According to Selvin, a problem with the crystal structure is that it only offers a static snapshot of what the protein looks like and provides only limited information about how different parts of the protein move. Another concern is that the conditions used to obtain protein crystals sometimes alter the original structure of the protein.
In the new study, postdoctoral researcher David Posson worked with Selvin to put the models of voltage sensor movement to the test.
They studied the voltage sensor segment in a specific ion channel called the Shaker potassium channel. This protein was first discovered in fruit flies after researchers observed that a mutation in the channel caused the flies to vigorously shake.
To preserve channels in their original state, Posson studied ion channels inserted into the membranes of frog eggs. He tested the two models using a fluorescence technique called Lanthanide resonance energy transfer (LRET) which allowed him to measure small movements in proteins. The technique involves the use of a special pair of molecular bulbs that glow either brightly or dimly depending on how far apart they are. The measurement is sensitive to movements as small as one-billionth of a meter. Posson also needed a way to control the voltage across the membrane.
He used an approach called electrophysiology that involves inserting electrodes into the frog egg. This gave him the ability to change the voltage across the membrane and regulate channel opening.
"Our approach brings together two distinct biophysical techniques, electrophysiology and fluorescence, which have been independently useful for the study of proteins," Posson said.
|Contact: Kaushik Ragunathan|
University of Illinois at Urbana-Champaign