A prespore-cell-inducing factor in Dictyostelium discoideum: its purification and characterization

Under starvation conditions, amoebae of Dictyostelium discoideum aggregate to form multicellular masses; the aggregates are then initiated to differen tiate. We have reported previously that a signal substance exists in condit ioned medium of D. discoideum, and we named it prespore-cell-inducing facto r (psi, Psi factor) [Oohata, Nakagawa, Tasaka, and Fujii (1997) Development 124, 2781-2787]. The factor can induce isolated amoebae to differentiate i nto prespore cells. Moreover, we suggested that it caused not only cell dif ferentiation but also cell division. In the present study, we have purified Psi factor from the conditioned medium and characterized it. The purified Psi factor induced both prespore cell differentiation and cell division of prespore cells. Its apparent molecular mass was 180 kDa by gel filtration a nd 106 kDa by SDS/PAGE. Based on these results, Psi factor exists as a dime r in normal conditions. Periodic acid/Schiff staining showed that Psi facto r was a glycoprotein. It was ascertained by Edman degradation that Psi fact or is blocked at the N-terminal. Treatment with pyroglutamate aminopeptidas e removed the N-terminal block and allowed determination of the amino-acid sequence of Psi factor. Moreover, three internal amino-acid sequences were determined in limited proteolysis experiments using trypsin and endoprotein ase Lys-C. The homology search for these sequences supports the fact that P si factor is a novel differentiation factor.