Coiled-Coil Formation Conveys a STIM1 Signal from ER Lumen to Cytoplasm.

STIM1 and STIM2 are endoplasmic reticulum (ER) membrane proteins that sense decreases in ER-luminal free Ca2+ and, through a conformational change in the STIM cytoplasmic domain, control gating of the plasma membrane Ca2+ channel ORAI1. To determine how STIM1 conveys a signal from the ER lumen to the cytoplasm, we studied the Ca2+-dependent conformational change of engineered STIM1 proteins in isolated ER membranes and, in parallel, physiological activation of these proteins in cells. We find that conserved "sentinel" features of the CC1 region help to prevent activation while Ca2+ is bound to STIM ER-luminal domains. Reduced ER-luminal Ca2+ drives a concerted conformational change, in which STIM luminal domains rearrange and the STIM transmembrane helices and initial parts of the CC1 regions pair in an extended coiled coil. This intradimer rearrangement overcomes the relatively weak CC1-SOAR/CAD interactions that hold STIM in an inactive conformation, releasing the SOAR/CAD domain to activate ORAI channels.

TMEM110 regulates the maintenance and remodeling of mammalian ER-plasma membrane junctions competent for STIM-ORAI signaling.

The stromal interaction molecule (STIM)-ORAI calcium release-activated calcium modulator (ORAI) pathway controls store-dependent calcium entry, a major mechanism of physiological calcium signaling in mammalian cells. The core elements of the pathway are the regulatory protein STIM1, located in the endoplasmic reticulum (ER) membrane, the calcium channel ORAI1 in the plasma membrane, and sites of close contact between the ER and the plasma membrane that permit the two proteins to interact. Research on calcium signaling has centered on STIM1, ORAI1, and a few proteins that directly modulate STIM-ORAI function. However, little is known about proteins that organize ER-plasma membrane junctions for STIM-ORAI-dependent calcium signaling. Here, we report that an ER-resident membrane protein identified in a previous genome-wide RNAi screen, transmembrane protein 110 (TMEM110), regulates the long-term maintenance of ER-plasma membrane junctions and the short-term physiological remodeling of the junctions during store-dependent calcium signaling.

Secretagogin, a hexa EF-hand calcium-binding protein: high level bacterial overexpression, one-step purification and properties.

Secretagogin (SCGN), a hexa EF-hand calcium-binding protein, is highly expressed in the endocrine cells (especially in pancreatic islets) and in restricted neuronal sub-populations, albeit at comparatively low level. Since SCGN is predicted to be a potential neuroendocrine marker in carcinoid tumors of lung and gastrointestinal tract, it is of paramount importance to understand the features of this protein in different environment for assigning its crucial functions in different tissues and under pathophysiological conditions. To score out the limitation of protein for in vitro studies, we report a one-step, high purity and high level bacterial purification of secretagogin by refolding from the inclusion bodies yielding about 40mg protein per litre of bacterial culture. We also report previously undocumented Ca(2+)/Mg(2+) binding and hydrodynamic properties of secretagogin.

Liaison between myristoylation and cryptic EF-hand motif confers Ca(2+) sensitivity to neuronal calcium sensor-1.

Many members of the neuronal calcium sensor (NCS) protein family have a striking coexistence of two characteristics, that is, N-myristoylation and the cryptic EF-1 motif. We investigated the rationale behind this correlation in neuronal calcium sensor-1 (NCS-1) by restoring Ca(2+) binding ability of the disabled EF-1 loop by appropriate mutations. The concurrence of canonical EF-1 and N-myristoylation considerably decreased the overall Ca(2+) affinity, conformational flexibility, and functional activation of downstream effecter molecules (i.e., PI4Kß). Of a particular note, Ca(2+) induced conformational change (which is the first premise for a CaBP to be considered as sensor) is considerably reduced in myristoylated proteins in which Ca(2+)-binding to EF-1 is restored. Moreover, Ca(2+), which otherwise augments the enzymatic activity of PI4Kß (modulated by NCS-1), leads to a further decline in the modulated PI4Kß activity by myristoylated mutants (with canonical EF-1) pointing toward a loss of Ca(2+) signaling and specificity at the structural as well as functional levels. This study establishes the presence of the strong liaison between myristoylation and cryptic EF-1 in NCS-1. Breaking this liaison results in the failure of Ca(2+) specific signal transduction to downstream effecter molecules despite Ca(2+) binding. Thus, the EF-1 disability is a prerequisite in order to append myristoylation signaling while preserving structural robustness and Ca(2+) sensitivity/specificity in NCS-1.