IDENTIFICATION OF DARLIN, A DICTYOSTELIUM PROTEIN WITH ARMADILLO-LIKEREPEATS THAT BINDS TO SMALL GTPASES AND IS IMPORTANT FOR THE PROPER AGGREGATION OF DEVELOPING CELLS

We purified from Dictyostelium lysates an 88-kDa protein that bound to a subset of small GTPases, including racE, racC, cdc42Hs, and TC4ran, but did not bind to R-ras or rabB. Cloning of the gene encoding this 88-kDa protein revealed that it contained multiple armadillo-like repe ats most closely related to the mammalian GTP exchange factor smgGDS. We named this protein darlin (Dictyostelium armadillo-like protein). D isruption of the gene encoding darlin demonstrated that this protein i s not essential for cytokinesis, pinocytosis, phagocytosis, or develop ment. However, the ability of darlin null cells to aggregate in respon se to starvation is severely affected. When starved under liquid mediu m, the mutant cells were unable to form aggregation centers and stream s, possibly because of a defect in cAMP relay signaling. This defect w as not due to an inability of the darlin mutants to activate adenylate cyclase in response to G protein stimulation. These results suggest t hat the darlin protein is involved in a signaling pathway that may mod ulate the chemotactic response during early development.