Spatial regulation of Cdc42 activity is essential to maintain polarized growth. Fission yeast Rga6 is a new Cdc42 GAP that collaborates with Rga4, the only known Cdc42 GAP, in the spatial restriction of active Cdc42 at the cell tip. Both GAPs localize preferentially at the nongrowing areas of the membrane in different clusters.

F-BAR domain proteins act as linkers between the cell cortex and cytoskeleton, and are involved in membrane binding and bending. Rga7 is one of the seven F-BAR proteins present in the fission yeast Schizosaccharomyces pombe. In addition to the F-BAR domain in the N-terminal region, Rga7 possesses a Rho GTPaseactivating protein (GAP) domain at its C-terminus. We show here that Rga7 is necessary to prevent fragmentation of the contracting ring and incorrect septum synthesis. Accordingly, cultures of cells lacking Rga7 contain a higher percentage of dividing cells and more frequent asymmetric or aberrant septa, which ultimately might cause cell death. The Rga7 F-BAR domain is necessary for the protein localization to the division site and to the cell tips, and also for the Rga7 roles in cytokinesis. In contrast, Rga7 GAP catalytic activity seems to be dispensable. Moreover, we demonstrate that Rga7 cooperates with the two F-BAR proteins Cdc15 and Imp2 to ensure proper cytokinesis. We have also detected association of Rga7 with Imp2, and its binding partners Fic1 and Pxl1. Taken together, our findings suggest that Rga7 forms part of a protein complex that coordinates the late stages of cytokinesis.

Highlights d Pxl1 recruits calcineurin (CN) phosphatase to the cell division site d CN collaborates with Bgs1 in septum ingression d CN increases the concentration of Pxl1 and other Cdc15 binding partners d CN dephosphorylates Cdc15 both in vivo and in vitro

bThe fission yeast small GTPase Rho2 regulates morphogenesis and is an upstream activator of the cell integrity pathway, whose key element, mitogen-activated protein kinase (MAPK) Pmk1, becomes activated by multiple environmental stimuli and controls several cellular functions. Here we demonstrate that farnesylated Rho2 becomes palmitoylated in vivo at cysteine-196 within its carboxyl end and that this modification allows its specific targeting to the plasma membrane. Unlike that of other palmitoylated and prenylated GTPases, the Rho2 control of morphogenesis and Pmk1 activity is strictly dependent upon plasma membrane localization and is not found in other cellular membranes. Indeed, artificial plasma membrane targeting bypassed the Rho2 need for palmitoylation in order to signal. Detailed functional analysis of Rho2 chimeras fused to the carboxyl end from the essential GTPase Rho1 showed that GTPase palmitoylation is partially dependent on the prenylation context and confirmed that Rho2 signaling is independent of Rho GTP dissociation inhibitor (GDI) function. We further demonstrate that Rho2 is an in vivo substrate for DHHC family acyltransferase Erf2 palmitoyltransferase. Remarkably, Rho3, another Erf2 target, negatively regulates Pmk1 activity in a Rho2-independent fashion, thus revealing the existence of cross talk whereby both GTPases antagonistically modulate the activity of this MAPK cascade.

Chitin synthesis occurs in most fungi through the action of di¡erent chitin synthase (CS) isoenzymes. In Schizosaccharomyces pombe the chs2 + gene codes for a protein with signi¢cant similarity to CS enzymes, but lacking most of the residues considered to be essential for activity, including the QRRRW domain. Here we show that chs2p is a functional protein that localises to the growing edge of the septum but is not a CS enzyme. Strong over-expression is lethal, while moderate expression leads to a severe defect in septum formation. These results suggest that chs2p has remained through evolution to play an alternative role in septation. ß

In Schizosaccharomyces pombe cytokinesis requires the function of a contractile actomyosin ring. Fission yeast Chs2p is a transmembrane protein structurally similar to chitin synthases that lacks such enzymatic activity. Chs2p localisation and assembly into a ring that contracts during division requires the general system for polarised secretion, some components of the actomyosin ring, and an active septation initiation network. Chs2p interacts physically with the type-II myosin Myo3p revealing a physical link between the plasma membrane and the ring. In chs2Δ mutants, actomyosin ring integrity is compromised during the last stages of contraction and it remains longer in the midzone. In synchronous cultures, chs2Δ cells exhibit a delay in septation with respect to the control strain. All these results show that Chs2p participates in the correct functioning of the medial ring.

Cytokinesis, which enables the physical separation of daughter cells once mitosis has been completed, is executed in fungal and animal cells by a contractile actin- and myosin-based ring (CAR). In the fission yeast Schizosaccharomyces pombe the formin For3 nucleates actin cables and also co-operates for CAR assembly during cytokinesis. Mitogen-Activated Protein Kinases (MAPKs) regulate essential adaptive responses in eukaryotic organisms to environmental changes. We show that the Stress Activated Protein Kinase pathway (SAPK) and its effector, MAPK Sty1, downregulates CAR assembly in S. pombe when its integrity becomes compromised during cytoskeletal damage and stress by reducing For3 levels. Accurate control of For3 levels by the SAPK pathway may thus represent a novel regulatory mechanism of cytokinesis outcome in response to environmental cues. Conversely, SAPK signalling favours CAR assembly and integrity in its close relative S. japonicus, revealing a remarkable evolutionary divergence of this response within the fission yeast clade.

Coordination between microtubule and actin cytoskeletons plays a crucial role during the establishment of cell polarity. In fission yeast, the microtubule cytoskeleton regulates the distribution of actin assembly at the new growing end during the monopolar-to-bipolar growth transition. Here, we describe a novel mechanism in which a myosin V modulates the spatial coordination of proteolysis and microtubule dynamics. In cells lacking a functional copy of the class V myosin, Myo52, the plus ends of microtubules fail to undergo catastrophe on contacting the cell end and continue to grow, curling around the end of the cell. We show that this actin-associated motor regulates the efficient ubiquitin-dependent proteolysis of the Schizosaccharomyces pombe CLIP-170 homologue, Tip1. Myo52 facilitates microtubule catastrophe by enhancing Tip1 removal from the plus end of growing microtubules at the cell tips. There, Myo52 and the ubiquitin receptor, Dph1, work in concert to target Tip1 for degradation. Supplementary material available online at