Understanding How Cells Divide

Cytokinesis is the seemingly simple step of cell division or mitosis, during which the parent cell physically divides into two daughter cells. Most textbooks describe this process simply as the “pinching of the cell in two.”

Geneticists know that more than 60 genes are responsible for creating the proteins involved in this process in fission yeast. Still, until recently, next to nothing was known as to exactly where, when, and how these proteins work in concert to divide the cell.

Thomas Pollard, the Sterling Professor of Molecular, Cellular, and Developmental Biology, has discovered that “pinching” hardly does justice to what has been discovered to be an extremely complex mechanism. Along with Jian-Qui Wu, and undergraduate Steven Hao, Pollard has sought to investigate the details of cytokinesis in collaboration with Ben O’Shaughnessy and Dimitrios Vavylonis of Columbia University.

The team began by attempting to determine the order of events and protein interactions during cytokinesis. A three-step model was proposed: node formation, actin polymerization, and pulling. They discovered that approximately 65 nodes, or groups of associated proteins, form around the equator of the cell at the outset of cytokinesis.

Long chains of actin protein extend in random directions and “search” for other nodes where they are latched onto by the protein myosin. After this binding, myosin literally walks along the length of the actin filament bringing the nodes closer and constricting the conractile ring. This model was confirmed experimentally and their findings published in Science.

Interestingly, the team also discovered this is not a continuous process. Actin-myosin attachments are in no way permanent. Rather, myosin binds and releases many times, and as a result, the nodes seem to move about in random directions in a start-stop dance rather than in a single, smooth convergence.

This actually represents a repair mechanism to protect against incorrect connections. On average, the nodes tend to converge towards the center of the cell, thereby splitting it in two.

“It shows one way in which a completely undirected stochastic process could assemble a cellular structure, and how random processes can create order,” commented Pollard. Despite the disorder, “It’s amazingly efficient; virtually every one of them succeeds.”