Three-dimensional Rearrangements within Inositol 1,4,5-Trisphosphate Receptor by Calcium

Abstract

, as reconstructed by three-dimensional electron microscopy, had a "mushroom-like" appearance consisting of a large square-shaped head and a small channel domain linked by four thin bridges. The projection image of the "headto-head" assembly comprising two particles confirmed the mushroom-like side view. The "windmill-like" form of IP 3 R1 with Ca 2؉ also contains the four bridges connecting from the IP 3 -binding domain toward the channel domain. These data suggest that the Ca 2؉ -specific conformational change structurally regulates the IP 3 -triggered channel opening within IP 3 R1.Inositol 1,4,5-trisphosphate receptor (IP 3 R) 1 acts as a highly controlled decoder transducing extracellular stimuli into intracellular calcium (Ca 2ϩ ) signals, a process that plays an integral role in vital morphogenesis and physiological plasticity (1). IP 3 Rs are structurally divided into three regions: 1) a large cytoplasmic region with an IP 3 binding pocket close to the N terminus; 2) a transmembrane region near the C terminus; and 3) a short C-terminal tail (2). The most characterized IP 3 R, type 1 IP 3 R (IP 3 R1), a predominant type in the cerebellar endoplasmic reticulum (ER) and spine apparatus, is a 2749-amino acid polypeptide (2) containing the IP 3 binding core (residues 226 -578) (3, 4), a transmembrane region composed of putative six-spanning domains (residues 2276 -2589) (5, 6). The transmembranous segments are essential for the tetramerization (7, 8) to form a channel domain as in the case of the voltage-, Ca 2ϩ -, and cyclic nucleotide-gated ion channel superfamily (9, 10).The IP 3 never fully opened the channel of IP 3 R without Ca 2ϩ . Previous functional analysis indicates that the low Ca 2ϩ level acts as an essential co-agonist for IP 3 -gated Ca 2ϩ release (11)(12)(13)(14). The requirement of the dual ligands is considered to be an important basis of the signaling cross-talk involved in the cell function (15). Thus, it would be useful to understand how the binding of co-agonists structurally opens the channel of IP 3 R. Recently, the x-ray structure of the crystallized IP 3 binding core has revealed the spatial relationship among a specific IP 3 -binding site and two plausible Ca 2ϩ -binding sites (4). The local allosteric coupling of IP 3 -and Ca 2ϩ -binding sites in this domain is thought to be one of the possible elements for the requirement of dual agonists to open the channel. However, additional six Ca 2ϩ -binding sites have been detected by extensive screening of expressed IP 3 R1-fragments outside the IP 3 binding core (16,17) and, based on the primary sequence, a critical residue for the Ca 2ϩ -activated channel opening was located far from the IP 3 binding core (18,19). Furthermore, allosteric sites for regulations were widely distributed outside the IP 3 binding core (20). Thus, it is quite important to visualize the overall behavior of tetrameric IP 3 R to elucidate the mechanism underlying channel gating by IP 3 and Ca 2ϩ . Knowledge of the spatial interrelations and rearrangements of the...

Mechanistic basis of bell-shaped dependence of inositol 1,4,5-trisphosphate receptor gating on cytosolic calcium

“…In contrast, Ca 2+ binding functions to decrease FRET efficiency, indicating that Ca 2+ binding induces a relaxation of tetrameric channel complexes. This Ca 2+ -induced decrease in FRET efficiency is consistent with the results of single particle analysis showing that purified IP 3 R tetramers change their shape from a tight "square" form to a relaxed "windmill" form after Ca 2+ binding (25,27). In this study, the magnitude of the FRET change evoked by Ca 2+ was larger than that evoked by IP 3 ( Fig.…”

“…In this study, the magnitude of the FRET change evoked by Ca 2+ was larger than that evoked by IP 3 ( Fig. 2 C and D), a feature consistent with the results of single particle analysis in which the effect of IP 3 addition on IP 3 R conformation was undetectably small (25,27). In the presence of both IP 3 and Ca 2+ , the conformational changes within the channel subunits are not a simple summation of those evoked by each ligand, because the dual-ligand-induced FRET signal changes were not as predicted for a summative response, as shown in Fig.…”

IP3 Receptors: Toward Understanding Their Activation

“…Functional IP 3 Rs are tetrameric, assembled either from identical subunits or from mixtures of the three subtypes and their many splice variants Foskett et al 2007). Several structures of the entire IP 3 R1 have been published, each derived from single particle analysis of images from electron microscopy (Hamada and Mikoshiba 2002;Jiang et al 2002a;da Fonseca et al 2003;Hamada et al 2003;Serysheva et al 2003;Sato et al 2004). These studies confirm the tetrameric state of IP 3 R, but variability between the structures and their relatively low resolution ( 30 Å ) have, so far, limited any realistic interpretation of the structural basis of IP 3 R activation (Fig.…”

Aberrant calcium signaling by transglutaminase-mediated posttranslational modification of inositol 1,4,5-trisphosphate receptors

“…Two intrinsic determinants stabilizing the subunit configurations can be considered a priori-IP 3 and Ca 2+ -because channel activation requires both IP 3 and Ca 2+ . In fact, the recent X-ray structures of the IBD indicate that conformational changes occur upon IP 3 binding (7,8); our group and others have demonstrated Ca 2+ -induced structural changes (84,85,94,95). Fluorescence resonance energy transfer (FRET) analysis of recombinant IP 3 Rs also showed reversible structural changes induced by physiological concentrations of IP 3 and Ca 2+ (96).…”

“…From our early EM studies of negatively stained IP 3 R1 using heparin-gold labeling, we proposed a long-range coupling of IBD to the channel domain by Ca 2+ -dependent structural changes (84)(85)(86). However, direct binding of the IBD to the channel domain was postulated, and a model was proposed in which an N-terminal suppressor domain directly binds a loop between putative transmembrane helices (87)(88)(89)(90).…”