Pom1 regulation of Cdr2 membrane association and interaction with Mid1 prevents Cdr2 assembly into stable nodes in the cell tip region, which ensures proper positioning of cytokinetic ring precursors and accurate division plane positioning in fission yeast.

c Spatial control of cytokinesis is essential for proper cell division. The molecular mechanisms that anchor the dynamic assembly and constriction of the cytokinetic ring at the plasma membrane remain unclear. In the fission yeast Schizosaccharomyces pombe, the cytokinetic ring is assembled in the cell middle from cortical node precursors that are positioned by the anillin-like protein Mid1. During mitotic entry, cortical nodes mature and then compact into a contractile ring positioned in the cell middle. The molecular link between Mid1 and medial cortical nodes remains poorly defined. Here we show that Blt1, a previously enigmatic cortical node protein, promotes the robust association of Mid1 with cortical nodes. Blt1 interacts with Mid1 through the RhoGEF Gef2 to stabilize nodes at the cell cortex during the early stages of contractile ring assembly. The Blt1 N terminus is required for localization and function, while the Blt1 C terminus promotes cortical localization by interacting with phospholipids. In cells lacking membrane binding by both Mid1 and Blt1, nodes detach from the cell cortex and generate aberrant cytokinetic rings. We conclude that Blt1 acts as a scaffolding protein for precursors of the cytokinetic ring and that Blt1 and Mid1 provide overlapping membrane anchors for proper division plane positioning. Cell division requires the spatial and temporal coordination of many cellular activities. During cytokinesis, the final act of the cell cycle, a contractile actomyosin ring constricts to separate the two daughter cells. The contractile ring must be properly assembled and positioned to ensure equal segregation of cellular materials to each daughter cell. The contracting cytokinetic ring maintains association with the cell cortex, which undergoes dramatic remodeling and membrane bending during this process. The mechanisms that position and anchor components of the cytokinetic ring in the plasma membrane have been the subject of intense study, as defects in this process can lead to a range of cellular defects and disease states (1-4).Many insights into eukaryotic cytokinesis have come from work on the fission yeast Schizosaccharomyces pombe (1, 5). These rod-shaped cells grow in a linear manner at the cell ends and then position the contractile ring precisely in the cell middle at division. This positioning occurs through the combination of inhibitory signals emanating from the cell ends and positive cues from the nucleus in the cell middle. These positional cues act largely through the protein Mid1, which is similar to anillin in metazoans (6, 7). During interphase, Mid1 localizes to the nucleus and to a band of cortical nodes that are positioned in the cell middle. These interphase nodes are organized by the protein kinase Cdr2 and are spatially restricted to the cell middle by inhibitory cues from the cell tips (8-12). During interphase, these cortical nodes also contain a cell cycle regulatory network that couples mitotic entry with cell size (11, 13). Proteomic studies have revealed additional ...

Cell growth and division are tightly coordinated to maintain cell size constant during successive cell cycles. In Schizosaccharomyces pombe, the SAD kinase Cdr2 regulates the cell size at division and the positioning of the division plane. Cdr2 forms nodes on the medial cortex containing factors that constitute an inhibitory pathway for Wee1. This pathway is regulated by polar gradients of the DYRK kinase Pom1, and involves a direct inhibitor of Wee1, the SAD kinase Cdr1. Cdr2 also interacts with the anillin Mid1, which defines the division plane, and with additional components of the medial cortical nodes, including Blt1, which participate in the mitotic-promoting and cytokinetic functions of nodes. Here, we show that the interaction of Cdr2 with Wee1 and Mid1 requires the UBA domain of Cdr2, which is necessary for its kinase activity. In contrast, Cdr1 associates with the C-terminus of Cdr2, which is composed of basic and KA-1 lipid-binding domains. Mid1 also interacts with the C-terminus of Cdr2 and might bridge the N-and C-terminal domains, whereas Blt1 associates with the central spacer region. We propose that the association of Cdr2 effectors with different domains might constrain Cdr1 and Wee1 spatially to promote Wee1 inhibition upon Cdr2 kinase activation.