IIGP, a member of the IFN inducible and microbial defense mediating 47 kDa GTPase family, interacts with the microtubule binding protein hook3

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Abstract
Innate immunity against intracellular pathogens is critically determined by an as yet unknown interferon (IFN)-inducible mechanism exerted by members of the 47 kDa GTPase family. The association of IGTP and IIGP with membranous compartments, the endoplasmic reticulum and, in addition in case of IIGP, the Golgi, implicate these GTPases in intracellular membrane trafficking or processing. We identified the cytoplasmic linker molecule hook3 as an interactor for IIGP by yeast two-hybrid screening. The physical complex between these molecules was present in lysates of IFNgamma-stimulated macrophages as demonstrated by co-immunoprecipitation. Only a minor subfraction of total cellular IIGP or hook3 was co-purified, indicating that this interaction is either transient and/or involves distinct subpopulations of the total cellular pools of these molecules. Binding of IIGP to hook3 depends on a GTP-bound conformation. Hook3 is a microtubule-binding protein which participates in the organization of the cis-Golgi compartment. Both proteins were detected in the Golgi-membrane-enriched fraction upon subcellular fractionation. Apart from the Golgi localization of both proteins, hook3 was detected in perinuclear regions in close spatial proximity to IIGP, associated with the endoplasmic reticulum. Our experiments identify hook3 as the first cooperation partner of a member of the 47 kDa GTPase protein family and indicate that hook3 links in an IFNgamma-inducible fashion to cytoskeleton-based membrane trafficking.