STIM calcium sensing and conformational change

Abstract

The control of calcium influx at the plasma membrane by endoplasmic reticulum (ER) calcium stores, a process common to invertebrates and vertebrates, is central to physiological calcium signalling and cellular calcium balance. Stromal interaction molecule 1 (STIM1) is a calcium sensor and regulatory protein localized to the ER. ORAI1 is a calcium channel in the plasma membrane (PM). In outline, STIM1 senses an ER-luminal calcium decrease, relocalizes to ER-PM junctions, and recruits and gates ORAI1 channels. Recent work, reviewed here, has offered detailed insight into the process of sensing and communicating ER calcium-store depletion, and particularly into the STIM1 conformational change that is the basis for communication between the ER and the PM.

Keywords: ORAI; STIM; calcium; conformational change; store depletion.

FIGURE 1

STIM functional organization and sequence alignments. A, STIM protein domains or regions discussed in the text. The EFSAM domain in the ER lumen is connected by the transmembrane helix with the CC1 region, SOAR/CAD, and polybasic tail in the cytoplasm. The residue numbering is for human STIM1. B, Alignment of D melanogaster STIM with human STIM1. The locations of EFSAM, the transmembrane helix, CC1, and SOAR/CAD in human STIM1 are marked. Identities are indicated by vertical lines, and gaps have been introduced as necessary to maintain the alignment. C, Alignment of C elegans STIM with human STIM1. D, Alignment of N vectensis STIM with human STIM1. The N- and C-terminal sequences of N vectensis STIM have not been annotated. In (B)–(D), the stated percent identity refers to the region from EFSAM through SOAR/CAD.

FIGURE 2

Cartoon representation of the transmembrane helices (TM) and the initial portions of CC1 pairing in a coiled coil, as in the STIM active conformation. The coiled-coil core residues (red), discontinuities in the heptad repeat (grey), and sentinel residues are marked. These features are conserved across most invertebrates and all vertebrates, and collectively constitute the hallmarks of an ancestral STIM activation mechanism [Hirve et al 2018]. The cartoon does not portray the local distortions in the coiled coil that will necessarily occur at the heptad discontinuities. Residue numbering for human STIM1 is shown for reference. The figure was created using PyMOL [Schrödinger LLC 2010].

FIGURE 3

Tentative evidence that the calcium-sensing mechanism is ancestral. A, Regions experimentally linked to function in human EFSAM [PDB ID 2K60]. Red highlighting marks the EF-hands, where mutations are known to disrupt calcium sensing [Liou et al 2005; Zhang et al 2005; Spassova et al 2006; Stathopulos et al 2008], and the EFSAM segments most affected by the presence or absence of calcium in HDX–MS experiments [Gudlur et al 2018]. B, Segments of EFSAM with a notable clustering of sequence identities between H sapiens STIM1 and N vectensis STIM. H sapiens–N vectensis identities were mapped as a binary function to human EFSAM and smoothed with a triangle function of width 9, which amounts to assigning each residue in EFSAM a weighted sum of the binary map values that fall within four residues. Red highlighting marks residues for which this weighted sum equals or exceeds 0.6 [see inset]. The figure was created using PyMOL [Schrödinger LLC 2010].

FIGURE 4

Two models for the STIM1 activation mechanism. Upper, An earlier view. A, The resting state of STIM has a well structured ER-luminal (EFSAM) domain with calcium bound at a single EF-hand site. B, Store depletion triggers loss of calcium from the single calcium-binding site and unfolding of the luminal domain. C, Oligomerization of the unfolded EFSAM domains drives higher-order oligomerization of STIM and relocalization to ER-PM junctions. Lower, An updated view based on the findings reviewed here. A, The resting state of STIM has a well structured EFSAM domain with 5–6 calcium-binding sites. The cytoplasmic domain is folded back on the CC1 region. B and C, store depletion triggers a concerted change in STIM involving dissociation of calcium from all EFSAM sites, rearrangement of the paired luminal domains to a well folded dimer, and assembly of a coiled coil in the initial region of CC1. The SOAR/CAD domain and polybasic tail are released from the restraints that hold them near the ER membrane in resting STIM, and the STIM cytoplasmic domain extends toward the PM. D, Extended STIM dimers may oligomerize further through interactions of their cytoplasmic domains.