Characterization of hydromedusan Ca2+-regulated photoproteins as a tool for measurement of Ca2+concentration

Calcium ion is a ubiquitous intracellular messenger, performing this function in many eukaryotic cells. To understand calcium regulation mechanisms and how disturbances of these mechanisms are associated with disease states, it is necessary to measure calcium inside cells. Ca²⁺-regulated photoproteins have been successfully used for this purpose for many years. Here we report the results of comparative studies on the properties of recombinant aequorin from Aequorea victoria, recombinant obelins from Obelia geniculata and Obelia longissima, recombinant mitrocomin from Mitrocoma cellularia, and recombinant clytin from Clytia gregaria as intracellular calcium indicators in a set of identical in vitro and in vivo experiments. Although photoproteins reveal a high degree of identity of amino acid sequences and spatial structures, and, apparently, have a common mechanism for the bioluminescence reaction, they were found to differ in the Ca²⁺ concentration detection limit, the sensitivity of bioluminescence to Mg²⁺, and the rates of the rise of the luminescence signal with a sudden change of Ca²⁺ concentration. In addition, the bioluminescence activities of Chinese hamster ovary cells expressing wild-type photoproteins also differed. The light signals of cells expressing mitrocomin, for example, slightly exceeded the background, suggesting that mitrocomin may be hardly used to detect intracellular Ca²⁺ without modifications improving its properties. On the basis of experiments on the activation of endogenous P2Y₂ receptor in Chinese hamster ovary cells by ATP, we suggest that wild-type aequorin and obelin from O. longissima are more suitable for calcium detection in cytoplasm, whereas clytin and obelin from O. geniculata can be used for calcium measurement in cell compartments with high Ca²⁺ concentration. Figure

Hydromedusan photoproteins differ in Ca²⁺ concentration detection limit, sensitivity of bioluminescence to Mg²⁺, and rates of rise of luminescence signal with a sudden change of [Ca²⁺] despite a high degree of identity of their amino acid sequences and spatial structures, and, apparently, a common mechanism for the bioluminescence reaction.