In one of the cell's key controls of internal calcium levels, channels at the cell membrane open in response to changes in calcium levels in the endoplasmic reticulum (ER). These plasma membrane channels, called calcium-release activated calcium (CRAC) channels, were first identified electrophysiologically about 20 years ago. Because their tiny conductance (about 100 times lower than that of any other known calcium channel) makes individual channel currents very difficult to detect, however, both the molecular mechanism of this signaling pathway and the identity of the channel, have until recently remained a mystery.
Just two years ago, shortly after the discovery of a protein called stromal interacting molecule (STIM), which acts as the ER calcium sensor that activates CRAC channels...
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After the discovery of Orai, researchers wondered about how the sensor and the channel communicate with each other. Upon calcium store depletion, STIM proteins reorganize from their diffuse positions throughout the ER membrane to form discrete punta close to the plasma membrane. Richard Lewis' group at the Stanford University School of Medicine showed that activated Orai colocalizes with STIM, concentrating calcium influx to the membrane region containing both proteins.
Still, other views are out there. One lab suggests that a second messenger delivers the signal from STIM to Orai. Other researchers implicate a different group of proteins as a key element of the CRAC signaling pathway, and possibly even as the CRAC channel itself (instead of Orai). The results of those studies, however, remain "a minority view," says Lewis.
Several parts of the molecular picture are still missing, starting with a clear understanding of Orai's stoichiometry. The only clues so far, from Trevor Shuttleworth's group at the University of Rochester, suggest that the channel is a tetramer.
What's clear, researchers agree, is that the channel is very strange, with no homology to any previously described ion channels. Still, says Lewis, progress in just two years has been tremendous.
Knockout studies are exploring the channel's cellular function, in particular with mouse knockouts for the two STIM variants and the three Orai variants found in mammals. In February 2008, Jean-Pierre Kinet's group at Harvard published the first results of an Orai1 knockout.
Meanwhile, researchers say pharmaceutical companies have begun to investigate the CRAC channel blockers to reduce the toxicity of immunosuppressant drugs such as cyclosporine A, used to prevent host rejection after organ transplantation. "We and others think there's a real opportunity for immunosuppression to be mediated by Orai1," says Cahalan.