engleski

Dictyostelium IQGAP-related proteins negatively regulate roles of small GTPases in promoting actin protrusions

Small GTPases from the Ras superfamily are monomeric GTP-hydrolyzing proteins that act as molecular switches, governing a variety of cellular processes, e.g. cell motility, large-scale endocytosis, cell division, and membrane trafficking, by alternating between active (GTP-bound) and inactive (GDP-bound) forms. Small GTPases are primarily deactivated by GTPase-activating proteins (GAPs). In Dictyostelium amoeba, GTPases from the Ras and Rho families regulate the formation of transient actin-based assemblies such as pseudopodia, filopodia and endocytotic cups. Mammalian IQGAP proteins are multi-domain scaffolding proteins that harbor a GAP-related domain (GRD) highly homologous to the catalytic domain of RasGAPs, but rendered catalytically inactive by substitution of key residues. Therefore, IQGAPs do not exhibit GAP activity toward small GTPases, but interact with Cdc42 and Rac1. Dictyostelium encodes four IQGAP-like proteins, DGAP1, GAPA, IqgC and IqgD. All IQGAP-related proteins in Dictyostelium contain a GRD domain, and it was shown that DGAP1 binds Rac1 and does not exhibit GAP activity. Here we show that IQGAP-related proteins in Dictyostelium suppress specific Ras and Rac activities that promote actin protrusions. Rac1-GTP binds to the Scar/WAVE complex and thereby stimulates actin polymerization at the cell’s leading edge. Remarkably, it also recruits DGAP1 into a quaternary complex with actin-bundling proteins cortexillin I and II, which is localized at the cell back. By sequestering active Rac1, DGAP1 suppresses its role in promoting actin-based protrusions. Consistently, DGAP1-overexpressing cells move more slowly and phagocyte less efficiently than wild-type cells, whereas dgap1-null cells move faster and have increased content of F-actin. Untypically for an IQGAP-related protein, IqgC does not interact with Rho-family GTPases, but instead acts as a RasG-specific GAP and colocalizes with RasG at endocytotic cups. Fluid and particle uptake is enhanced in iqgC-null cells and diminished in IqgC-overexpressing cells, indicating that IqgC suppresses actin protrusions in large-scale endocytosis. Taken together, DGAP1 and IqgC negatively regulate activity of Rac1 and RasG in promoting actin protrusions, respectively, by non-canonical mechanisms.

Dictyostelium ; DGAP1 ; IqgC

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