Pseudo-phosphatase Sbf1 contains an N-terminal GEF homology domain that modulates its growth regulatory properties

Sbf1 (SET binding factor 1) is a pseudo-phosphatase related to the myotubul arin family of dual specificity phosphatases, some of which have been impli cated in cellular growth and differentiation by virtue of their mutation in human genetic disorders. Sbf1 contains germline-encoded alterations of its myotubularin homology domain that render it non-functional as a phosphatas e. We report here the complete structure of Sbf1 and further characterizati on of its growth regulatory properties. In addition to its similarity to my otubularin, the predicted full-length Sbf1 protein contains pleckstrin (PH) and GEF homology domains that are conserved in several proteins implicated in signaling and growth control. Forced expression of wild-type Sbf1 in NI H 3T3 cells inhibited their proliferation and altered their morphology. The se effects required intact PH, GEF and myotubularin homology domains, imply ing that growth inhibition may be an intrinsic property of wild-type Sbf1. Conversely, deletion of its conserved N-terminal 44 amino acids alone was s ufficient to convert Sbf1 from an inhibitor of cellular growth to a transfo rming protein in NIH 3T3 cells. Oncogenic forms of Sbf1 partially localized to the nucleus, in contrast to the exclusively cytoplasmic subcellular loc alization of endogenous Sbf1 in all cell lines and mammalian tissues tested . These data show that the N-terminal GEF homology domain serves to inhibit the transforming effects of Sbf1, possibly sequestering the protein to the cytoplasm, and suggest that this region may be a modulatory domain that re lays growth control signals.