Identification of a region on the outer surface of the CBFβ-SMMHC myeloid oncoprotein assembly competence domain critical for multimerization

In the core binding factor (CBF)β-smooth muscle myosin heavy chain (SMMHC) acute myeloid leukemia (AML) oncoprotein, CBFβ lies N-terminal to the α-helical rod domain of SMMHC. Deletion of the SMMHC assembly competence domain (ACD), conserved among skeletal, smooth and nonmuscle myosins, prevents multimerization, inhibition of CBF and inhibition of cell proliferation. To define the amino acids critical for ACD function, three outer surface residues of ACD helices A-D, the subsequent helices E-H or the more N-terminal X or Z helices were now mutated. Variants were assessed for multimerization in low ionic strength in vitro and for nuclear localization as a measure of in vivo multimerization. Mutation of individual helices C-H reduced multimerization, with alteration of the outer surface of helices D or E having the greatest effect. The ability of these SMMHC variants to slow murine myeloid progenitor proliferation largely paralleled their effects on multimerization. Divergence at the boundaries of the ACD may reflect quantitative differences between in vitro and in vivo filament assembly. Each helix mutant retained the ability to bind the mSin3A corepressor. Agents interacting with the outer surface of the CBFβ-SMMHC ACD that prevent multimerization may be effective as novel therapeutics in AML.