Cytokinesis involves an actin-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 plane are placed in the cell. In animal cells, microtubules of the mitotic spindle seem to instruct the cell where to divide. In S. pombe, the cell division site appears to be determined by the position of the medial nucleus.

S. pombe is now one of the "leading" model organisms for studying molecular aspects of cytokinesis (Bathe and Chang, 2010). Genetic screens for cytokinesis-defective mutants have identified many factors required for assembly of this contractile ring and its placement. Remarkably, many of these gene products are conserved from yeast to mammals. There are upwards of 150-200 proteins at the division site in fission yeast. Now a goal is to understand how the set of ring factors assemble together in a ring structure and cause the cell to divide.

Formin cdc12p in the ring.
Cdc12p, one of the first formins identified, is responsible for assembling actin filaments for the ring (Chang et al., 1997; Pelham and Chang, 2002). Recent work showing that a truncated cdc12 fragment can drive ring assembly during interphase suggests that cdc12p has a key function also in coordinating the initiation of ring assembly (Yonetani and Chang, 2010).

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, mid1p is present in a series of membrane-bound complexes near the nucleus. In mitosis, mid1p recruits myosin and other contractile ring proteins to these sites for ring assembly (Figure 3). A key unanswered question is how this mid1p is positioned on the cortex. In addition to positive spatial cues from the nucleus, there are also negative cues at the cell tips that prevent mid1p from binding at the cell tips. One of the negative cues is a gradient of pom1p (a DYRK kinase)(Padte et al., 2006). We are pursuing a quantitative description of how the pom1p gradient is formed and how it functions. In addition, recent work shows surprisingly, that the endoplasmic reticulum, which is located just below the plasma membrane, serves as a membranous network to that positions mid1p.

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). We are asking what these dynamics are for and what can they tell us about how the ring is assembled.