Characterization and purification of lipovitellin-like egg yolk protein in a stony coral, Acropora aff. tenuis

Three egg proteins (EP1, EP2, and EP3) were detected in ovulated eggs of Acropora aff. tenuis, a reef-building stony coral found in tropical and subtropical areas. The proteins were separated into different fractions by gel filtration chromatography, and different patterns were observed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and Western blotting with antiserum against A. aff. tenuis egg extract (anti-AtE). EP2 was purified from ovulated egg extracts using a combination of hydroxylapatite and gel filtration chromatography. The molecular mass of native EP2 was estimated to be 286 kDa. SDS-PAGE revealed bands at 70.5, 67.7, 49.6, and 22.9 kDa, with the 70.5 and 67.7 kDa bands staining positive for lipid (Sudan black B). These results indicate that purified EP2 is a lipoprotein in A. aff. tenuis eggs, resembling lipovitellin, the major yolk protein derived from the egg yolk precursor, vitellogenin (Vtg), in oviparous vertebrates. Furthermore, EP2 and its bands were digested with trypsin and analyzed by liquid chromatography using a quadrupole time-of-flight tandem mass spectrometer (LC-QTOF-MS). All fragments were identified as A. aff. tenuis Vtg. This is the first report of the purification and characterization of a lipovitellin-like egg yolk protein in non-bilaterian oviparous animals.