Catecholaminergic polymorphic ventricular tachycardia-related mutations R33Q and L167H alter calcium sensitivity of human cardiac calsequestrin

Two missense mutations, R33Q and L167H, of hCASQ2 (human cardiac calsequestrin), a protein segregated to the lumen of the sarcoplasmic reticulum, are linked to the autosomal recessive form of CPVT (catecholaminergic polymorphic ventricular tachycardia). The effects of these mutations on the conformational, stability and Ca2+ sensitivity properties of hCASQ2, were investigated. Recombinant WT (wild-type) and mutant CASQ2s were purified to homogeneity and characterized by spectroscopic (CD and fluorescence) and biochemical (size-exclusion chromatography and limited proteolysis) methods at 500 and 100 mM KCl, with or without Ca2+ at a physiological intraluminal concentration of 1 mM; Ca2+-induced polymerization properties were studied by turbidimetry. In the absence of Ca2+, mutations did not alter the conformation of monomeric CASQ2. For L167H only, at 100 mM KCl, emission fluorescence changes suggested tertiary structure alterations. Limited proteolysis showed that amino acid substitutions enhanced the conformational flexibility of CASQ2 mutants, which became more susceptible to tryptic cleavage, in the order L167H>R33Q>WT. Ca2+ at a concentration of 1 mM amplified such differences: Ca2+ stabilized WT CASQ2 against urea denaturation and tryptic cleavage, whereas this effect was reduced in R33Q and absent in L167H. Increasing [Ca2+] induced polymerization and precipitation of R33Q, but not that of L167H, which was insensitive to Ca2+. Based on CASQ2 models, we propose that the Arg33→Gln exchange made the Ca2+-dependent formation of front-to-front dimers more difficult, whereas the Leu167→His replacement almost completely inhibited back-to-back dimer interactions. Initial molecular events of CPVT pathogenesis begin to unveil and appear to be different depending upon the specific CASQ2 mutation.