Oct. 27, 2000 University of California, San Francisco scientists have stripped the fundamental decision-making apparatus of a cell down to its bare essentials, revealing the inner workings of one of life's smallest "decision nodes" - the biochemical switches by which cells take in multiple signals and integrate them, leading to a course of action. The decision node they studied leads cells to initiate movement - such as a white blood cell of the immune system migrating towards an invading bacterium.
Defects in this key decision node protein are known to cause immunodeficiency and other severe human illnesses. In addition, oncogenes, the normal genes that can be converted into cancer genes, probably code for proteins which function as decision-making nodes, underscoring the potential value in better understanding these pivotal proteins, says Wendell Lim, PhD, UCSF associate professor of cellular and molecular pharmacology and senior author on a report on the research.
The UCSF study is published in the October 27 issue of Science. Survival of all organisms from microbes to humans depends on the ability to react to changing conditions, and scientists have already described many of the responses made constantly by every cell of the body. But the mechanisms by which these decisions are made has been unclear. How, for example, does a white blood cell detect the presence of a bacterium and resolve to move towards it? Such decisions often involve coordination of two or more inputs.
Rather than a simple direct-line action such as "If A, then B," for example, the decision might instead be "If A and B, then C." The workings of this fundamental cellular decision-making unit - similar to the decision-making components in a computer - has eluded researchers until now.
The UCSF study examined the key element in the decision-making protein N-WASP, a relative of the Wiskott Aldrich Syndrome Protein (WASP), known for the fatal childhood disease that occurs when people have a defective gene for this protein.
The pivotal step in N-WASP's decision-making process - its ability to integrate two incoming signals and direct the cell to take an action - involves "cooperation" between the two signals, the scientists found. Only when each of the signaling molecules binds to distinct regions, or domains, on the N-WASP protein, can N-WASP can take the first step in the process that leads to cell movement. Cell movement involves the growth, or polymerization, of the protein actin. Chains of actin act as the engine that pushes the cell forward.
The scientists found that when the decision switch is "off" - that is, when instructions for adding actin are not needed - the key domains of the N-WASP protein physically torque the "working end" of the protein so that it cannot trigger actin polymerization. But when the two signal molecules bind to their separate target domains on the N-WASP protein, their combined effect frees the protein to trigger actin growth.
The actin trigger is difficult to pull, Lim says, and the researchers were able to show that neither incoming signal by itself is able to activate the trigger. Only when the signals "cooperate" - both binding to their respective N-WASP domains - is the protein able to trigger the actin formation that can lead to cell movement.
"It's a sophisticated, well-designed two-way switch that can monitor complex conditions," marvels Lim. He says the model probably applies to many cell decision nodes. In order to respond to shifting conditions, the cell needs a mechanism to translate multiple signals into action, he says, and the nodes accomplish this by relying on the power of cooperative action between two - and probably often more than two - incoming signals.
"We are only starting to understand this signal integrating protein, and it is likely that in this case as in many others, the two domains actually act in concert with at least one other domain to make a three-way switch," Lim says.
In their study, the researchers synthesized what they call a "mini-N-WASP protein" -- the bare bones of the node protein that is still capable of integrating signals. They used this to tease apart the essential steps in cellular decision-making.
"Much as we can understand how a computer works by breaking it down to its transistors and other components, so we can understand the complex circuits of a cell by studying its signaling proteins like N-WASP," said Kenneth Prehoda, PhD, lead author on the article and a post-doctoral scientist in Lim's lab. Co authors include R. Dyche Mullins, PhD, assistant professor of cellular and molecular pharmacology, and Jessica A. Scott, BS, a research associate in Lim's lab, all at UCSF.
The research is funded by the National Institutes of Health, the Burroughs Wellcome Young Investigator Program, the Searle Scholars Program, and the Packard Foundation.
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