Cross Talk between NDR Kinase Pathways Coordinates Cytokinesis with Cell Separation in Schizosaccharomyces pombe

Abstract

NDR (nuclear Dbf-2-related) kinases constitute key regulatory nodes in signaling networks that control multiple biological processes such as growth, proliferation, mitotic exit, morphogenesis, and apoptosis. Two NDR pathways called the septation initiation network (SIN) and the morphogenesis Orb6 network (MOR) exist in the fission yeast Schizosaccharomyces pombe. The SIN promotes cytokinesis, and the MOR drives cell separation at the end of cytokinesis and polarized growth during interphase. We showed previously that cross talk exists between these two pathways, with the SIN inhibiting the MOR during cytokinesis through phosphorylation of the MOR component Nak1 by the SIN Sid2 kinase. The reason for this inhibition remained uncertain. We show here that failure to inhibit MOR signaling during cytokinesis results in cell lysis at the site of septum formation. Time-lapse analysis revealed that MOR signaling during cytokinesis causes cells to prematurely initiate septum degradation/cell separation. The cell lysis phenotype is due to premature initiation of cell separation because it can be rescued by mutations in genes required for cell separation/septum degradation. We also shed further light on how the SIN inhibits the MOR. Sid2 phosphorylation of the MOR proteins Sog2 and Nak1 is required to prevent cell lysis during cytokinesis. Together, these results show that SIN inhibition of the MOR enforces proper temporal ordering of cytokinetic events. In the fission yeast Schizosaccharomyces pombe, cytokinesis and other late mitotic events are regulated by the septation initiation network (SIN) pathway, an NDR (nuclear Dbf-2-related) kinase signaling network analogous to the hippo pathway in mammalian cells and the mitotic exit network (MEN) in the budding yeast Saccharomyces cerevisiae (1-4). SIN signaling is essential for actomyosin ring assembly and constriction as well as for septum formation. The SIN also regulates mitotic entry, spindle checkpoint inactivation, spindle elongation, telophase nuclear positioning, and inhibition of polarized growth during cytokinesis (5-9). Recent work has begun to identify targets of the SIN, which include cytoskeletal and signaling proteins (5,8,(10)(11)(12)(13). SIN signaling is under precise temporal control; the SIN is activated in the anaphase of mitosis and inactivated upon completion of actomyosin ring constriction and septum formation (14). Regulation of other cell cycle signaling networks by the SIN may be important for properly coordinating cell cycle events. For example, the SIN inhibits a second conserved NDR kinase pathway called the morphogenesis Orb6 network (MOR) (9), which is analogous to the RAM (regulation of Ace2 and morphogenesis) network in budding yeast (3). The MOR normally drives septum degradation and cell separation following actomyosin ring constriction as well as polarized growth during the interphase by promoting localization of the actin cytoskeleton to cell tips (15). In mitosis, the SIN and other proteins cause actin to reorganize at the cell divisio...

Spatial control of translation repression and polarized growth by conserved NDR kinase Orb6 and RNA-binding protein Sts5

“…Consistent with this idea, we found that sts5∆ cells are prone to rupture at the site of cell separation (Figure 8—figure supplement 4A,d and B) similarly to cells that express ectopically active Orb6 during mitosis (Gupta et al, 2013, Gupta et al, 2014). …”

A cell separation checkpoint that enforces the proper order of late cytokinetic events

“…The authors demonstrated that cells failing to inactivate the MOR early during septation prematurely degraded the septum leading to lethality. Similar to our results here, this phenotype was reversed by deletion of wall degrading enzymes (Gupta et al, 2014). While fission yeast lack a sequence homolog of Fir1, a functional ortholog may exist.…”

Pombe's thirteen – control of fission yeast cell division by the septation initiation network

“…Important for CAR assembly. Control of polarized growth Nak1p and Sog2p Gupta et al, 2014;Gupta et al, 2013;Ray et al, 2010 Proposed mutual inhibition between SIN and MOR prevents conflicts between polar growth and cytokinesis.…”

“…Sid2p phosphorylates Nak1p, and mutation of the phosphorylation sites on Nak1p results in continued cell elongation during mitosis and lysis at the time of septation (Gupta et al, 2014;Gupta et al, 2013). Pmo25p localisation to the SPBs requires SIN activity (Kanai et al, 2005;Mendoza et al, 2005), and it has been suggested that SINdependent sequestration of Pmo25p and Nak1p at the SPB prevents MOR activation (Ray et al, 2010).…”