Proteomics of Shell Matrix Proteins from the Cuttlefish Bone Reveals Unique Evolution for Cephalopod Biomineralization

In contrast to the external shells in bivalves and gastropods,
most cephalopods are missing this external protection. The cuttlefish,
belonging to class cephalopod, has an internal biomineralized structure
made of mainly calcium carbonate for controlling buoyancy. However,
the macromolecules, especially proteins that control cuttlebone mineral
formation, are not sufficiently understood, limiting our understanding
of the evolution of this internal shell. In this study, we extracted
proteins from the cuttlebone of pharaoh cuttlefish Sepia pharaonis and performed liquid chromatography-tandem
mass spectrometry to identify the shell matrix proteins (SMPs). In
total, 41 SMPs were identified. Among them, hemocyanin, an oxygen-carrying
protein, was the most abundant SMP. By comparison with SMPs of other
marine biominerals, hemocyanin, apolipophorin, soul domain proteins,
transferrin, FL-rich, and enolase were found to be unique to the cuttlebone.
In contrast, typical SMPs of external shells such as carbonic anhydrase
complement control protein, fibronectin type III, and G/A-rich proteins
were lacking from the cuttlebone. Furthermore, the cluster analysis
of biomineral SMPs suggests that the SMP repertoire of the cuttlebone
does not resemble that of other species with external shells. Taken
together, this study implies a potential relationship of the cuttlefish
internal shell with other internal biominerals, which highlights a
unique shell evolutionary pathway in invertebrates.