Characterization of Two Human Skeletal Calsequestrin Mutants Implicated in Malignant Hyperthermia and Vacuolar Aggregates Myopathy [Protein Structure and Folding]

September 28th, 2015 by Lewis, K. M., Ronish, L. A., Rios, E., Kang, C.

Calsequestrin 1 (hCasq1) is the principal Ca2+ storage protein of the sarcoplasmic reticulum of skeletal muscle. Its inheritable D244G mutation causes a myopathy with vacuolar aggregates, while its M87T variant is weakly associated with malignant hyperthermia (MH). We characterized the consequences of these mutations with studies of the human proteins in vitro. Equilibrium dialysis and turbidity measurements showed that D244G and, to a lesser extent, M87T partially lose Ca2+ binding exhibited by wild type hCasq1 at high Ca2+ concentrations. D244G aggregates abruptly and abnormally, a property that fully explains the protein inclusions that characterize its phenotype. D244G crystallized in low Ca2+ concentrations lacks two Ca2+ ions normally present in wild type, which weakens the hydrophobic core of Domain II. D244G crystallized in high Ca2+ concentrations regains its missing ions and Domain II order, but shows a novel dimeric interaction. The M87T mutation causes a major shift of the α-helix bearing the mutated residue, significantly weakening the back-to-back interface essential for tetramerization. D244G exhibited the more severe structural and biophysical property changes, which matches the different pathophysiological impacts of these mutations.
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