Queue Gaps Among the IQGAPs in Dictyostelium discoideum

Based on their domain organisation, four proteins from the protist Dictyostelium discoideum have been assigned to the IQGAP family of scaffold proteins. Although these proteins are shorter than animal IQGAPs, their involvement in the regulation of the actin cytoskeleton in cell motility, macroendocytosis, cytokinesis, and adhesion appears to be broadly conserved between these evolutionarily distant organisms. In this article, we show that the putative three-dimensional structure of Dictyostelium IQGAP-related proteins, as predicted by AlphaFold 3, closely corresponds to the C-terminal half of human IQGAP1, thus supporting their common origin. IqgD is the largest IQGAP-related protein in Dictyostelium, with an overall domain organisation similar to human IQGAPs. IqgD is localised in the cell cortex, interacts with F-actin and Rac1 GTPases, and primarily supports cell adhesion to the underlying surface and cell growth on bacterial lawns. DGAP1 and GAPA are truncated proteins that have retained a 700-residue-long C-terminal region of homology compared to their animal relatives. They play important, yet opposite, roles in regulating contractile cortical assemblies comprising F-actin, myosin II, and the actin-bundling proteins cortexillins, which are especially important for cytokinesis and epithelial morphogenesis. Finally, IqgC, although structurally resembling other IQGAPs, turns out to be more closely related to GAP1 proteins from fungi. This multifaceted protein carries RasGAP activity, interacts with several other small GTPases, and positively regulates macroendocytosis and cell-substratum adhesion.