CORE BINDING-FACTOR BETA-SMOOTH MUSCLE MYOSIN HEAVY-CHAIN CHIMERIC PROTEIN INVOLVED IN ACUTE MYELOID-LEUKEMIA FORMS UNUSUAL NUCLEAR ROD-LIKE STRUCTURES IN TRANSFORMED NIH 3T3 CELLS

Patients with the M4Eo subtype of acute myeloid leukemia almost invari ably are found to have an inversion of chromosome 16 in their leukemic cells, which results in a gene fusion between the transcription facto r called core binding factor beta (CBF beta) on 16q and a smooth muscl e myosin heavy chain (SMMHC) gene on 16p, Subcellular localizations of the wild-type CBF beta and the CBF beta-SMMHC fusion protein were det ermined by immunofluorescence of NIH 3T3 cells that overexpress wild-t ype or fusion protein. Normal CBF beta showed an unexpected perinuclea r pattern consistent with primary localization in the Golgi complex, T he CBF beta-SMMHC fusion protein had a very different pattern. Nuclear staining included rod-like crystalline structures as long as 11 mu m. The heterodimeric partner of CBF beta, CBF alpha, formed part of this complex. Cytoplasmic staining included stress fibers that colocalized with actin, probably as a consequence of the myosin heavy chain compo nent of the fusion protein, Deletion of different regions of the CBF b eta portion of the fusion protein showed that binding to CBF alpha was not required for nuclear translocation, However, deletion of parts of the SMMHC domain of the fusion protein involved in myosin-mediated fi lament formation resulted in proteins that did not form rod-like struc tures. These observations confirm previous indirect evidence that the CBF beta-SMMHC fusion protein is capable of forming macromolecular nuc lear aggregates and suggests possible models for the mechanism of leuk emic transformation.