Identification of calcium binding sites on calsequestrin 1 and their implications for polymerizationElectronic supplementary information (ESI) available. See DOI: 10.1039/c3mb25588c

• Main Authors: Kumar, Amit, Chakravarty, Harapriya, Bal, Naresh C, Balaraju, Tuniki, Jena, Nivedita, Misra, Gauri, Bal, Chandralata, Pieroni, Enrico, Periasamy, Muthu, Sharon, Ashoke
• Format: Article
• Language: English
• ISSN: 1742-206X; 1742-2051
• DOI: 10.1039/c3mb25588c

Biophysical studies have shown that each molecule of calsequestrin 1 (CASQ1) can bind about 7080 Ca 2+ ions. However, the nature of Ca 2+ -binding sites has not yet been fully characterized. In this study, we employed in silico approaches to identify the Ca 2+ binding sites and to understand the molecular basis of CASQ1Ca 2+ recognition. We built the protein model by extracting the atomic coordinates for the back-to-back dimeric unit from the recently solved hexameric CASQ1 structure (PDB id: 3UOM ) and adding the missing C-terminal residues (aa350364). Using this model we performed extensive 30 ns molecular dynamics simulations over a wide range of Ca 2+ concentrations ([Ca 2+ ]). Our results show that the Ca 2+ -binding sites on CASQ1 differ both in affinity and geometry. The high affinity Ca 2+ -binding sites share a similar geometry and interestingly, the majority of them were found to be induced by increased [Ca 2+ ]. We also found that the system shows maximal Ca 2+ -binding to the CAS (consecutive aspartate stretch at the C-terminus) before the rest of the CASQ1 surface becomes saturated. Simulated data show that the CASQ1 back-to-back stacking is progressively stabilized by the emergence of an increasing number of hydrophobic interactions with increasing [Ca 2+ ]. Further, this study shows that the CAS domain assumes a compact structure with an increase in Ca 2+ binding, which suggests that the CAS domain might function as a Ca 2+ -sensor that may be a novel structural motif to sense metal. We propose the term D n -motif for the CAS domain. Sites of Ca 2+ binding on the CASQ1 back-to-back dimer.