Hydrophilic Shell Matrix Proteins of Nautilus pompilius and The Identification of a Core Set of Conchiferan Domains

Despite being a member of the shelled mollusks (Conchiferans), most members of extant cephalopods have lost their external biomineralized shells, except for the Nautiloids. Here, we report the result of our study to identify major Shell Matrix Proteins and their domains in the Nautiloid Nautilus pompilius, in order to gain a general insight into the evolution of Conchiferan Shell Matrix Proteins. In order to do so, we conducted transcriptomics of the mantle, and proteomics of the shell of N. pompilius simultaneously. Analyses of obtained data identified 61 distinct shell-specific sequences. Of the successfully annotated 27 sequences, protein domains were predicted in 19. Comparative analysis of Nautilus sequences with four Conchiferans for which Shell Matrix Protein data were available (the pacific oyster, the pearl oyster, the limpet, and the Euhadra snail) revealed that three proteins and six domains of the shell proteins are conserved in all Conchiferans. Interestingly, when the terrestrial Euhadra snail was excluded, another five proteins and six domains were found to be shared among the four marine Conchiferans. Phylogenetic analyses indicated that most of these proteins and domains were present in the ancestral Conchiferan, but employed in shell formation later and independently in most clades. Although further studies utilizing deeper sequencing techniques to obtain genome and full-length sequences, and functional analyses, must be done in the future, our results here provide important pieces of information for the elucidation of the evolution of Conchiferan shells at the molecular level.