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

HRC (histidine-rich Ca2+binding protein) has been identified from skeletal and cardiac muscle and shown to bind Ca2+ 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 Ca2+-sensitive. Our data suggest that HRC may play a role in the regulation of Ca2+release from the sarcoplasmic reticulum by interaction with triadin.