engleski

IQGAP-related protein IqgC terminates Ras signalling during macropinocytosis and phagocytosis

Ras proteins are major regulators of cell growth, proliferation and survival and their genes are most commonly mutated oncogenes in human cancers. Amongst different signalling pathways they control, Ras proteins also regulate large-scale endocytosis, i.e. bulk fluid uptake (macropinocytosis) and large particles uptake (phagocytosis). Ras positively regulates actin-driven membrane ruffling and endocytic cup formation and it is well established that Ras-driven tumours have upregulated fluid uptake in order to support growth in a nutrient-deprived microenvironment. We investigated fine regulation of Ras activity by an IQGAP-related protein IqgC during macropinocytosis and phagocytosis in model amoeba Dictyostelium discoideum. Confocal microscopy demonstrated that IqgC is recruited almost exclusively to macropinocytic cups, where it co-localizes with active Ras, and to a lesser extent to phagocytic cups. Interaction studies identified Dictyostelium RasG as a sole endogenous Ras binding partner, whereas an in vitro biochemical assay showed that IqgC acts as a Ras GTPase activating protein (RasGAP) toward RasG, thus terminating activity of this GTPase. Consistently, functional tests performed on iqgC-null and IqgC-overexpressing cells further implicated IqgC as a negative regulator of both macropinocytosis and phagocytosis. Microscopy assays revealed that IqgC supresses fluid uptake by restraining the size of macropinosomes, without affecting the frequency of their generation. Altogether, our results define IqgC as a RasG-specific GAP that suppresses Ras signalling specifically during large-scale endocytosis. Besides being the first protein with RasGAP activity involved in the regulation of Ras on macropinosomes and phagosomes in axenic Dictyostelium strains, IqgC was also proven to be an atypical IQGAP protein family member. Namely, IQGAP proteins have lost RasGAP activity and they don’t bind Ras proteins. Thus, our results suggest that IqgC is not a genuine IQGAP and should be reassigned to the RasGAP family of proteins.

IQGAP ; Ras ; RasGAP ; macropinocytosis ; phagocytosis

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