New Technique Captures Calcium Sensor in Action

photo: getting a handle on calmodulin

A specially designed atomic-force
microscope is used to manipulate
calmodulin; TUM Dept. of Physics/
M. Rief

Calmodulin plays a role in physiologically important processes ranging from gene transcription to nerve growth and muscle contraction. But just how it distinguishes between target proteins is not well understood.

In recent experiments the researchers compared the sequences of structural and kinetic changes involved in binding two different kinases. The results reveal new details of how calmodulin binds and regulates its target proteins.

As a so-called signaling protein and "calcium sensor," calmodulin gives start and stop signals for a great number of intracellular activities by binding and releasing other proteins. Calmodulin can bind up to four calcium ions, and the three-dimensional spatial structure of calmodulin varies with the number of calcium ions bound to it. This structure in turn helps to determine which amino acid chains – peptides and proteins – the calmodulin will bind.

Techniques such as X-ray structural analysis offer snapshots, at best, of steps in this intracellular work flow. But single-molecule atomic-force spectroscopy has opened a new window on such dynamic processes.

Professor Matthias Rief and colleagues at the Technical University Munich had previously shown that they could fix a single calmodulin molecule between a surface and the cantilever tip of a specially built atomic-force microscope, expose it to calcium ions in solution, induce peptide binding and unbinding, and measure changes in the molecule's mechanical properties as it did its work.

"What is special about our technique," Rief says, "is that we can work directly in aqueous solution. We can make our measurements in exactly the conditions under which the protein works in its natural environment. So we can directly observe how the calmodulin snatches the amino acid chain and folds itself, to hold its target fast."; Source: Technical University Munich