Biological implications of filamin A-bound PEBP2beta/CBFbeta retention in the cytoplasm

Multiple mechanisms regulate dynamic cytoplasmic-to-nuclear transport of transcription factors. However, little is known about the involvement of cytoskeletal proteins in this process. The heterodimeric transcription factor PEBP2/CBF is composed of a DNA-binding subunit, Runx1, and a non-DNA-binding subunit, PEBP2beta/CBFbeta. The Runx1 protein possesses nuclear localization signals and is found exclusively in the nucleus, whereas PEBP2beta is located in the cytoplasm in most cells and tissues examined thus far. We investigated the mechanism by which PEBP2beta localizes to the cytoplasm and found that it associates with filamin A, an actin-binding cytoskeletal protein. Filamin A retains PEBP2beta in the cytoplasm, thereby hindering its engagement as a Runx1 partner. When filamin A is absent, PEBP2beta moves into the nucleus and enhances Runx1-dependent transcription. These observations highlight the significance of the subcellular localization of PEBP2beta in regulating its activity as a component of the PEBP2/CBF transcription factor. In humans, PEBP2beta is frequently targeted in the leukemia-associated chromosomal abnormality, inversion 16 (inv 16). Thus, identifying the factors that mediate the subcellular localization of the PEBP2beta-derived chimeric transcription factor produced by inv 16 is an important issue that will need to be resolved in order to understand the mechanism(s) involved in inv 16-induced leukemogenesis.