpha interaction domain and the smooth muscle myosin heavy chain (SMMHC) segment of CBFbeta-SMMHC are both required to slow cell proliferation.

We have expressed several variants of core binding factor beta (CBFbeta)-smooth muscle myosin heavy chain (SMMHC) from the metallothionein promoter in Ba/F3 cells. Deletion of amino acids 2-11 from the CBFbeta segment, required for interaction with CBFalpha, prevented CBFbeta-SMMHC from inhibiting CBF DNA binding and cell cycle progression. Deletion of 283 carboxyl-terminal residues from the SMMHC domain, required for multimerization, also inactivated CBFbeta-SMMHC. Nuclear expression of CBFbeta(Delta2-11)-SMMHC was decreased relative to CBFbeta-SMMHC. CBFbeta(Delta2-11)-SMMHC linked to a nuclear localization signal still did not slow cell growth. The ability of each CBFbeta-SMMHC variant to inhibit CBF DNA binding and cell proliferation correlated with its ability to inhibit transactivation by an AML1-VP16 fusion protein. Thus, CBFbeta-SMMHC slows cell cycle progression from G1 to S phase by inhibiting CBF DNA binding and transactivation.