Protein thermodynamics, measured on the move

The precise biological work of folding a protein can be undone by any number of environmental stresses—heat, acidity, and mechanical strain, to name a few. To identify which conditions cause a protein to lose its shape, biologists typically perform a titration: They gradually adjust some input variable, say, temperature, while looking for a sudden physical change indicative of a folding transition. But distinguishing that change from naturally occurring variations in the background requires sampling a wide, potentially damaging range of the input variable. For instance, when Martin Gruebele (University of Illinois at Urbana-Champaign) and colleagues heated the cancer cell shown here, its emission spectrum shifted from yellow to green, an indication that some of its fluorescent-tagged enzymes had unfolded. But at the high temperatures required to complete the titration, the cell died. Now, Gruebele and coworkers have developed a kinder, gentler approach. Instead of slowly adjusting the input variable—in their case, temperature—they administered small, swift jumps. The background signal equilibrates rapidly; the proteins’ folding structure equilibrates much more slowly. By isolating that slow response and plotting its amplitude as a function of temperature, the researchers pinpointed folding transitions more accurately and at milder temperatures than they could have via titration. That means they should now be able to study temperature effects on protein structure in vivo—zebrafish are a likely subject—and no animals need be harmed in the process. (K. Girdhar et al., J. Chem. Phys. 135, 015102, 2011 ; image courtesy of Martin Gruebele.)—Ashley G. Smart