title: Trix, a novel Rac guanine-nucleotide exchange factor from Dictyostelium discoideum is an actin-binding protein and accumulates at endosomes.
abstract: Small Rho family GTPases are involved in regulation of actin cytoskeleton dynamics. These molecular switches are themselves mainly controlled by specific GTPase-activating proteins (GAPs) and guanine-nucleotide exchange factors (GEFs). We have cloned and initially characterized a novel putative RhoGEF from Dictyostelium discoideum. The predicted 135-kDa protein displays a unique domain organization in its N-terminus by harboring two type3 calponin homology (CH) domains followed by a single type1 CH domain. The C-terminal region encompasses a diffuse B-cell lymphoma homology/pleckstrin homology tandem domain that is typically found in RhoGEFs. We therefore refer to this protein as Trix (triple CH-domain array exchange factor). A recombinant N-terminal region of Trix carrying all three CH domains binds to F-actin and bundles actin filaments. Trix-null mutants are viable and display only subtle defects when compared to wild-type cells with the exception of a substantial decrease in exocytosis of a fluid-phase marker. GFP fusions with the full-length protein or the N-terminal part containing all three CH domains revealed that Trix localizes to the cortical region and strongly accumulates on late endosomes. Our results suggest that Trix is specifically involved in a Rho GTPase-signaling pathway that is required for regulation of the actin cytoskeleton during exocytosis.
authors.affiliation: A. Butenandt-Institut/Zellbiologie, Ludwig-Maximilians-Universität, Schillerstr. 42, D-80336 München, Germany.
authors.forename: Axel
authors.initials: A
authors.lastname: Strehle
authors.forename: Michael
authors.initials: M
authors.lastname: Schleicher
authors.forename: Jan
authors.initials: J
authors.lastname: Faix
mesh_terms: D008841:Actin Cytoskeleton / Q000378:metabolism; D000595:Amino Acid Sequence; D000818:Animals; D003599:Cytoskeleton / Q000378:metabolism; D004023:Dictyostelium / Q000166:cytology / Q000235:genetics / Q000378:metabolism*; D011992:Endosomes / Q000378:metabolism* / Q000648:ultrastructure; D004789:Enzyme Activation; D020662:Guanine Nucleotide Exchange Factors / Q000145:classification / Q000235:genetics / Q000378:metabolism*; D008840:Microfilament Proteins / Q000145:classification / Q000235:genetics / Q000378:metabolism*; D008969:Molecular Sequence Data; D010587:Phagocytosis / Q000502:physiology; D010802:Phylogeny; D017434:Protein Structure, Tertiary; D015800:Protozoan Proteins / Q000145:classification / Q000235:genetics / Q000378:metabolism*; D011993:Recombinant Fusion Proteins / Q000235:genetics / Q000378:metabolism; D064067:Rho Guanine Nucleotide Exchange Factors; D016415:Sequence Alignment; D015398:Signal Transduction / Q000502:physiology; D020744:rac GTP-Binding Proteins / Q000378:metabolism
chemical_list: D020662:Guanine Nucleotide Exchange Factors; D008840:Microfilament Proteins; D015800:Protozoan Proteins; D011993:Recombinant Fusion Proteins; D064067:Rho Guanine Nucleotide Exchange Factors; D020744:rac GTP-Binding Proteins