PLoS ONE 9, e112326 (2014)

Credit: PLoS ONE

Biomineralization can be described as the controlled crystallization of metals by biological molecules into materials such as shells, bone and diatom cell walls. The crystal structures present in these materials, such as calcite and aragonite, can often be identified. However, defining the biomolecules present in or near the crystals, much less establishing their role in crystallization, is a more challenging task, as the comparatively delicate biomolecules are often damaged by any manipulation to or chemical treatment of the crystals. Biomolecules known to be involved in biomineralization in pearl oyster include the polysaccharide chitin and proteins rich in β-sheets and acidic residues. Naganuma et al. had previously identified a lectin, PPL1, in Pteria penguin pearl shells with a potential role in biodefense. However, the authors suspected that other lectins could be present, and given that glycans are present in the shell matrix, the authors further suspected that lectins could have a role in mineralization. To test these ideas, the authors returned to their previous affinity chromatography workflow, pulling out three additional lectins, PPL2A, PPL2B and PPL2C. These proteins are the first invertebrate homologs to the jacalin-related lectins (JRLs), though their substrate specificities diverge from both known JRL preferences and each other: PPL2A's activity in a hemagglutination assay was strongly inhibited by isomaltose and trehalose, whereas PPL2B's activity was affected by larger substrates such as sialoglycoproteins. Interestingly, both trehalose and PPL2A were found to be abundant in the P. penguin mantle—where mineralization occurs—and the nacreous (iridescent) layer. Finally, knockdown of PPL2A but not PPL1 caused defects in shell formation during P. penguin larva development, and both PPL2A and PPL2B altered the morphological populations of calcium carbonate crystals in vitro. A more detailed understanding of this interesting system awaits further study.