Cytokinesis
Cytokinesis involves an actin-myosin based contractile ring that forms at the division site and aids in cleaving the cell into two. We are interested in the questions of how the ring is assembled and how cell division planes are placed in the cell. In animal cells, microtubules of the mitotic spindle seem to instruct the cell where to divide. In fission yeast, the cell division site appears to be determined by the position of the medial nucleus.
S. pombe is a popular model organism for studying molecular aspects of cytokinesis. Genetic screens for cytokinesis-defective mutants have led to the identification of many conserved factors required for assembly of the contractile ring and its placement (Chang et al., 1995). There are upwards of 150-200 proteins at the division site in fission yeast, with many conserved from yeast to mammals.
Positioning the Division Plane
A key question in cytokinesis is how the division site is properly positioned. While the mitotic apparatus specifies the division site in animal cells, fission yeast (and other eukaryotes such as plants) use the nucleus as a positive spatial cue (Daga and Chang, 2005). A key protein in division site positioning is an anillin-like protein Mid1p (Paoletti and Chang, 2000). In interphase, Mid1 is present in "nodes," membrane-bound protein clusters near the nucleus. In mitosis, Mid1 recruits myosin and other contractile ring proteins to nodes for ring assembly. A key unanswered question is how these nodes are positioned near the nucleus. In addition to positive spatial cues from the nucleus, there are also negative cues at the cell tips that prevent Mid1 from binding at the cell tips, such as a gradient of Pom1 (a DYRK kinase)(Padte et al., 2006).
Formin cdc12p in the ring
Cdc12, one of the first formins identified, is responsible for assembling actin filaments for the ring (Chang et al., 1997; Pelham and Chang, 2002; Yonetani et al, 2008). Expression of a Cdc12 fragment can actually drive ring assembly during interphase, indicating that Cdc12 has a key function also in coordinating the initiation of ring assembly (Yonetani and Chang, 2010). Learning how this works will provide inroads into understanding the initiation of cytokinesis.
Dynamics of the ring
The actin-based contractile ring is a highly dynamic structure. Components such as actin, myosin and formins are all coming on and off the ring in about a minute - even when the ring itself is at "steady state" (Pelham and Chang, 2002). Why is the ring dynamic? Could the turnover be part of a mechanism that ensures that the ring is of the proper size and shape? How does it contribute to force generation and remodeling during constriction?
Function of the ring-dependent force in regulation of the cell wall
The acto-myosin contractile ring is generally believed to provide the primary force for cytokinesis. However, there are hints that the mechanics of the process may not be so simple. In fission yeast, cytokinesis involves the growth of the cell wall septum as well as closure of the ring. We found that ingression continues even after the ring is disassembled, suggesting that the ring is not the primary force for cleavage (Proctor et al., 2012). Instead, the growth of the cell wall may drive the process.
What is the function of the ring then? We found that in the absence of the ring, the septum still grows inward, but its shape is abnormal. This leads to a model in which the ring provides a contractile force that coordinates cell wall assembly around the leading edge. Cell wall assembly is dependent on local curvature and the presence of the ring, supporting the notion that this process is mechano-sensitive (Zhou et al., 2015).
Ongoing interests address the dynamic organization of the ring, its mechanics and function. For instance, we are interested in understanding how pulling forces from the ring may modulate the mechanosensitive activity of cell wall synthases, investigations into cleavage furrows of different shapes, and mechanisms of dynamic ring homeostasis and constriction.