Hercyk, B. S. | bioRxiv Science

bioRxiv2021

Cdc42 GTPase activating proteins Rga4 and Rga6 coordinate septum synthesis and membrane trafficking at the division plane during cytokinesis

Fission yeast cytokinesis is driven by simultaneous septum synthesis, membrane furrowing and actomyosin ring constriction. The septum consists of a primary septum flanked by secondary septa. First, delivery of the glucan synthase Bgs1 and membrane vesicles initiate primary septum synthesis and furrowing. Next, Bgs4 is delivered for secondary septum formation. It is unclear how septum synthesis is coordinated with membrane furrowing. Cdc42 promotes delivery of Bgs1 but not Bgs4. We find that after primary septum initiation, Cdc42 inactivators Rga4 and Rga6 localize to the division site. In rga4{Delta}rga6{Delta} mutants Cdc42 activity is enhanced during late cytokinesis and cells take longer to separate. Electron micrographs of the division site in these mutants exhibit malformed septum with irregular membrane structures. These mutants have a larger division plane with enhanced Bgs1 delivery but fail to enhance accumulation of Bgs4 and several exocytic proteins. Additionally, these mutants show endocytic defects at the division site. This suggests that Cdc42 regulates only specific membrane trafficking events. Our data indicate that while active Cdc42 promotes primary septum synthesis, as cytokinesis progresses Rga4 and Rga6 localize to the division site to decrease Cdc42 activity. This couples specific membrane trafficking events with septum formation to allow proper septum morphology. Abstract Figure O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=160 SRC="FIGDIR/small/472679v1_ufig1.gif" ALT="Figure 1"> View larger version (35K): org.highwire.dtl.DTLVardef@ef042aorg.highwire.dtl.DTLVardef@1fa81adorg.highwire.dtl.DTLVardef@157a4a5org.highwire.dtl.DTLVardef@fa516d_HPS_FORMAT_FIGEXP M_FIG C_FIG SynopsisThe GTPase Cdc42 regulates cytokinesis in cell-walled fission yeast. Active Cdc42 promotes the initiation of septum (new cell wall) synthesis to physically divide daughter cells. Here we show that Cdc42 activity must be decreased at the later stages of cytokinesis to enable proper septum formation. Mutants lacking Cdc42 inactivators, Rga4 and Rga6, lead to membrane trafficking defects and malformed septa consequently delaying cell separation.

cell biology↗

bioRxiv2019

F-BAR Cdc15 Promotes Gef1-mediated Cdc42 Activation During Cytokinesis and Cell Polarization in S. pombe

Cdc42, a Rho-family GTPase, is a master regulator of cell polarity. Recently it has been shown that Cdc42 also facilitates proper cytokinesis in the fission yeast, Schizosaccharomyces pombe. Cdc42 is activated by two partially redundant GEFs Gef1 and Scd1. Although both the GEFs activate Cdc42, their deletion mutants display distinct phenotypes, indicating that they are differentially regulated, by an unknown mechanism. During cytokinesis, Gef1 localizes to the division site and activates Cdc42 to initiate ring constriction and septum ingression. Here we report that the F-BAR domain containing Cdc15 promotes Gef1 localization to its functional sites. We show that cdc15 promotes Gef1 association with the cytokinetic nodes to activate Cdc42 during ring assembly. Moreover, cdc15 phospho-mutants phenocopy polarity phenotypes of gef1 mutants. In a hypermorphic cdc15 mutant, Gef1 localizes precociously to the division site, and is readily detected at the cortical patches and the cell cortex. Correspondingly, the hypermorphic cdc15 mutant shows increased bipolarity during interphase and precocious Cdc42 activation at the division site during cytokinesis. Finally, loss of gef1 in hypermorphic cdc15 mutants abrogates the increased bipolarity and precocious Cdc42 activation phenotype. We did not see any change in the localization of the other GEF Scd1 in a Cdc15-dependent manner. Taken together our data indicates that Cdc15 promotes Cdc42 activation specifically via Gef1 localization to the division site to facilitate proper cytokinesis and to the cell cortex to promote bipolarity.

cell biology↗