The researchers saw the strong muscle contraction, but they only saw a small increase in MLCK activation, not enough to account for the muscle response. They discovered that because MLCK was competing for calmodulin with other calmodulin-binding proteins, there was only enough calmodulin available in this system to activate a small portion of the MLCK.
"Surprisingly, there is not enough calmodulin for all of its targets," Dr. Stull said. "So the signaling system has recruited a second pathway to enhance the limited activation of MLCK, which leads to a strong muscle contraction."
At the end of the primary chemical pathway, an enzyme called phosphatase can remove the phosphate from the myosin, hampering the muscle cell contraction. But the second chemical pathway inhibits the phosphatase from removing the phosphate.
"In this second pathway, the phosphates are no longer taken away from the myosin, which allows more phosphorylated myosin to remain, leading to a stronger muscle contraction," Dr. Stull said.
To track the progress of this intricate chemical dance, researchers genetically engineered a mouse containing a fluorescent molecule, or biosensor that directly monitors the calcium/calmodulin activation of MLCK in real time in smooth-muscle cells.
"These studies demonstrate the feasibility of producing transgenic biosensor mice for investigations of signaling processes in intact systems," Dr. Stull said.
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Contact: Amanda Siegfried
Amanda.siegfried@utsouthwestern.edu
214-648-3404
UT Southwestern Medical Center
5-Apr-2004