The protein in this study, neuronal Wiskott-Aldrich syndrome protein (N-WASP), is naturally found in every cell in the body and is known to be involved in a wide range of cellular processes. One of its key functions is believed to be guiding cellular growth and movement within the body, including when tumor cells metastasize, or spread, from one organ to another.
"To our knowledge this is the first probe of its kind that allows us to actually see in a living system where, when and how proteins are activated," says first author Michael E. Ward, a graduate student in anatomy and neurobiology. "This is significant progress in moving from examining the biochemistry of ground up cells to being able to study it in an intact cell."
The study was led by Yi Rao, Ph.D., associate professor of anatomy and neurobiology. It appears online in the early edition of the Proceedings of the National Academy of Sciences and will be featured on the cover of the Jan. 27 print edition of the journal.
To design this new probe, the team took advantage of the fact that N-WASP folds in half when it is inactivated. They latched two fluorescent proteins onto the opposing ends of N-WASP -- one yellow and one cyan (greenish-blue).
When stimulated by a particular wavelength of light, fluorescent proteins normally release energy in the form of light. In the case of yellow and cyan proteins, the light emitted appears either yellow or cyan. Under certain circumstances, light energy from the cyan protein can be transferred to the yellow protein since cyan is a higher energy light than yellow and energy naturally jumps from high- to low-energy states. The team hypothesized that, as N-WASP becomes activated and folds, the two ends would be brought closer together, resulting in an increase in the brightness of the yellow protein and a decrease in the brightness of the cyan protein. This phenomenon is called fluorescence resonance energy transfer.
While this phenomenon has been used previously to examine the activity of proteins other than N-WASP, this is the first study in which the natural folding and unfolding of a single protein was observed. All former efforts relied on artificially tethering two separate proteins together, which can produce deceptive results.
As they had hoped, the ratio of cyan to yellow light did accurately reflect N-WASP activity. Normally, N-WASP, so named because it belongs to a family of proteins implicated in the rare genetic disorder Wiskott-Aldrich syndrome, is only marginally activated by one of two proteins, PIP2 and CDC42. However, it becomes highly activated when simultaneously stimulated by the two proteins. In accordance with this synergistic effect, activation with only one of these proteins resulted in only a modest decrease in cyan light and increase in yellow light, while simultaneous activation with both resulted in a much more dramatic effect.
"It was exciting to discover that we could not only visualize N-WASP activation but also could visualize the specific integration of PIP2 and CDC42 stimulation," Ward says. "This supports the idea that our probe is sensitive to normal cellular signaling processes."
Using their new technique, the team recorded preliminary observations of N-WASP activation throughout living cells placed in a petri dish.
Traditionally, N-WASP was thought to be significantly active in filopodia, thin filaments that protrude from cells to help navigate through the body. As expected, N-WASP activity was high in these compartments.
However, several of the team's other observations surprised them.
First, N-WASP and its stimulator proteins CDC42 and PIP2 all were active in "ruffles," animated ridges on the cell membrane that also help cells move forward. According to Ward, research on N-WASP has never highlighted its potential role in ruffling.
Second, some of the highest levels of N-WASP activity were in the nucleus, despite the general assumption that the protein's main functions are in cell movement, which occurs in the periphery of the cell.
"Because we were able to visualize where N-WASP is activated, we were able to show it's activated in certain unexpected cellular compartments," Ward says. "Now that we've demonstrated this technique is effective, we hope to further examine this protein's activity and also to see whether similar probes can help us visualize other folding proteins."
Ward ME, Wu JY, Rao Y. Visualization of spatially and temporally regulated N-WASP activity during cytoskeletal reorganization in living cells. Proceedings of the National Academy of Sciences, Jan. 27, 2004.
Funding from the National Institutes of Health, the Society for Progressive Supranuclear Palsy, the National Brain Tumor Foundation, the Muscular Dystrophy Association and the Leukemia Society of America supported this research.