The Orai channel is characterized by voltage independence, low conductance, and high Ca 2+ selectivity and plays an important role in Ca 2+ influx through the plasma membrane (PM). How the channel is activated and promotes Ca 2+ permeation is not well understood. Here, we report the crystal structure and cryo-electron microscopy (cryo-EM) reconstruction of a Drosophila melanogaster Orai (dOrai) mutant (P288L) channel that is constitutively active according to electrophysiology. The open state of the Orai channel showed a hexameric assembly in which 6 transmembrane 1 (TM1) helices in the center form the ion-conducting pore, and 6 TM4 helices in the periphery form extended long helices. Orai channel activation requires conformational transduction from TM4 to TM1 and eventually causes the basic section of TM1 to twist outward. The wider pore on the cytosolic side aggregates anions to increase the potential gradient across the membrane and thus facilitate Ca 2+ permeation. The open-state structure of the Orai channel offers insights into channel assembly, channel activation, and Ca 2+ permeation.

It is known that tRNAs play an essential role in genetic information transfer from DNA to protein. The maturation of tRNA precursors is performed by the endoribonuclease RNase P, which classically consists of a main RNA segment and accessory proteins. However, the newly identified human mitochondrial RNase P-like protein (MRPP123) complex is unique in that it is composed of three proteins without RNA. Here, we determined the crystal structure of MRPP123 complex subunit 3 (MRPP3), which is thought to carry out the catalytic reaction. A detailed structural analysis in combination with biochemical assays suggests that MRPP3 is in an auto-inhibitory conformation in which metal ions that are essential for catalysis are excluded from the active site. Our results indicate that further regulation is necessary to rearrange the conformation of the active site of MRPP3 and trigger it, thus providing important information to understand the activation of MRPP123.

Store-operated calcium entry (SOCE) is a major pathway for calcium ions influx into cells and has a critical role in various cell functions. Here we demonstrate that calcium-bound calmodulin (Ca2+-CaM) binds to the core region of activated STIM1. This interaction facilitates slow Ca2+-dependent inactivation after Orai1 channel activation by wild-type STIM1 or a constitutively active STIM1 mutant. We define the CaM-binding site in STIM1, which is adjacent to the STIM1–Orai1 coupling region. The binding of Ca2+-CaM to activated STIM1 disrupts the STIM1–Orai1 complex and also disassembles STIM1 oligomer. Based on these results we propose a model for the calcium-bound CaM-regulated deactivation of SOCE.

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