Light at the end of the Ca²⁺-release channel tunnel: structures and mechanisms involved in ion translocation in ryanodine receptor channels

Abstract

RyR and InsP3R are Ca(2+)-release channels. When induced to open by the appropriate stimulus, these channels allow Ca2+ to leave intracellular storage organelles at an astonishing rate. Investigations of the ion-handling properties of isolated RyR channels have demonstrated that, at least in comparison to voltage-gated channels of surface membranes, these channels display limited powers of discrimination between physiologically relevant cations and this relative lack of selectivity is likely to contribute to the ability of Ca(2+)-release channels to maintain high rates of cation translocation without compromising function. A range of ion-handling properties in RyR are consistent with the proposal that this channel functions as a single-ion channel and theoretical considerations indicate that the high rates of ion translocation monitored for RyR would require the pore of such a structure to be short and possess a large capture radius. Measurements of the dimensions of regions of RyR involved in ion conduction and discrimination indicate that this is likely to be the case. In each monomer of RyR/InsP3R, residues making up the last two trans-membrane spanning domains and a luminal loop linking these two helices contribute to the formation of the channel pore. The luminal loops of both RyR and InsP3R contain amino acid sequences similar to those known to form the selectivity filter of K+ channels. In addition the luminal loops of both Ca(2+)-release channels contain sequences that are likely to form helices that may be analogous to the pore helix visualised in KcsA. The correlation in structural elements of the luminal loops of RyR/InsP3R and KcsA has prompted us to speculate on the tertiary arrangement for this region of the Ca(2+)-release channels using the established structure of KcsA as a framework.

Methanethiosulfonate Ethylammonium Block of Amine Currents through the Ryanodine Receptor Reveals Single Pore Architecture

“…These dimensions can be compared with those obtained from the three-dimensional structure deduced from single particle reconstructions (26,27). When the solid body representation of the three-dimensional structure is viewed from the side, the RyR resembles a mushroom with a "cap" composed of the cytoplasmic domains of the component monomers and a "stalk" made up of the putative membranespanning domain (8). When viewed from the cytoplasm, the cap appears as a square slab (270 ϫ 270 Å) with a depth of 110 Å (28,8).…”

“…When the solid body representation of the three-dimensional structure is viewed from the side, the RyR resembles a mushroom with a "cap" composed of the cytoplasmic domains of the component monomers and a "stalk" made up of the putative membranespanning domain (8). When viewed from the cytoplasm, the cap appears as a square slab (270 ϫ 270 Å) with a depth of 110 Å (28,8). The size of the vestibule deduced from functional studies is 25 Å wide and 10.4 Å tall, dimensions that would easily fit into the cap as drawn from three-dimensional images (8).…”

“…When viewed from the cytoplasm, the cap appears as a square slab (270 ϫ 270 Å) with a depth of 110 Å (28,8). The size of the vestibule deduced from functional studies is 25 Å wide and 10.4 Å tall, dimensions that would easily fit into the cap as drawn from three-dimensional images (8). The stalk is also square in appearance with a height through the membrane of ϳ65 Å and lateral dimensions of ϳ120 Å on a side (8).…”

“…Further support for the single pore model of the RyR tetramer comes from co-expression of a modified RyR construct designed to produce a low conductance channel with the wild type RyR. The expressed heteromeric proteins produced channel currents that could be separated into six groups where the unitary conductance ranged from wild type to that of the homomeric mutant (8). The six conductance groups were then correlated with six possible arrangements of mutant and wild type RyR subunits.…”

IP3 Receptors: Toward Understanding Their Activation

“…In effect, the ER Ca 2þ pump (SERCA), by creating a steep Ca 2þ concentration gradient across the ER membrane, assumes responsibility for determining which cations flow through an open IP 3 R. Indeed, the K þ permeability of IP 3 R and RyR may facilitate rapid Ca 2þ release byallowing K þ to move into the ER to electrically compensate the efflux of Ca 2þ (Gillespie and Fill 2008). The pore of the IP 3 R, like that of RyR, is formed by the final pair of TMD (TMD5-6) and the luminal loop that links them from each of the four subunits (Ramos-Franco et al 1999;Williams et al 2001) (Fig. 2C).…”