Little is known about the mechanisms that establish the position of the division plane in eukaryotic cells.Wild-type fission yeast cells divide by forming a septum in the middle of the cell at the end of mitosis. Dmfl mutants complete mitosis and initiate septum formation, but the septa that form are positioned at random locations and angles in the cell, rather than in the middle. We have cloned the dmfl gene as a suppressor of the cdc7-24 mutant. The dmfl mutant is allelic with mid1. The gene encodes a novel protein containing a putative nuclear localization signal, and a carboxy-terminal PH domain. In wild-type cells, Dmflp is nuclear during interphase, and relocates to form a medial ring at the cell cortex coincident with the onset of mitosis. This relocalization occurs before formation of the actin ring and is associated with increased phosphorylation of Dmflp. The Dmflp ring can be formed in the absence of an actin ring, but depends on some of the genes required for actin ring formation. When the septum is completed and the cells separate, Dmflp staining is once again nuclear. These data implicate Dmflp as an important element in assuring correct placement of the division septum in Schizosaccharomyces pombe cells. [Key Words: dmfl/midl; S. pombe; cytokinesis]Received April 10, 1996; revised version accepted August 22, 1996.Cells must assure correct spatial and temporal coordination of mitosis with cytokinesis so that each daughter cell will receive a nucleus and an appropriate complement of cytoplasmic material following cell division. A number of studies indicate that the position of the cleavage plane is determined during mitosis, but in most systems very little is known about the nature of the signal or the proteins involved (for review, see Strome 1993). Cytokinesis requires assembly of the division apparatus at an appropriate site on the cell cortex. In higher eukaryotes, a contractile ring forms around the equator of the parent cell at the end of mitosis. The sliding of actin and myosin filaments generates a force that pulls the plasma membrane inward to create the cleavage furrow, which narrows to form the midbody as division is completed (for review, see Satterwhite and Pollard 1992;Fishkind and Wang 1995). A structure called the telophase disc, which forms prior to the appearance of the contractile ring, has been proposed to organize contractile ring formation (Margolis and Andreassen 1993).The mitotic apparatus plays a critical role in determining where cell cleavage will occur. The site of cell division is defined by the plane of the metaphase plate, and is at right angles to the long axis of the mitotic spindle.

The S. pombe cdc15 gene is essential for cell division. cdc15ts mutants do not form a septum, but growth and nuclear division continue, leading to formation of multinucleate cells. The earliest step in septum formation and cytokinesis, rearrangement of actin to the center of the cell, is associated with appearance of hypophosphorylated cdc15p and formation of a cdc15p ring, which colocalizes with actin. Loss of cdc15p function impairs formation of the actin ring. The abundance of cdc15 mRNA varies through the cell division cycle, peaking in early mitosis before septation. Expression of cdc15 in G2-arrested cells induces actin rearrangement to the center of the cell. These data implicate cdc15p as a key element in mediating the cytoskeletal rearrangements required for cytokinesis.

Schizosaccharomyces pombe divides by means of a centrally placed division septum. The initiation of septation must be tightly coordinated with events in mitosis, as premature formation of the septum can lethally cut the undivided nucleus. The Spg1p GTPase and the Cdc7p kinase, with which it interacts, play a central role in signaling the initiation of septum formation. Loss-of-function mutations in either gene prevent septation, whereas inappropriate activation of Spg1p can induce septum formation from G 1 or G 2 interphase cells. Increased expression of either gene leads to multiple rounds of septation without cell cleavage, emphasizing the need for precise cell cycle regulation of their activity. To understand the mechanisms underlying this regulation, we have investigated whether these key initiators of septum formation are controlled by changes in their activity and/or location during mitosis and cytokinesis. We demonstrate that Spg1p localizes to the spindle pole body in interphase and to both spindle poles during mitosis. In contrast, Cdc7p shows no discrete localization during interphase, but early in mitosis it associates with both spindle pole bodies and, as the spindle extends, is seen on only one pole of the spindle during anaphase B. Spg1p activity is required for localization of Cdc7p in vivo but not for its kinase activity in vitro. Staining with an antiserum that recognizes preferentially GDP-Spg1p indicates that activated GTP-Spg1p predominates during mitosis when Cdc7p is associated with the spindle pole body. Furthermore, staining with this antibody shows that asymmetric distribution of Cdc7p may be mediated by inactivation of Spg1p on one spindle pole. Deregulated septation in mutant cells correlates with segregation of Cdc7p to both spindle poles.

We have investigated proteins which interact with the PEST-type protein tyrosine phosphatase, PTP hematopoietic stem cell fraction (HSCF), using the yeast two-hybrid system. This resulted in the identification of proline, serine, threonine phosphatase interacting protein (PSTPIP), a novel member of the actin- associated protein family that is homologous to Schizosaccharomyces pombe CDC15p, a phosphorylated protein involved with the assembly of the actin ring in the cytokinetic cleavage furrow. The binding of PTP HSCF to PSTPIP was induced by a novel interaction between the putative coiled-coil region of PSTPIP and the COOH-terminal, proline-rich region of the phosphatase. PSTPIP is tyrosine phosphorylated both endogenously and in v-Src transfected COS cells, and cotransfection of dominant-negative PTP HSCF results in hyperphosphorylation of PSTPIP. This dominant-negative effect is dependent upon the inclusion of the COOH-terminal, proline-rich PSTPIP-binding region of the phosphatase. Confocal microscopy analysis of endogenous PSTPIP revealed colocalization with the cortical actin cytoskeleton, lamellipodia, and actin-rich cytokinetic cleavage furrow. Overexpression of PSTPIP in 3T3 cells resulted in the formation of extended filopodia, consistent with a role for this protein in actin reorganization. Finally, overexpression of mammalian PSTPIP in exponentially growing S. pombe results in a dominant-negative inhibition of cytokinesis. PSTPIP is therefore a novel actin-associated protein, potentially involved with cytokinesis, whose tyrosine phosphorylation is regulated by PTP HSCF.

In both animal cells and fungi, cytokinesis proceeds via a contractile actomyosin ring (CAR). Many CAR components and regulators are evolutionarily conserved. In Schizosaccharomyces pombe, the spatial cue for cytokinesis is provided by Mid1p/Anillin, whereas temporal coordination is ensured by the septation initiation network (SIN). However, neither Mid1p nor the SIN is considered to be essential for CAR assembly per se. Here, using 4D imaging, we reveal an unanticipated, novel role for the SIN in CAR assembly. We demonstrate that CAR assembly involves three, genetically separable steps: establishment of a cortical network of CAR proteins, its lateral condensation, and finally, the formation of a homogeneous CAR. We show that SIN mutants fail to form a homogeneous CAR; we identify hypophosphorylation and recruitment of the conserved PCH-family protein Cdc15p to the CAR as critical steps requiring SIN function. Furthermore, we show that in the absence of Mid1p, CAR assembly proceeds via an actomyosin filament, rather than a cortical network of CAR proteins. This mode of assembly is totally dependent on SIN signaling, thereby demonstrating a direct role for the SIN in CAR formation. Taken together, these data establish that Mid1p and the SIN are the key regulators that orchestrate CAR assembly.[Keywords: S. pombe; cytokinesis; septation initiation network; Mid1p/anillin; contractile actomyosin ring] Supplemental material is available at http://www.genesdev.org.