The histidine-rich calcium-binding protein of sarcoplasmic reticulum (HRC) may play a role in the regulation of calcium sequestration or release in the SR of skeletal and cardiac muscle []. This protein is very acidic (31% of Asp and Glu) and rich in histidine (13%). The sequenceof HRC contains 10 tandem repeats of a 26 to 29 amino acid residues domain. This domain starts with an invariant hexapeptide (HRHRGH), followed by a stretch of acidic residues. The end of the domain consists of an almost invariant nonapeptide (STESDRHQA). The highly acidic central cores of each repeat are likely to constitute the calcium-binding sites of HRC.
This entry represents a histidine-rich calcium-binding repeat which appears in proteins called histidine-rich-calcium binding proteins (HRC). HRC is a high capacity, low affinity Ca2+-binding protein, residing in the lumen of the sarcoplasmic reticulum. HRC binds directly to triadin. This binding interaction occurs between the histidine-rich region of HRC and multiple clusters of charged amino acids, named KEKE motifs, in the lumenal domain of triadin. This repeat is found in the acidic region of the protein, which can be long and variable. There is also a cysteine-rich region further towards the C terminus []. HRC may regulate sarcoplasmic reticular calcium transport, play a critical role in maintaining calcium homeostasis, and function in the heart. HRC is a candidate regulator of sarcoplasmic reticular calcium uptake.
The species Pseudomonas syringae encompasses plant pathogens with differing host specificities and corresponding pathovar designations. P. syringae requires the Hrp (type III protein secretion) system, encoded by a 25-kb cluster of hrp and hrc genes, in order to elicit the hypersensitive response (HR) in nonhosts or to be pathogenic in hosts. The exact function of HrpF is unknown but the protein is needed for pathogenicity [].
