A novel prespore-cell-inducing factor in Dictyostelium discoideum induces cell division of prespore cells

In Dictyostelium discoideum strain V12M2, at a very low cell density (approximately 10(2) cells/cm2), most amoebae differentiate into prespore cells in a salt solution containing cAMP if an adequately diluted conditioned medium (CM) is provided (Oohata, A. A. (1995) Differentiation 59, 283-288). This finding suggests the presence of factor(s) released into the medium that are involved in inducing prespore cell differentiation. In the present study, we report the presence of two types of factors that function synergistically in prespore cell induction; one is a heat-stable and dialysable factor(s) and the other is a heat-labile and non-dialysable factor termed psi (psi) factor (prespore-inducing factor). We purified and characterized the psi factor. Its relative molecular mass was determined to be 106x10(3) Mr by SDS-PAGE and 180x10(3) Mr by gel filtration HPLC, respectively. These results indicate that psi factor exists as a dimer under native conditions. In addition to inducing prespore cell differentiation, psi factor induced cell division of prespore cells in submerged culture. Our results suggest that psi factor plays important roles not only in prespore cell differentiation but also in the progress of the cell cycle in the prespore pathway in normal development.