Junctin – the quiet achiever

Abstract

Recent advances in understanding skeletal and cardiac muscle function have evolved with recognition of the active role played by the intracellular sarcoplasmic reticulum (SR) Ca2+ store in contraction. The key proteins in this store are the Ca2+ binding protein calsequestrin (CSQ), the ryanodine receptor (RyR) Ca2+ release channel and triadin and junctin (Beard et al. 2004). The CSQ–triadin–junctin–RyR complex (Fig. 1) in the SR lumen forms a ‘Ca2+ transduction machine’ that is central to EC coupling and to normal muscle development. Other proteins in the lumen of the SR, including the histidine rich calcium binding protein (HRC) (Suk et al. 1999), JP-45 (Anderson et al. 2003) and SRP-27 (Bleunven et al. 2008), must also contribute to control of SR intraluminal Ca2+ load, but the precise nature of their role remains undetermined. JP-45 in particular is ideally placed to communicate store load to the excitation–contraction (EC) coupling process as it binds to both CSQ and the dihydropyridine receptor (DHPR) in the surface/transverse tubule membrane. The importance of the luminal proteins has been underlined by the recent discovery that changes in Ca2+ signalling due to mutations in CSQ or to lack of its expression can result in sudden cardiac death (Viatchenko-Karpinski et al. 2004). Furthermore, studies in animal models show that changes in CSQ, junctin, triadin and HRC expression can lead to defective Ca2+ signalling. The review by Pritchard & Kranius (2009) examines the roles of junctin and HRC in Ca2+ cycling and their potential significance in heart failure. This commentary focuses on protein-protein interactions between CSQ, triadin, junctin and RyR proteins that may underlie their roles in Ca2+ cycling. Figure 1 Illustration of the possible interactions between triadin, junctin, CSQ1 and RyR1 in the lumen of the SR