February 28, 2010 | 2
Even in sleep, the human body is rarely still—and within it, there is the constant motion of the contents of our cells and the proteins within.
Until now, scientists have had to estimate the speed of complex but common actions such as protein folding (which turns an unorganized polypeptide strand into a complex and useful three-dimensional protein). They could watch the action unfold, so to speak, in a test tube but weren’t sure how close the pace conformed to real life.
A group of researchers at the University of Illinois at Urbana–Champaign, however, have developed a system to move the observation out of in vitro and into in vivo.
"This is the first experiment that allows us to observe the dynamics of a protein folding in a live cell," Martin Gruebele, a chemist at Illinois and co-author of the study, said in a prepared statement. "Now we have the capability of looking at how fast biological processes occur as a function of time."
Getting a glimpse of proteins at work inside a cell provides a much more nuanced picture than previous experiments that have tracked the big molecules in a test tube. "You have a very simple, very homogenous environment when you study proteins in vitro," Gruebele said. "In a living cell, 30 to 40 percent of the contents are solids of some kind" that physically constrain the protein. That means that when researchers perform the same experiment in a living cell, "we find we get very different answers in different parts of the cell," he said.
Aside from the variability, Gruebele and his colleagues found that a protein was more stable and its folding slower in vivo than in vitro, suggesting that within the cell there was "higher viscosity or higher activation energy," the researchers wrote in the study, published online February 28 in Nature Methods (Scientific American is part of Nature Publishing Group).
To cue the protein actions of folding or unfolding, they needed to create rapid changes of temperature that would still stay in a range safe for cell survival. So the team used laser pulses to move the temperature of the cell’s environment quickly within the range of 96 to 100 degrees Fahrenheit. "It’s like we give them a little bit of a fever," Gruebele said.
The quick temperature shifts had been used before, he noted, "but you don’t get any imagery that lets you see if proteins fold faster in one region [of the cell] and slower in another." He and his team solved that problem by using fluorescence microscopy to "take images of cells and see inside them."
Using both the laser pulses and fluorescent microscopy "combines two worlds: chemical dynamics and the ability to study reactions as they occur; and biological environments, where cell biologists observe how reactions occur in cells," Gruebele noted. He and his team have named the technique Fast Relaxation Imaging.
Though these first assays were performed only on two cells (human osteosarcoma and cervical carcinoma cells), the new technique could have broad applications for drug screening and the study of biological processes in diseases such as Alzheimer’s and Huntington’s. "We can take these proteins that cause these diseases, actually put them into the kind of cells where they cause these disease, give them a heat shock and…see if they bind differently," Gruebele noted. "We’ll be able to follow these events in real time and give researchers an idea of if this is a possible pathway through which disease could occur."
Image courtesy of Simon Ebbinghaus