For an unknown reason, several genes expressed during Dictyostelium development are regulated by cell density. This is mediated by an 80-kD glycoprotein, conditioned medium factor (CMF), which is slowly secreted and simultaneously sensed by starved cells. To examine further this eukaryotic cell density-sensing mechanism, we have isolated a cDNA encoding CMF. The derived amino acid sequence of CMF shows no obvious similarity to any known protein and thus may represent a new class of eukaryotic intercellular signal. CMF antisense transformants do not aggregate, whereas normal development is restored by the addition of purified CMF protein. This suggests that CMF might synchronize the onset of development in Dictyostelium by triggering aggregation when a majority of the cells in a given area have starved, as signaled by CMF secretion.

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 80 × 10(3) Mr glycoprotein and the other is a heterogeneous group of molecules, with relative molecular masses between 6.5 × 10(3) and 0.65 × 10(3). Interestingly, the two classes of molecules do not need to be combined for activity. We find that the 80 × 10(3) Mr CMF but not the small CMF is sequestered in vegetative cells. The 80 × 10(3) Mr CMF is then secreted by cells during early development, while the small CMF appears only during late development. Like the 80 × 10(3) Mr CMF, the small CMFs are trypsin-sensitive and contain N- and O-linked glycosylation. The breakdown products of a fraction containing 80 × 10(3) Mr CMF cochromatographed from a Sephadex G-50 column and a reverse-phase HPLC column with small CMFs. The specific activity of CMF increases roughtly 100-fold upon breakdown. The results suggest that, during differentiation, the slowly diffusing 80 × 10(3) 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 80 × 10(3) Mr CMF to a faster-diffusing, higher specific activity form then might enable cells farther from the aggregation center to differentiate.