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

Abstract

1. Introduction 62

2. Channel structure 63

2.1 Isoforms, primary structure and topology of Ca²⁺-release channels 63

2.2 Identification of ligand binding sites within the primary sequence of RyR 65

2.2.1 Calcium 66

2.2.2 Calmodulin 66

2.2.3 FK506-binding proteins 66

2.2.4 L-type Ca²⁺ channel 66

2.2.5 Ryanodine 67

2.3 The three-dimensional structure of the RyR channel 68

3. Channel function 70

3.1 RyR channel gating 70

3.2 Ion translocation and discrimination 71

3.2.1 Monovalent inorganic cations 71

3.2.2 Divalent inorganic cations 74

3.2.3 Organic monovalent cations 75

3.2.4 Permeant ion translocation can be blocked 75

3.3 Summary of ion handling in RyR 76

3.4 Where is the pore and what components of RyR are involved in its formation? 76

3.5 The mechanisms underlying ion translocation and discrimination in RyR 79

3.6 Does RyR employ ion–ion repulsion to attain high unitary conductance? 82

3.6.1 Conductance–activity relationships 83

3.6.2 Concentration dependence of reversal potential 83

3.6.3 The dependence of unitary conductance on mole-fraction 83

3.6.4 Effective valence of channel-blocking cations 84

3.6.5 Modelling ionic conduction 84

3.7 Factors influencing maximum conductance 85

3.8 Factors influencing ion entry 86

3.9 Theoretical design for the pore of RyR 88

4. What do we know about the structure of the conduction pathway in RyR? 88

4.1 The narrowest region of the conduction pathway – the ‘selectivity filter’ of RyR 89

4.2 The voltage drop across RyR – the length of the ‘pore’ 89

4.3 Mechanisms for ion discrimination in RyR 93

4.4 Musings on the structure of the RyR/InsP₃R pore 95

5. Summary 98

6. Acknowledgements 98

7. References 98

The purpose of this article is to provide a description of the current state of our understanding of certain aspects of the relationship between the structure and function of the ryanodine receptor (RyR). RyR is an ion channel found in the membranes of the intracellular Ca²⁺ storage organelles, the endoplasmic reticulum (ER) and its counterpart in muscle cells the sarcoplasmic reticulum (SR), where it provides a regulated pathway for the release of stored Ca²⁺ during Ca²⁺ signalling processes such as fertilization and muscle contraction. RyR possesses both high- and low-affinity binding sites for the plant alkaloid ryanodine; however, whilst this ligand gives the channel its name and is of toxicological and pharmacological interest, physiological regulation of channel gating is mediated by the binding of cytosolic ligands (primarily Ca²⁺, ATP, Mg²⁺) and in some cases by direct coupling with a surface membrane voltage sensor.