Bioluminescent and biochemical properties of Cys-free Ca2+-regulated photoproteins obelin and aequorin

Abstract

Bioluminescence of a variety of marine coelenterates is determined by Ca2+-regulated photoproteins. A strong interest in these proteins is for their wide analytical potential as intracellular calcium indicators and labels for in vitro binding assays. The presently known hydromedusan Ca2+-regulated photoproteins contain three (aequorin and clytin) or five (obelin and mitrocomin) cysteine residues with one of them strictly conserved. We have constructed Cys-free aequorin and obelin by substitution of all cysteines to serine residues. Such mutants should be of interest for researchers by the possibility to avoid the incubation with dithiothreitol (or β-mercaptoethanol) required for producing an active photoprotein that is important for some prospective analytical assays in which the photoprotein is genetically fused with a target protein sensitive to the reducing agents. Cys-free mutants were expressed in Escherichia coli, purified, and characterized regarding the efficiency of photoprotein complex formation, functional activity, and conformational stability. The replacement of cysteine residues has been demonstrated to affect different properties of aequorin and obelin. Cys-free aequorin displays a two-fold lower specific bioluminescence activity but preserves similar activation properties and light emission kinetics compared to the wild-type aequorin. In contrast, Cys-free obelin retains only ~10% of the bioluminescence activity of wild-type obelin as well as binding coelenterazine and forming active photoprotein much less effectively. In addition, the substitution of Cys residues drastically changes the bioluminescence kinetics of obelin completely eliminating a “fast” component from the light signal decay curve. At the same time, the replacement of Cys residues increases conformational flexibility of both aequorin and obelin molecules, but again, the effect is more prominent in the case of obelin. The values of thermal midpoints of unfolding (Tm) were determined to be 53.3±0.2 and 44.6±0.4°C for aequorin and Cys-free aequorin, and 49.1±0.1 and 28.8±0.3°C for obelin and Cys-free obelin, respectively. Thus, so far only Cys-free aequorin is suitable as a partner for fusing with a tag sensitive to reducing agents since the aequorin mutant preserves almost 50% of the bioluminescent activity and can be produced with a substantial yield.