Dictyostelium counting-factor group-size control

The secreted-signal feedback system that sets the number of cells per aggregate (and hence fruiting-body size) in Dictyostelium discoideum. Aggregating cells secrete "counting factor" (CF), a large multisubunit complex whose principal component is Countin (CtnA); high CF activity decreases cell-cell adhesion and the size of cAMP-relay streams, breaking large aggregation territories into smaller groups. The cytosolic protein SmlA negatively regulates the secretion of counting factor: smlA-null cells oversecrete CF and form many small aggregates, whereas countin-null cells undersignal and form few large ones. The module is thus a two-element size rheostat — SmlA restrains CF output, CtnA is the secreted size-limiting signal. Phrased over the reviewed D. discoideum proteins. OUT OF SCOPE: the other CF subunits (CF45-1, CF50) which are not yet reviewed, and the downstream glucose/adhesion effectors of CF.

Dictyostelium-specific group-size control module grounded to SmlA (P54661, no determined molecular function) and Countin/CtnA (Q86IV5, GO:0005102). SmlA acts intracellularly and is not itself secreted; it negatively regulates CF secretion (GO:0050709 in the smlA review). The other counting-factor subunits (CF45-1, CF50) are named but not grounded because they have not been reviewed. Named to signal Dictyostelium scope.