Fms signal transduction: p150(Ship), A signal transduction molecule with inositol 5-phosphatase activity

Hematopoietic cell development is controlled by a series of cytokines and growth factors that guide the production of eight different blood cell lineages from a common stem cell pool. The later stages in the production of monocytes and macrophages are regulated by the macrophage colony-stimulating factor (M-CSF) and its receptor (Fms). Binding of homodimeric M-CSF to Fms initiates the dimerization and transphosphorylation of Fms on at least four known tyrosine residues. Two of these tyrosines, Y697 and Y721 for murine c-Fms are involved directly in Fms signalling. The SH2 domain(s) of the p85 subunit of PI3 kinase binds to the phosphorylated Y721 resulting in its activation and translocation to the plasma membrane. The SH2 domain of another adapter protein, Grb2, binds to the phosphorylated Y697 resulting in the translocation of the nucleotide exchange factor mSos to the plasma membrane where it activates Ras by exchanging GDP for GTP. Tyrosine to phenylalanine mutations on Fms that abolish its interactions with PI3K (Y721F) and/or Grb2/Sos (Y697F) do not exhibit any adverse effects of M-CSF dependent growth or differentiation in myeloid cells, suggesting the existence of redundant signalling pathways. {dollar}rm p52sb{lcub}Shc{rcub}{dollar} phosphorylation provides a binding site for Grb2/Sos, while the Shc PTB domain binds another tyrosine phosphorylated protein of about 150 kDa (p150). A partial cDNA for p150 was obtained using a modified yeast two hybrid system that contained a tyrosine kinase. Using this partial cDNA as a probe, a full length cDNA for p150 was identified containing an open reading frame translating for a 1190 amino acid protein. Structural features include an N-terminal SH2 domain and a C-terminal tail containing two potential Shc-PTB binding motifs (NPXY) and at least three proline rich sequences in good agreement with concensus motifs for binding to SH3 domains. A central domain is highly homologous (81%) to the 51C gene product (INPLP) and although only about 25-35% identity exists between p150 and the known 5-phosphatases, OCRL and inositol triphosphate 5-phosphatase type II (IT5P2), p150 is a bonafide inositol 5-phosphatase. It removes the D5 phosphate specifically from the second messangers PtdIns(3,4,5){dollar}rm Psb3{dollar} and/or Ins(1,3,4,5){dollar}rm Psb4{dollar} both of which are the end products of PI3K. p150 is the only 5-Pase currently known to contain an SH2 domain and was proposed to be named {dollar}rm p150sp{lcub}Ship{rcub}{dollar} for {dollar}rm{lcub}underline SH{rcub}2{dollar} containing Inositol Phosphatase. {dollar}rm p150sp{lcub}Ship{rcub}{dollar} antibodies recognize a 150 kDa protein in hematopoietic cells but {dollar}rm p150sp{lcub}Ship{rcub}{dollar} is not detected in fibroblasts. Although, stable fibroblast cell lines expressing exogenous {dollar}rm p150sp{lcub}Ship{rcub}{dollar} were easily obtained, overexpression of Ship in hematopoietic cells was not possible. These preliminary results suggest that in myeloid cells, {dollar}rm p150sp{lcub}Ship{rcub}{dollar} might function in providing a negative growth regulatory signal by inhibiting the growth stimulatory effect of PI3K.