Structure of the chromoprotein ovorubin from the golden apple snail ( <i>Pomacea canaliculata</i> )

The golden apple snail (Pomacea canaliculata), an invasive gastropod, produces distinctly bright pink egg masses. The astaxanthin-binding ovorubin (P. canaliculata ovorubin [PcOvo]) is a 300-kDa glycoprotein responsible for the egg coloration. Here, we determine the three-dimensional structure of PcOvo using cryo-electron microscopy (cryo-EM). PcOvo is a heterodecameric protein consisting of two copies of each PcOvo1-5 subunit. The subunits have similar repeated ferredoxin-like structures. N-linked glycosylation sites are identified. Solution x-ray scattering data support the overall architecture of the complex. PcOvo3 and PcOvo5 have hydrophobic pockets that likely bind various hydrophobic compounds. The cryo-EM map and binding experiments suggest that PcOvo5 binds astaxanthin, which is responsible for the pink color of the protein. Our structure provides molecular insights into the nature of the gastropod egg coloration and lays a foundation for further investigation of ovorubin biology and evolution.