The Multiple Functions of Rho GTPases in Fission Yeasts

Abstract

The Rho family of GTPases represents highly conserved molecular switches involved in a plethora of physiological processes. Fission yeast Schizosaccharomyces pombe has become a fundamental model organism to study the functions of Rho GTPases over the past few decades. In recent years, another fission yeast species, Schizosaccharomyces japonicus, has come into focus offering insight into evolutionary changes within the genus. Both fission yeasts contain only six Rho-type GTPases that are spatiotemporally controlled by multiple guanine-nucleotide exchange factors (GEFs) and GTPase-activating proteins (GAPs), and whose intricate regulation in response to external cues is starting to be uncovered. In the present review, we will outline and discuss the current knowledge and recent advances on how the fission yeasts Rho family GTPases regulate essential physiological processes such as morphogenesis and polarity, cellular integrity, cytokinesis and cellular differentiation.

Keywords: Cdc42; GTPase-activating protein (GAP); Rho GTPases; Rho1; cellular integrity; crosstalk; cytokinesis; cytoskeleton; fission yeasts; guanine–nucleotide exchange factor (GEF); phosphorylation; polarity; sexual differentiation; signaling.

Figure 2

Role of Rho GTPases during S. pombe polarized growth (adapted from [77,146,147]). (A) Schematic representation of the main molecular players involved in the local activation of Cdc42 (GTP-Cdc42, dark pink) during polarized growth initiation. GAPs are shown in different shades of red, and GEFs in different shades of green. (−): negative regulation on Cdc42; (+): positive regulation on Cdc42; P: protein is phosphorylated. Please see text for details. (B) Activated Cdc42 binds and activates formin For3 promoting actin cytoskeleton reorganization. Exocytic vesicles can reach the cell tips either transported along actin cables by myosin V Myo52 or by random walk. Once there, Sec3 and Exo70 tether the exocyst and the vesicle by binding PIP₂, Cdc42 and Rho3. Exo represents Sec5, Sec6, Sec8, Sec10, Sec15 and Exo84. (C) Cell wall remodeling is initiated after secretory vesicles containing the enzymes necessary for cell wall remodeling reach the poles. These enzymes are activated by GTP-Rho1 and -Rho2. The Cell Integrity MAPK Pathway (CIP) is involved in the regulation of cell wall homeostasis, and the polarity landmarks Tea1, Tea4 and Pom1 functionally interact. Dotted arrows mean that the molecular links have not been established. Please see main text for more details.

Figure 3

Rho GTPases involved in the regulation of CW integrity (adapted from [5,200]). (A) Cell wall synthesis at the cell tips during interphase is mediated by α-GS Mok1 (α-glucan) and β-GS Bgs4 (branched β-1,3-glucan). Other β-GS (Bgs1 and 3) also localize to the poles where their role is less well understood. The main function of Bgs1 is the synthesis of the linear β-1,3-glucan of the primary septum, whereas Bgs4 (branched β-1,3-glucan) and Mok1 (α-glucan) synthesize the polymers that form the secondary septum. The Rho GTPases involved in the activation of each GS are indicated in the figure. PM: plasma membrane; CW: cell wall; Glucose units (,); PS: primary septum; SS: secondary septum. (B) Schematic representation of S. pombe CIP activation. Wsc1 and Mtl2 activate Rho1 through the GEF Rgf1. Rho1 activates the β-GS and stabilizes Pck1 and Pck2. After activation by the phospholipid-dependent kinase Ksg1, both kinases activate the GS. Rho2 regulates Mok1 via Pck2. Rho2-Pck2 are the main activators of the CIP. Please see main text for more details.

Figure 1

S. pombe Rho GTPases and their regulators. See main text and tables for details on each protein.

Figure 4

Rho signaling during cytokinesis. Schematic representation of fission yeast cytokinesis. (A) CAR precursors are assembled into nodes () composed of multiple proteins localized into the cell middle by anillin-like protein Mid1. After synthesis of the F-actin network by formins Cdc12 and For3, nodes condense into the CAR through actomyosin interactions. The fully formed ring matures by recruitment of additional cytokinesis proteins (mature car is indicated in magenta). Cdc42 effector Pak1 promotes phosphorylation of CAR components Mid1, Cdc15 and Rlc1. Cdc42 mediates Bgs1 recruitment at the ring during anaphase. Rho1 GTPase participates in a cytokinetic checkpoint that delays CAR maturation after cell wall damage. (B) CAR maturation is followed by constriction and cell wall deposition (primary septum, in green, and secondary septum, in brown), which facilitates proper ring closure. Rho1 and Rho2 bind and activate the enzymes involved in septum synthesis. Question mark denotes that the molecular mechanism by which SIN might activate Rho1 remains unsolved. (C) Cells separation occurs by digestion of the primary septum and the surrounding cell wall by Agn1 (endo-α-1,3 glucanase) and Eng1 (endo-β-1,3 glucanase). Secretion of Agn1 and Eng1 depends on the activity of Rho4 (activated by Gef3). Both enzymes are targeted to the septum by the exocyst subunits Sec8 and Exo70 regulated by Rho4 and Rho3. Agn1 and Eng1 distribution at the ring is influenced by Cdc42. Please see main text for more details.

Figure 5

Functions of Rho GTPases during sexual differentiation (adapted from [7]). (A) GPCR (G-protein-coupled receptors) activation promotes MAPK signaling by the Gα Gpa1 and the Ras GTPase. Ras1 also promotes Cdc42 activation for polarization during mating. (B) Cells form dynamic polarization zones that contain pheromone release and perception machinery that probe the environment for partner choice (exploration). The dynamic behavior requires patch destabilization promoted by negative control on Ras1. These zones are stabilized by opposite-type pheromones (represented in blue and yellow semicircles). Higher pheromone sensing promotes Gα activation, which leads to enhanced Ste6 and Ras1 activation and patch stabilization. Upon cell–cell contact, cell wall remodeling allows plasma membrane contact, fusion pore formation and pore expansion for zygote formation. Fus1 is a formin essential for fusion and for actin localization at the shmoo tip. The type V myosins serve to concentrate the cell wall glucanases to drive local cell wall dissolution for cell fusion. (C) Diagrammatic representation of sporulation stages in S. pombe. After cell pairing and fusion, the two nuclei fuse as well (karyogamy) and the resulting diploid nucleus undergoes meiosis. The zygote develops into an ascus, containing four dormant ascospores. The initiation of forespore membrane (FSM) assembly occurs during meiosis II. Shortly after, FSM extension encapsulates nucleus and cytoplasm and closure occurs. Finally, spore wall is synthesized. The role of several Rho GTPases during the different stages is shown in the figure. Dotted arrows mean that the molecular links have not been established. See main text for more details.