A microscopic green alga helped scientists at the Salk Institute for Biological Studies identify a novel function for the retinoblastoma protein (RB), which is known for its role as a tumor suppressor in mammalian cells. By coupling cell size with cell division, RB ensures that cells stay within an optimal size range.
Their findings, which will be published in the October 13 online edition of PLoS Genetics, show that RB blocks cells from dividing before they reach a minimum size and could provide new insights into the origins of cancer.
"Being the right size is very important for cells because their physiology changes quite dramatically when the surface-to-volume ratio changes," explains senior author James Umen, Ph.D., an assistant professor and Hearst Endowment Chair in Salk's Plant Biology Laboratory. "The human body is composed of trillions of cells, each of which must coordinate its growth and division in order to maintain size equilibrium," he adds.
This process is very tightly regulated and any given cell type will always stay within a very narrow size range, but the means by which cell size is determined remain mysterious. In proliferating cells, control mechanisms termed checkpoints are thought to prevent cells from dividing until they reach a specific size, but the nature of the checkpoints has proved difficult to dissect.
Understanding how cells balance the opposing processes of growth and division in order to achieve size control is more than just a fascinating intellectual pursuit for cell biologists: loss of size control is a hallmark of cancer cells, which exhibit severe defects in regulating growth and division.
"In mammalian cells it is very hard to separate size control from cell cycle control because it is very easy to mess up cell size as an indirect consequence of manipulating cell cycle rates," says Umen.