Among the reasons Hahn and co-authors at Scripps Research Institute in La Jolla, Calif., decided to study the Rho proteins was that different members of the protein family each control a different aspect of cellular movement of extension and retraction. One family controls extension of the edge, another the formation of fibers, and still another controls tail retraction.
"And the key to understanding this mechanism is to see where in time and space each of these is turned on and how it's all coordinated," Hahn said. "So there's a really good reason to look at this in live cells. You can't understand spatio-temporal control if you look at this in a test tube."
Another reason to study Rho proteins is that their activation is necessary to induce essentially opposite behaviors. "They're activated for proliferation and for cell death (apoptosis), also for motility. So it may be that this spatio-temporal control is what's producing these differences."
Some of the study's biological findings in that latter regard were tantalizing. Cdc42 induced formation of cell extensions called filopodia when it was activated around the filopodia base and not within the lengths, Hahn said.
"When we looked at extension and retraction, we found that Cdc42 activation was remarkably correlated with both. It was activated at exact locations relative to cell extensions and was turned off in exact parallel with retraction."
Further experiments showed that this coordination was produced by "upstream signals" regulating both retraction and extension.
"The use of fluorescent labeling of molecules in live cells was pioneered over a decade ago here at UNC," Hahn said. "My work with these new dyes is an extension of that work, it grew out of that."