Signal transduction in cells: Researchers describe new model for regulation

In order to adapt to changing environments and eventually to survive, cells have to change their shape constantly: they bend, stretch, and move. In doing so, the cells depend on a network of filaments called the cytoskeleton just beneath the plasma membrane, which surrounds the cells. One key regulator of the cytoskeleton is the protein ROCK, which is crucial from embryonic development on. However, while the biological roles of ROCK have been studied extensively, the structure and the mechanisms by which its activity is controlled are not well understood.

Size and position matter

The group of Thomas Leonard has now determined the structure of ROCK, showing that a 107 nm long coiled-coil tether links the kinase domain, which is responsible for the enzymatic activity of the protein, and the membrane-binding domain. As a reference, the diameter of the smallest known unicellular organism is just 200 nm. Postdoc Linda Truebestein and her colleagues show that ROCK activity in cells depends on the length of this coiled-coil tether, which therefore acts like a "molecular ruler." This mechanism represents a completely new type of spatial control.

Similarities to a car engine

"Our findings indicate that the activity of the Rho kinases is regulated by the spatial positioning of the kinase domain and substrate in the cell, much like the clutch in a car engine determines whether the car is in gear or not. The engine, or kinase, is always running, but the car, or cell, doesn't move unless the clutch, or substrate, is engaged. This represents a hitherto unknown mechanism in the regulation of protein kinases such as ROCK, whereby substrate specificity and corresponding activity are simply governed by the precise spatial positioning of enzyme and substrate," says MFPL group leader Thomas Leonard.

Challenges for the future

The future challenges of the MFPL researchers are to find out what this "molecular ruler" is actually bridging and whether there are additional functions of the ruler beyond its role in positioning the kinase domains -- the part of the molecule that carries out its function. "We could show that the molecular ruler is conserved throughout evolution, but we don't know yet if the ruler is truly sequence independent," says Linda Truebestein, who conducted the studies together with PhD student Daniel Elsner.