IqgD is a Rac1-interacting IQGAP required for efficient growth of Dictyostelium discoideum on bacterial lawns

IQGAPs are large multidomain scaffold proteins that interact with the Rho family GTPases Cdc42 and Rac1, functioning both as their effectors and as regulators by stabilizing their active GTP-bound state. In this study, we analyzed the function of IqgD, an IQGAP-related protein from the professional phagocyte Dictyostelium discoideum. IqgD contains a calponin homology domain (CHD), a GAP-related domain (GRD), and a RasGAP C-terminal (RGCt) domain. We show that the CHD is essential for F-actin binding and cortical localization, whereas the GRD and RGCt domains mediate interactions with Rac1 GTPases and the actin-bundling proteins cortexillins. Moreover, similar to mammalian IQGAPs, IqgD maintains Rac1 in its active conformation. IqgD is enriched in macropinocytic and phagocytic cups and co-localizes with F-actin and active Rac1 in the ring-like structure that forms around surface-bound particles at the cell bottom. Loss of IqgD results in markedly reduced growth on bacterial lawns and significantly smaller cell size. While mutant cells internalize bacteria from suspension as efficiently as wild-type cells, they display a strong defect in phagocytosis of surface-bound particles, accompanied by decreased adhesion to the cell substrate. Together, our data show that although IqgD localizes to macroendocytic cups, it is dispensable for macropinocytosis and phagocytosis of suspended particles. Instead, IqgD is specifically required for efficient phagocytosis of surface-bound particles, likely by facilitating robust F-actin polymerization at the cell bottom to generate the force necessary for detachment of surface-bound bacteria. Significance StatementPhagocytosis of surface-bound microbes is essential for host defense and environmental feeding strategies, yet its underlying mechanisms remain poorly understood. We identify the IQGAP-related protein IqgD in D. discoideum as a key factor required for efficient uptake of bacteria attached to solid surfaces. IqgD localizes to an F-actin- and Rac1-rich circular structure analogous to the phagocytic adhesion ring (PAR) recently described in mammalian macrophages, suggesting that this mode of force-driven particle detachment is evolutionarily conserved. Our findings provide mechanistic insight into substrate-dependent phagocytosis and establish IqgD as a central regulator of this process.