Regulation and processing of a secreted protein that mediates sensing of cell density in Dictyostelium

Abstract

During Dictyostelium development, the expression of some genes is dependent on cell density. This effect is mediated by soluble factors referred to as conditioned medium factors (CMFs) which the developing cells secrete at very low rates and simultaneously sense. There are at least two classes of CMFs: one is an 80X103 Mr glycoprotein and the other is a heterogeneous group of molecules, with relative molecular masses between 6.5X103 and 0.65X103. Interestingly, the two classes of molecules do not need to be combined for activity. We find that the 80X103 Mr CMF but not the small CMF is sequestered in vegetative cells. The 80X103 Mr CMF is then secreted by cells during early development, while the small CMF appears only during late development. Like the 80X103 Mr CMF, the small CMFs are trypsin-sensitive and contain N- and O-linked glycosylation. The breakdown products of a fraction containing 80X103 Mr CMF cochromatographed from a Sephadex G-50 column and a reverse-phase HPLC column with small CMFs. The specific activity of CMF increases roughly 100-fold upon breakdown. The results suggest that, during differentiation, the slowly diffusing 80X103 Mr CMF is first produced from a precursor pool already present in vegetative cells, allowing differentiation of only those cells in the immediate vicinity of the aggregation center. The breakdown of 80X103 Mr CMF to a faster-diffusing, higher specific activity form then might enable cells farther from the aggregation center to differentiate.