Question: Does the histidine-rich Ca-binding protein (HRC) interact with triadin?

Context:
The HRC effects on RyR may be regulated by the Ca(2+)-sensitivity of its interaction with triadin.
In rabbit skeletal and cardiac muscles, HRC binds to sarcoplasmic reticulum (SR) membranes via triadin, a junctional SR protein. 
HRC may play a key role in the regulation of SR Ca cycling through its direct interactions with SERCA2 and triadin, mediating a fine cross talk between SR Ca uptake and release in the heart.
Histidine-rich calcium binding protein (HRC) is located in the lumen of sarcoplasmic reticulum (SR) that binds to both triadin (TRN) and SERCA affecting Ca(2+) cycling in the SR.
HRC is a SR luminal Ca(2+) binding protein known to associate with both triadin and the sarcoplasmic reticulum Ca(2+)-ATPase, and may thus mediate the crosstalk between SR Ca(2+) uptake and release.
The histidine-rich Ca(2+) binding protein (HRC) is a high capacity Ca(2+) binding protein in the sarcoplasmic reticulum (SR). Because HRC appears to interact directly with triadin, HRC may play a role in the regulation of Ca(2+) release during excitation-contraction coupling.
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.
A direct binding of HRC (histidine-rich Ca(2+)-binding protein) to triadin, the main transmembrane protein of the junctional sarcoplasmic reticulum (SR) of skeletal muscle, seems well supported.
The present study documents the binding interaction of skeletal muscle sarcoplasmic reticulum (SR) transmembrane protein triadin with peripheral histidine-rich, Ca(2+)-binding protein (HCP).
In addition, the intra-luminal histidine-rich calcium binding protein (HRC) has been shown to interact with both SERCA2a and triadin.
Notably, there is physical and direct interaction between these protein players, mediating a fine-cross talk between SR Ca-uptake, storage and release.
The histidine-rich calcium-binding protein (HRCBP) is expressed in the junctional SR, the site of calcium release from the SR. HRCBP is expressed exclusively in muscle tissues and binds calcium with low affinity and high capacity. In addition, HRCBP interacts with triadin, a protein associated with the ryanodine receptor and thought to be involved in calcium release. Its calcium binding properties, localization to the SR, and interaction with triadin suggest that HRCBP is involved in calcium handling by the SR.
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. 
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.
Further support for colocalization of HRC with triadin cytoplasmic domain is provided here by experiments of mild tryptic digestion of tightly sealed TC vesicles.
We demonstrate that HRC can be isolated as a complex with triadin, following equilibrium sucrose-density centrifugation in the presence of mM Ca(2+).
Here, we characterized the COOH-terminal portion of rabbit HRC, expressed and purified as a fusion protein (HRC(569-852)), with respect to Ca(2+)-binding properties, and to the interaction with triadin on blots, as a function of the concentration of Ca(2+).
Our results identify the polyglutamic stretch near the COOH terminus, as the Ca(2+)-binding site responsible, both for the acceleration in mobility of HRC on SDS-PAGE in the presence of millimolar concentrations of Ca(2+), and for the enhancement by high Ca(2+) of the interaction between HRC and triadin cytoplasmic segment.
In addition to providing further evidence that HCP coenriches with RyR1, FKBP-12, triadin and calsequestrin (CS) in sucrose-density-purified TC vesicles, using specific polyclonal antibody, we show it to be expressed as a single protein species, both in fast-twitch and slow-twitch fibers, and to identically localize to the I-band.
Colocalization of HCP and triadin at junctional triads is supported by the overlapping staining pattern using monoclonal antibodies to triadin. We show a specific binding interaction between digoxigenin-HCP and triadin, using ligand blot techniques.
Suggesting that triadin dually interacts with HCP and with CS, at distinct sites, we have found that triadin-CS interaction in overlays does not require the presence of Ca2+.
These differential effects form the basis for the hypothesis that HCP anchors to the junctional membrane domain of the SR, through binding to triadin short cytoplasmic domain at the NH2 terminus.
Although the function of this interaction, as such, is not well understood, it seems of potential biological interest within the more general context of the structural-functional role of triadin at the triadic junction in skeletal muscle.
BACKGROUND: Histidine-rich calcium binding protein (HRC) is located in the lumen of sarcoplasmic reticulum (SR) that binds to both triadin (TRN) and SERCA affecting Ca(2+) cycling in the SR. Chronic overexpression of HRC that may disrupt intracellular Ca(2+) homeostasis is implicated in pathogenesis of cardiac hypertrophy.
Interaction of HRC (histidine-rich Ca(2+)-binding protein) and triadin in the lumen of sarcoplasmic reticulum.
The histidine-rich Ca-binding protein (HRC) is an SR component that binds to triadin and may affect Ca release through the ryanodine receptor.
The histidine-rich Ca-binding protein (HRC) is an SR component that binds to triadin and may affect Ca release through the ryanodine receptor
Because HRC appears to interact directly with triadin, HRC may play a role in the regulation of Ca(2+) release during excitation-contraction coupling
A direct binding of HRC (histidine-rich Ca(2+)-binding protein) to triadin, the main transmembrane protein of the junctional sarcoplasmic reticulum (SR) of skeletal muscle, seems well supported
The histidine-rich Ca-binding protein (HRC) is an SR component that binds to triadin and may affect Ca release through the ryanodine receptor

Answer: Yes. HRC may play a key role in the regulation of SR Ca cycling through its direct interactions with SERCA2 and triadin, mediating a fine cross talk between SR Ca uptake and release in the heart. A direct binding of HRC (histidine-rich Ca(2+)-binding protein) to triadin, the main transmembrane protein of the junctional sarcoplasmic reticulum (SR) of skeletal muscle, seems well supported.
Histidine-rich calcium binding protein (HRC) is located in the lumen of sarcoplasmic reticulum (SR) and binds to triadin (TRN), a protein associated with the ryanodine receptor and thought to be involved in calcium release.
Exact answer: yes