Research |  Structure-function relations of calcium channels (CRAC)



STIM & ORAI: two major players in the SOC(cer) field


It has been recognized for over two decades that depletion of the ER calcium store elicits calcium influx across the plasma membrane, a process known as "store-operated calcium entry (SOCE)". SOCE is best exemplified by the highly calcium-selective Ca release-activated Ca channels (CRAC) channels. CRAC channels contain a class of four-pass transmembrane proteins known as ORAI (named after the keepers of heaven's gate in Greek mythology) as their pore subunits, which are gated by ER-resident type I single-pass transmembrane proteins termed STIM. In addition, mitochondria play essential roles in maintaining CRAC channel activity by undergoing relocalization toward the plasma membrane, uptaking excessive calcium, diminishing calcium-dependent inactivation in activated cells, and thus prolonging the duration of SOCE. The residue E106 in the first transmembrane segment of ORAI1 plays a crucial role in calcium permeation and contributes to the high calcium selectivity of ORAI1 channels. Rare gene alleles that compromise CRAC channel expression or activation (e.g., R91W) result in severe immunodeficiency disease in human patients.

 

Following the identification of these two proteins, the major steps and certain regulatory mechanisms involved in the exquisite molecular choreography of SOCE have been under intense investigation. In particular, the structural basis, as well as the accompanying conformational changes underlying STIM-ORAI coupling between the ER and plasma membrane remains to be defined. Given that both CRAC channels and MCU have no discernible homology to other well-characterized channels, their ion permeation and gating mechanisms may represent totally new paradigms.


The discovery of SOCE mediated by ORAI and STIM also led to the recent acceleration of knowledge on the critical role of SOCE in cancer. Augmented SOCE has been found in a wide range of cancers and is significantly associated with the risk of tumor metastasis and survival. Therefore, the ORAI-STIM pathway could also serve as a novel target for anticancer therapeutic intervention.


Image courtesy of Ji Jing



STIM1-ORAI1 coclustering at ER-PM junctions
Top panel: before store depletion
Bottom: after store depletion
Image courtesy of Ji Jing

References:

1. Ma G*, Wei M*, He L*, Liu C*, Wu B, Zhang SL, Jing J, Liang X, Senes A, Tan P, Li S, Sun A, Bi Y, Zhong L, Si H, Shen Y, Li M, Lee MS, Zhou W, Wang J, Wang Y and Zhou YInside out Ca2+ signaling prompted by STIM1 conformational switch. Nature Communications2015,6:7826. 

2. Jing J*, He L*, Sun A*, Quintana A*, Ding Y*, Ma G, Tan P, Liang X, Zheng X, Chen L, Shi X, Zhang SL, Zhong L, Huang Y, Dong MQ, Walker CL, Hogan PG, Wang Y and Zhou Y. Proteomic mapping of ER-PM junctions identifies STIMATE as a regulator of Ca2+ influx. Nature Cell Biology. 2015,17:1339-4. 

3. Gudlur A, Quintana A, Zhou Y,  Hirve N, Mahapatra S and Hogan PG. STIM1 triggers a gating rearrangement at the extracellular mouth of the ORAI1 channel. Nature Communications. 2014,5:5164. 
4. Zhou Y, Srinivasan P, Razavi S, Seymour S, Meraner P, Gudlur A, Stathopulos PB, Ikura M Rao A and Hogan PG. Initial activation of STIM1, the regulator of store-operated calcium entry. NSMB. 2013,20:973-981. 
5.Gudlur A, Zhou Y, and Hogan PG.  STIM-ORAI interactions that control the CRAC channel.Current Topics in Membranes. 2013,71:33-58. 
6. Zhou Y, Ramachandran S, Oh-hora M, Rao A and Hogan PG. Pore architecture of the ORAI1 store-operated calcium channel. Proceedings of the National Academy of Sciences USA. 2010,107(11):4896-901. (PMID: 20194792)
7. Zhou Y, Meraner P, Kwon, HT, Machnes D, Oh-hora Masatsugu, Zimmer Jochen, Huang Y, Stura A, Rao A and Hogan PG. STIM1 gates the store-operated calcium channel ORAI1 in vitro. NSMB. 2010,17(1):112-116. (PMID: 20037597)


Inside-out calcium signaling | discovery of novel gain-of-function STIM1 mutations  

Image courtesy of Guolin Ma

References: 


1. Ma G*, Wei M*, He L*, Liu C*, Wu B, Zhang SL, Jing J, Liang X, Senes A, Tan P, Li S, Sun A, Bi Y, Zhong L, Si H, Shen Y, Li M, Lee MS, Zhou W, Wang J, Wang Y and Zhou YInside out Ca2+ signaling prompted by STIM1 conformational switch. 

Nature Communications2015,6:7826. DOI: 10.1038/ncomms8826. (PMID:26184105)


STIMATE | A novel regulator of calcium influx at ER-PM junctions 

STIMATE (for STIM-activating enhancer; encoded by TMEM110), is an ER-resident multi-transmembrane protein identified by our laboratory through a proteomic study on the ER-PM junctions. The ER-PM junctions are defined as specialized junctional sites that connect the endoplasmic reticulum (ER) and the plasma membrane (PM), and are closely implicated in controlling lipid and calcium homeostasis in mammalian cells.


STIMATE is a positive modulator of calcium flux mediated by the STIM-ORAI signaling in vertebrates. STIMATE can physically associate with STIM1 to promote conformational switch of STIM1 from inactive toward an activated state, thereby coupling to and gating the ORAI calcium channels on the plasma membrane.

Depletion of TMEM110 with RNAi knockdown or Cas9-mediated gene disruption substantially reduces the puncta formation of STIM1 at ER-PM junctions and remarkably inhibits the calcium/calcineurin/NFAT signaling axis. 


Genetic and biochemical studies are currently performed to uncover more uncharted functions of this ER-resident protein at ER-PM junctions.


References:
STIMATE reveals a STIM1 transitional state. Nat Cell Biol. 2015 Sep 30;17(10):1232-4. doi: 10.1038/ncb3245.
Proteomic mapping of ER-PM junctions identifies STIMATE as a regulator of Ca(2+) influx. Nat Cell Biol. 2015 Oct;17(10):1339-47. doi: 10.1038/ncb3234. 

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