Interaction of HRC (Histidine-rich Ca2+-Binding Protein) and Triadin in the Lumen of Sarcoplasmic Reticulum

Abstract

HRC (histidine-rich Ca2؉ binding protein) has been identified from skeletal and cardiac muscle and shown to bind Ca 2؉ with high capacity and low affinity. While HRC resides in the lumen of the sarcoplasmic reticulum, the physiological function of HRC is largely unknown. In the present study, we have performed co-immunoprecipitation experiments and show that HRC binds directly to triadin, which is an integral membrane protein of the sarcoplasmic reticulum. Using a fusion protein binding assay, we further identified the histidine-rich acidic repeats of HRC as responsible for the binding of HRC to triadin. These motifs may represent a novel protein-protein interaction domain. The HRC binding domain of triadin was also localized by fusion protein binding assay to the lumenal region containing the KEKE motif that was previously shown to be involved in the binding of triadin to calsequestrin. Notably, the interaction of HRC and triadin is Ca 2؉ -sensitive. Our data suggest that HRC may play a role in the regulation of Ca 2؉ release from the sarcoplasmic reticulum by interaction with triadin.

Regulation of myocardial function by histidine-rich, calcium-binding protein

“…Recently, HRC has been suggested to be an additional component of the SR quaternary structure involved in Ca release (18) and may play a similar role to CSQ in SR Ca storage (14,18). Furthermore, an interaction between HRC and TRI has been reported (18,27,28), although the apparent cross-talk between the players in the SR Ca release complex has not been elucidated.…”

“…Furthermore, an interaction between HRC and TRI has been reported (18,27,28), although the apparent cross-talk between the players in the SR Ca release complex has not been elucidated. Interestingly, overexpression of JNT was associated with downregulation of TRI protein levels (12,17) and vice verca (16), suggesting an important compensatory cross-talk, because the overall structures and interacting characteristics of JNT and TRI are similar (12,32).…”

“…There is a single isoform of HRC, identified in rabbit skeletal, cardiac, and arteriolar smooth muscles (10,24,26,28). The deduced amino acid sequence of HRC reveals structural features that are analogous to CSQ, suggesting that HRC may play a similar role to CSQ during excitation-contraction coupling, as a Ca storage protein (18). In addition, biochemical studies in skeletal muscle have shown that HRC associates with TRI in a Ca-dependent manner, implicating HRC to be involved in regulation of Ca release from the SR by an interaction with TRI (18,27,28).…”

“…This protein appears to be an additional component of the SR Ca release structure, composed of the RyR, calsequestrin (CSQ), triadin (TRI), and junctin (JNT) (18,33). HRC binds Ca (3,11,25) and has been proposed to be either in the lumen of the SR (10,15,18,30,31) or on the cytoplasmic surface of the SR (5,6,27,28). There is a single isoform of HRC, identified in rabbit skeletal, cardiac, and arteriolar smooth muscles (10,24,26,28).…”

HRC Is a Direct Transcriptional Target of MEF2 during Cardiac, Skeletal, and Arterial Smooth Muscle Development In Vivo

“…HRCBP binds calcium in vitro with low affinity and high capacity (20,52) and is present in the lumen of the junctional SR, the site of calcium release by the ryanodine receptor (15,20,29,55). The function of HRCBP is not known, but its expression pattern, subcellular localization to the lumen of the SR, and association with components of the calcium release channel complex suggest a possible role in calcium release during excitation-contraction coupling (15,20,21,25,29). The expression of HRC in cardiac, skeletal, and smooth muscle suggests the possibility that HRC is the target of a common transcriptional program in the three muscle lineages.…”