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 differentiate. We have reported previously that a signal substance exists in conditioned medium of D. discoideum, and we named it prespore-cell-inducing factor (psi, Psi factor) [Oohata, Nakagawa, Tasaka, and Fujii (1997) Development 124, 2781-2787]. The factor can induce isolated amoebae to differentiate into prespore cells. Moreover, we suggested that it caused not only cell differentiation 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 and 106 kDa by SDS/PAGE. Based on these results, Psi factor exists as a dimer in normal conditions. Periodic acid/Schiff staining showed that Psi factor was a glycoprotein. It was ascertained by Edman degradation that Psi factor is blocked at the N-terminal. Treatment with pyroglutamate aminopeptidase 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 endoproteinase Lys-C. The homology search for these sequences supports the fact that Psi factor is a novel differentiation factor.