Molecular caving

How is the ring assembled? A set of conserved contractile ring proteins have been identified and characterized using genetic, biochemical and proteomic approaches in model organisms ranging from yeasts, plants, worms, and flies to sea urchin eggs and mammalian cells. These proteins, many of which organize actin in some manner Actin filaments in the ring may be derived both from pre-formed filaments recycled from other actin structures and from new filaments newly polymerized by the ring's own actin nucleation centers. Myosin not only contributes force, but may also help bring the actin filaments together into a discrete ring. The small GTPase Rho is a key regulator of the process. How is the ring positioned? In animal cells, microtubules of the mitotic spindle somehow position the ring at a point equidistant from the two spindle poles. How this occurs is still highly controversial. In some cell types, overlapping astral microtubules from the two spindle poles that touch the cortex at the future division site may be the key spatial determinants, while in other cell types a bundle of microtubules known as the spindle midzone (or central spindle), may be responsible. Microtubules may regulate cleavage by sending out 'signals' that induce contractile ring formation, regulate contractility and direct membrane traffic. Other cell types use other spatial cues. For instance, fission yeast cells use the nucleus to position the cell division site, while budding yeast use cortical marks left over from previous cell divisions. How does the contractile ring divide the cell? A prevalent view is that that actin and myosin in the contractile ring exert squeezing forces leading to cleavage (at least in many cell types). Recent evidence shows that components of the ring are highly dynamic, suggesting that actin polymerization is also important for cleavage and may even contribute to force production. The ring also clearly has other roles, including organizing a membrane domain and targeting membrane insertion and trafficking. It seems that we don't know much about this pretty important universal process? Yes, cytokinesis is one of the frontiers of cell biology, filled with wildly different theories and controversies, studied in a large number of different model organisms in many different ways. Although certainly a universal process, it is becoming apparent that different cell types use similar but slightly different mechanisms. The advent of genomics, genetics, improved microscopy and proteomics promises rapid progress in sorting out the themes and variations …