GOOSECOID inhibits erythrocyte differentiation by competing with Rb for PU.1 binding in murine cells

Misexpression of the dorsal mesodermal patterning factor goosecoid on the v entral side of amphibian embryos results in inhibition of blood formation i n early embryogenesis, To investigate the mechanism of this inhibition, we ectopically expressed goosecoid in erythroleukemia cells. While erythroid d ifferentiation of these cells can be induced by activin, goosecoid expressi ng cells were unresponsive to activin, We demonstrate an in vitro interacti on between the oncogene PU.1, an ets family transcription factor thought to play a role in erythropoiesis, and the goosecoid protein (GSC). Interactio n with PU.1 was specific as GSC did not bind to the ets family members, Fli -1 or Ets-2, The ability of goosecoid expressing erythroleukemia cells to d ifferentiate in response to activin was rescued by coexpression of the GSC- binding N-terminal portion of PU.1. The N-terminal portion of PU.1 was coim munoprecipitated with anti-GSC antibodies as well. The N-terminal domain of PU.1 is the region recognized by the retinoblastoma protein (Rb), a tumor suppressor gene presumably involved in erythroid differentiation. We show t hat GSC competitively inhibits binding of Rb to PU.1, Our data suggest that the suppression of blood formation by GSC could, at least in part, be medi ated by binding to PU.1.