tsunami, the Dictyostelium homolog of the Fused kinase, is required for polarization and chemotaxis

Abstract

In a forward genetic screen for chemotaxis mutants in Dictyostelium discoideum, we identified a loss-of-function mutation, designated tsunami, encoding a homolog of the Fused kinase. Cells lacking tsuA function could not effectively perform chemotaxis and were unable to become polarized or correctly orient pseudopods in chemotactic gradients. While tsuA − cells were able to couple receptor occupancy to phosphatidylinositol (3,4,5) trisphosphate (PIP3) production and actin polymerization, the PIP3 response was prolonged and basal F-actin levels were increased. Interestingly, TsuA localizes to the microtubule network and puncta mainly found at the cell periphery. Analysis of the gene uncovered a novel C-terminal domain that we designated the Tsunami Homology (TH) domain. Both the kinase domain and the TH domain are required to rescue the phenotypic defects of tsuA − cells. While kinase activity is not required for localization to microtubules, the TH domain is essential. Thus, localization of kinase activity to microtubules is critical for TsuA function. We propose that functions in association with the microtubule network may underlie the divergent roles of Fused kinase proteins in different organisms. Chemotaxis refers to the capacity of cells to move directionally in a gradient of an external signaling molecule. This process guides the coordinated cell movements that produce morphogenesis during development and the migration of immune cells around the body and contributes to the pathology of cancer metastasis, asthma, and rheumatoid arthritis (Ridley et al. 2003). The mechanisms of chemotaxis are very similar in evolutionarily diverse organisms (Bagorda et al. 2006).Powerful genetic and cell biological approaches available in the model organism Dictyostelium discoideum have led to many conceptual advances in our understanding of directed cell migration (Franca-Koh et al. 2006;Willard and Devreotes 2006). In particular, the parallels between this simple amoebae and leukocytes are remarkable. In these two cell types, chemotaxis is mediated by G-protein-coupled receptors and can conceptually be divided into three interrelated processes: membrane extension, directional sensing, and polarization (Iglesias and Devreotes 2008). Membrane extensions are protrusions driven by actin polymerization that mediate cell motility. In the absence of a chemoattractant gradient, membrane extensions are formed randomly along the cell perimeter. In a gradient, however, the process of directional sensing generates intracellular asymmetries that serve to focus signaling events and membrane extensions in the direction of the gradient. Together, membrane extensions coupled to directional sensing are sufficient to mediate chemotaxis. The third process, polarization, refers to the elongation of cell morphology and stable asymmetric localization of internal components and serves to increase the efficiency and speed of chemotaxis (Devreotes and Janetopoulos 2003). Chemotaxing neutrophils and starved D. discoideum amoebae are highly polarize...

Moving towards a paradigm: common mechanisms of chemotactic signaling in Dictyostelium and mammalian leukocytes

“…In Dictyostelium cells the microtubule network is involved in polarity, as depolarization with benomyl or genetic manipulations of Lissencephaly protein I (LisI), Dynein, or Tsunami (TsuA) all cause defects in polarity [346,347]. Surprisingly, there appears to be an opposite trend in neutrophils, where disruption of the microtubule network with nocodazole induces polarity [348].…”

Mechanism and evolution of cytosolic Hedgehog signal transduction

“…This includes plants and single-celled eukaryotes such as Chlamydomonas reinhardtii. Every identified role of the Fu kinase family in eukaryotes involves microtubules and/or some aspect of cell polarity, even in organisms that lack cilia (Oh et al, 2005;Tang et al, 2008;Wilson et al, 2009b). Thus, Fu and ancestral Cos2/Kif7 might function in a basic cilium structural or polarity pathway that has been co-opted by the Hh pathway in organisms such as Drosophila or zebrafish (Fig.…”

CP250, a Novel Acidic Coiled Coil Protein of theDictyosteliumcentrosome, Affects Growth, Chemotaxis, and the Nuclear Envelope

“…Furthermore, development is delayed and fruiting bodies are smaller than in wild type. In this work the kinase was located to microtubules and release of the kinase to the cytosol during microtubule depolymerization caused a loss in cell polarity underlining the importance of the microtubule system for cell polarity and chemotaxis (Tang et al, 2008).…”

“…In mammalian cells, for example, the orientation of the MTOC is a marker for polarization in a wound-healing assay (Lü ke et al, 2008). Furthermore, disruption of the microtubule network or mutations in microtubule-associated proteins affect directed cell migration and motility (Williams et al, 2006;Li and Gundersen, 2008;Tang et al, 2008). We therefore analyzed chemotaxis and motility of the mutant cells in a cAMP gradient.…”