The contractile ring

Abstract

What is it? The contractile ring is a ring-shaped structure located just beneath the plasma membrane at the future division site in many, though not all, eukaryotic cell types. Composed of actin, myosin and many other proteins, it assembles in anaphase and contracts as cells divide. The contractile ring is responsible for cytokinesis in many eukaryotic cell types, and is thought to contribute to cell division by ‘squeezing’ the cell into two.How was it discovered? Theories of cytokinesis have been hotly debated for over a hundred years, ever since the first microscopists started observing dividing cells. Although a ‘muscle’-like band was proposed earlier, it wasn't until the late 1960s that Schroeder first observed filaments (of actin) at the cleavage furrow in sea urchin eggs by electron microscopy, and then showed that an anti-actin drug inhibits cytokinesis.Do all cells use this structure to divide? No. Although the contractile ring appears to be crucial for cytokinesis in many cell types, some cells do not have one or do not appear to need one. Although slime mold cells usually have a ring, myosin-deleted cells can still divide fairly normally on a plate even without a concentration of actin filaments at the cleavage furrow. Some animal cells, such as amphibian eggs, never have a complete ring, but form a spreading arc. Plants divide without one. How might cells divide without a ring? Depending on the cell type, membrane insertion through secretion also contributes to cytokinesis in varying degrees.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, include: actin, myosin, septins, formins, Arp2/3 complex, tropomyosin, coronin, anillin, profilin, IQGAP, filiamin, MLCK, ROCK and so on. 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 in this fundamental process.Figure 1Contractile rings in a big and small cell. Sea urchin embryo (top) stained for actin filaments with rhodamine phalloidin and fission yeast cell (bottom) expressing a myosin light chain-GFP fusion protein. The fission yeast cell is approximately fourteen times smaller than the sea urchin cell.View Large Image | View Hi-Res Image | Download PowerPoint SlideWhere can I find out more?