Our immune system consists of white blood cells, including lymphocytes. Lymphocytes are regulated by calcium permeable ion channels on their plasma membrane. These ion channels are believed to be composed of two major proteins, STIM1 and Orai1. Mutations in Orai1 cause severe immune deficiency syndrome.
Abstract from: J Biol Chem. 2009 May 15;284(20):13676-85. Epub 2009 Mar 16.
Maruyama Y, Ogura T, Mio K, Kato K, Kaneko T, Kiyonaka S, Mori Y, Sato C.
Neuroscience Research Institute and Biological Information Research Center, National Institute of Advanced Industrial Science and Technology (AIST), 1-1-4 Umezono, Tsukuba 305-8568, Japan.
The Ca(2+) release-activated Ca(2+) channel is a principal regulator of intracellular Ca(2+) rise, which conducts various biological functions, including immune responses. This channel, involved in store-operated Ca(2+) influx, is believed to be composed of at least two major components. Orai1 has a putative channel pore and locates in the plasma membrane, and STIM1 is a sensor for luminal Ca(2+) store depletion in the endoplasmic reticulum membrane. Here we have purified the FLAG-fused Orai1 protein, determined its tetrameric stoichiometry, and reconstructed its three-dimensional structure at 21-Å resolution from 3681 automatically selected particle images, taken with an electron microscope. This first structural depiction of a member of the Orai family shows an elongated teardrop-shape 150Å in height and 95Å in width. Antibody decoration and volume estimation from the amino acid sequence indicate that the widest transmembrane domain is located between the round extracellular domain and the tapered cytoplasmic domain. The cytoplasmic length of 100Å is sufficient for direct association with STIM1. Orifices close to the extracellular and intracellular membrane surfaces of Orai1 seem to connect outside the molecule to large internal cavities.