A cDNA encoding the Ca2+-regulated photoprotein of the bioluminescent marine hydroid Obelia geniculata was cloned and sequenced. The cDNA is a 774 bp fragment containing two overlapping open reading frames, one of which contained 585 bp encoding a 195 amino acid polypeptide which obviously has the primary structure of the apoprotein of a calcium-regulated photoprotein. Many of the residues are identical to those in other Ca2+-regulated photoproteins: 86% compared with that from Obelia longissima, 76% with that from Clytia (Phialidium), 64% with that from Aequorea, and 64% with that from Mitrocoma(Halistaura). The obelin from O. geniculata was overexpressed in Escherichia coli, refolded from inclusion bodies, and purified. The yield of highly purified recombinant protein was 55-80 mg/L of LB medium. O. geniculata obelin has absorption maxima at 280 and 460 nm and a shoulder at approximately 310 nm. The calcium-discharged protein loses visible absorption but exhibits a new absorption maximum at 343 nm. The bioluminescence of the obelin from O. geniculata is blue (lambda(max) = 495 nm). In contrast, the fluorescence of the calcium-discharged protein is yellow-green (lambda(max) = 520 nm; excitation at 340 nm). This is in sharp contrast to aequorin in which the bioluminescence and fluorescence emission spectra of the calcium-discharged protein are almost identical (lambda(max) = 465 nm). The Ca2+ concentration-effect curve for O. geniculata obelin is similar to those of many other photoproteins: at [Ca2+] below approximately 10(-8) M, calcium-independent luminescence is observed, and at [Ca2+] approximately 10(-3) M, the luminescence reaches a maximum. Between these extremes, the curve spans a vertical range of almost 8 log units with a maximum slope on a log-log plot of about 2.5. In the absence of Mg2+ the rate constant for the rise of bioluminescence determined by the stopped-flow technique is about 450 s(-1). The effects of Mg2+ on the kinetics of bioluminescence are complicated, but at all concentrations studied they are relatively small compared to the corresponding effects on aequorin luminescence. At least with respect to speed and sensitivity to Mg2+, the obelins from both O. longissima and O. geniculata would appear to be more suitable than aequorin for use as intracellular Ca2+ indicators.

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