Biology subjects

Filic, V.

Publications and source records attributed to Filic, V..

3 recordsLinked to original sources

bioRxiv2026

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.

cell biology↗

bioRxiv2024

The IQGAP-related RasGAP IqgC regulates cell-substratum adhesion in Dictyostelium discoideum

Proper adhesion of cells to their environment is essential for the normal functioning of single cells and multicellular organisms. To attach to the extracellular matrix (ECM), mammalian cells form integrin adhesion complexes consisting of many proteins that together link the ECM and the actin cytoskeleton. Similar to mammalian cells, the amoeboid cells of the protist Dictyostelium discoideum also use multiprotein adhesion complexes to control their attachment to the underlying surface. However, the exact composition of the multiprotein complexes and the signaling pathways involved in the regulation of adhesion in D. discoideum have not yet been elucidated. Here we show that the IQGAP-related protein IqgC is important for normal attachment of D. discoideum cells to the substratum. Mutant iqgC-null cells have impaired adhesion, whereas overexpression of IqgC promotes directional migration. A RasGAP C-terminal (RGCt) domain of IqgC is sufficient for its localization in the ventral adhesion focal complexes, while RasGAP activity of a GAP-related domain (GRD) is additionally required for the proper function of IqgC in adhesion. We identify the small GTPase RapA as a novel direct IqgC interactor and show that IqgC participates in a RapA-regulated signaling pathway targeting the adhesion complexes that include talin A, myosin VII and paxillin B. Based on our results, we propose that IqgC is a positive regulator of adhesion, responsible for the strengthening of ventral adhesion structures and for the temporal control of their subsequent degradation.

cell biology↗

bioRxiv2024

Oscillatory dynamics of Rac1 activity in Dictyostelium discoideum amoebae

Small GTPases of the Rho family play a central role in the regulation of cell motility by controlling the remodeling of the actin cytoskeleton. In the amoeboid cells of Dictyostelium discoideum, the active form of the Rho GTPase Rac1 regulates actin polymerases at the leading edge and actin filament bundling proteins at the posterior cortex of polarized cells. However, constitutive Rac1 dynamics in D. discoideum have not yet been systematically investigated. Therefore, we monitored the spatiotemporal dynamics of Rac1 activity in vegetative amoebae using a specific fluorescent probe. We observed that plasma membrane domains enriched in active Rac1 not only exhibited stable polarization, but also showed rotations and oscillations. To simulate the observed dynamics, we developed a mass-conserving reaction-diffusion model based on the circulation of Rac1 between the membrane and the cytoplasm in conjunction with its activation by GEFs, deactivation by GAPs and interaction with the Rac1 effector DGAP1. Our theoretical model accurately reproduced the experimentally observed dynamic patterns, including the predominant anti-correlation between active Rac1 and DGAP1. Significantly, the model predicted a new colocalization regime of these two proteins in polarized cells, which we confirmed experimentally. In summary, our results improve the understanding of Rac1 dynamics and reveal how the occurrence and transitions between different regimes depend on biochemical reaction rates, protein levels and cell size. This study not only expands our knowledge of the behavior of small GTPases in D. discoideum amoebae, but also provides a simple modeling framework that can be adapted to study similar dynamics in other cell types.

systems biology↗