What is the driving force within a living cell that allows separation of daughter cells during cytokinesis? In many eukaryotes cell division requires the formation of a contractile ring, composed of actin and myosin filaments,that generates force to cleave the cell. How this ring is formed and how it contracts is rather elusive, and the actomyosin organization and contraction was believed by many to resemble that of smooth muscles. Recently, Pelham and Change studied actin dynamics during this process in fission yeast, a model organism for cytokinesis research. The surprising conclusion of their studies is that cytokinesis is a more dynamic process than many of us previously thought.
The formation of the contractile ring is very complex and hard to dissect. Genetic screens have identified many of the genes that are essential for ring assembly and its regulation during mitosis. However, the screens did not identify any of the genes that encode the protein complex called Arp2/3, which is the only well characterized cellular mediator that initiates actin polymerization.
Pelham and Chang used an alternative approach to see if Arp2/3 was involved in ring formation. They genetically fused a green fluorescent protein with a component of the Arp2/3 complex and followed its cellular distribution by confocal laser microscopy. They found that the fusion protein localized to the contractile ring. The next question was whether the ring is an active site for actin polymerisation, or whether actin molecules are assembled elsewhere in the cell before incorporation into the contractile ring. This pivotal question was tackled with a series of tricky experiments mainly based on the incorporation of fluorescence-labeled actin monomers into nascent filaments. It turned out that a fluorescence signal had already appeared at initial stages of ring formation, and was not obtained when actin polymerization was blocked with an inhibitor. Since pre-formed labeled actin filaments were not incorporated, the results obtained indicated de novo actin polymerization at the contractile ring.
The actin filaments of the ring were believed to associate with myosin molecules, and both proteins were thought to form stable filaments that slid past each other during ring contraction. But, if actin molecules were actively polymerising at the ring, filament sliding could be affected. Pelham and Chang investigated ring dynamics more precisely by analyzing in vivoturnover rates of actin filaments and other ring components. Amazingly, the contractile ring appeared as a highly dynamic structure in which actin and other ring components exchanged roughly every minute! Moreover, experimental attenuation of actin polymerization influenced the cell cleavage rates, which suggests that actin dynamics might even contribute to ring closure.
In summary, Pelham and Cheng showed that the contractile ring is a highly dynamic structure with an actin polymerization activity that is dependent on other proteins and that might in fact contribute force for cleavage. Previous studies had hypothesised that ring formation requires the recruitment of pre-existing actin cables from other parts of the cell, but Pelham and Chang's results show this is not exclusively the case and provide an alternative model, in which actin dynamics contribute not only to assembly and maintenance but also to closure of the ring. Despite some significant differences in cytokinesis between fission yeast and other eucaryotic cells, many components of the molecular apparatus are highly conserved. Thus, the results obtained for fission yeast may be relevant to other eucaryotic cells, too.
