title: Pseudo-phosphatase Sbf1 contains an N-terminal GEF homology domain that modulates its growth regulatory properties.
abstract: Sbf1 (SET binding factor 1) is a pseudo-phosphatase related to the myotubularin family of dual specificity phosphatases, some of which have been implicated 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 phosphatase. We report here the complete structure of Sbfl and further characterization of its growth regulatory properties. In addition to its similarity to myotubularin, 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 Sbfl in NIH 3T3 cells inhibited their proliferation and altered their morphology. These effects required intact PH, GEF and myotubularin homology domains, implying that growth inhibition may be an intrinsic property of wild-type Sbf1. Conversely, deletion of its conserved N-terminal 44 amino acids alone was sufficient to convert Sbf1 from an inhibitor of cellular growth to a transforming protein in NIH 3T3 cells. Oncogenic forms of Sbf1 partially localized to the nucleus, in contrast to the exclusively cytoplasmic subcellular localization 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 relays growth control signals.
authors.affiliation: Department of Pathology, Stanford University School of Medicine, CA 94305, USA.
authors.forename: R
authors.initials: R
authors.lastname: Firestein
authors.forename: M L
authors.initials: ML
authors.lastname: Cleary
mesh_terms: D016475:3T3 Cells; D000595:Amino Acid Sequence; D000818:Animals; D002352:Carrier Proteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D002455:Cell Division; D002467:Cell Nucleus / Q000378:metabolism; D048429:Cell Size; D002471:Cell Transformation, Neoplastic / Q000378:metabolism / Q000473:pathology*; D002874:Chromosome Mapping; D017124:Conserved Sequence; D003593:Cytoplasm / Q000378:metabolism; D020662:Guanine Nucleotide Exchange Factors / Q000737:chemistry*; D006801:Humans; D047908:Intracellular Signaling Peptides and Proteins; D051379:Mice; D008969:Molecular Sequence Data; D017434:Protein Structure, Tertiary; D021381:Protein Transport; D017027:Protein Tyrosine Phosphatases / Q000737:chemistry*; D054558:Protein Tyrosine Phosphatases, Non-Receptor; D017384:Sequence Deletion / Q000235:genetics; D015398:Signal Transduction
chemical_list: D002352:Carrier Proteins; D020662:Guanine Nucleotide Exchange Factors; D047908:Intracellular Signaling Peptides and Proteins; C111446:SBF1 protein, human; D017027:Protein Tyrosine Phosphatases; D054558:Protein Tyrosine Phosphatases, Non-Receptor; C099588:myotubularin