https://phys.org/news/2018-10-protein-pair-cellular-calcium.html

"We've known for a decade that STIM1 protein moved toward the plasma membrane to open ORAI channels when ER calcium levels drop," says Hogan, a professor in the Division of Signaling and Gene Expression. "Our recent work shows how the STIM machinery functions on a molecular level. Understanding these mechanisms is critical, as calcium is important for a panoply of immune responses."

The Cell Reports paper reveals how STIM1 protein elongates as it switches from a resting to an activated state when calcium reserves drop. STIM1 is a transmembrane protein that spans the ER wall: one end pokes a calcium-sensing dipstick-like tail inside the reserve; in a resting, calcium-replete state, the other arm juts out from the ER but remains tucked up against the ER wall, out of touch with dormant ORA1 channels that dot the cell membrane.