Cells respond to and bind countin, a component of a multisubunit cell number counting factor

In Dictyostelium discoideum counting factor (CF), a secreted ∼450-kDa complex of polypeptides, inhibits group and fruiting body size. When the gene encoding countin (a component of CF) was disrupted, cells formed large groups. We find that recombinant countin causes developing cells to form small groups, with an EC₅₀ of ∼3 ng/ml, and affects cAMP signal transduction in the same manner as semipurified CF. Recombinant countin increases cell motility, decreases cell-cell adhesion, and regulates gene expression in a manner similar to the effect of CF. However, countin does not decrease adhesion or group size to the extent that semipurified CF does. A 1-min exposure of developing cells to countin causes an increase in F-actin polymerization and myosin phosphorylation and a decrease in myosin polymerization, suggesting that countin activates a rapid signal transduction pathway. ¹²⁵I-Labeled countin has countin bioactivity, and binding experiments suggest that vegetative and developing cells have ∼53 cell-surface sites that bind countin with a K_D of ∼1.5 ng/ml or 60 pM. We hypothesize that countin regulates cell development through the same pathway as CF and that other proteins within the complex may modify the activity of countin and/or have independent size-regulating activities.