engleski

Functions of Dictyostelium protein IqgC in macropinocytosis

Dictyostelium IqgC is an IQGAP-related protein. IQGAPs from higher eukaryotes are large, multidomain proteins whose name derives from the IQ motifs and a GAP-related domain (GRD) or RasGAP domain (GAP, GTPase-activating protein). Although they belong to a large group of RasGAP domain-containing proteins, they have lost catalytic activity, i.e. they are unable to promote GTP-hydrolytic activity of small GTPases from the Ras family. Instead, they interact with Rho family GTPases, Rac1 and Cdc42, and act as their effectors in the regulation of the actin cytoskeleton [1]. Dictyostelium discoideum harbours four IQGAP-related proteins that are substantially smaller than their mammalian relatives, but still contain conserved RasGAP-RasGAP_C-terminal domain architecture found only in IQGAPs. Dictyostelium DGAP1 (DdIQGAP1) and GAPA (DdIQGAP2) are Rac1 effectors that mediate the bundling of actin filaments. They are essential for normal cytokinesis and localize to the cleavage furrow of dividing cells and to the retracting tail of polarized interphase cells [2]. We have investigated the role of IqgC (DdIQGAP3) [3]. IqgC is an atypical IQGAP because it does not bind Rho GTPases and has preserved RasGAP activity exerted specifically toward RasG protein. Besides, unlike DGAP1 and GAPA, IqgC strongly localizes to protrusive structures, like forming macropinocytic and phagocytic cups, and nascent macropinosomes and phagosomes, where it colocalizes with active Ras. Moreover, we have demonstrated interaction with RasG to be indispensable for the recruitment of IqgC to membrane patches primed for macropinocytosis. Since RasG is one of the major regulators of macropinocytosis in D. discoideum, by suppressing its activity, IqgC negatively regulates fluid uptake. More specifically, we have demonstrated that IqgC limits the size of macropinocytic cups. Similarly, phagocytosis of large particles is more effective in cells deficient for IqgC. Next, we used lattice light-sheet microscopy to compare IqgC dynamics with the active Ras probe and revealed that IqgC lingers on the internalized macropinosome after Ras has dissociated from the vesicle. This finding suggests additional, RasG-independent functions of IqgC, probably during early macropinosome maturation. Indeed, using biochemical methods, we identified an early endosome marker, the GTPase Rab5A, as a direct binding partner of IqgC. Furthermore, we found Rab5A to colocalize with IqgC on the nascent macropinosome. However, a biological consequence of this interaction is not clear at the moment.

IqgC, IQGAP, Ras, Rab5A, macropinocytosis

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