The Biomineralization Proteome: Protein Complexity for a Complex Bioceramic Assembly Process

There are over 62 different biominerals on Earth and a diverse array of organisms that generate these biominerals for survival. This review will introduce the process of biomineralization and the current understanding of the molecular mechanisms of mineral formation, and then comparatively explore the representative secretomes of two well-documented skeletal systems: vertebrate bone (calcium phosphate) and invertebrate mollusk shell (calcium carbonate). It is found that both skeletal secretomes have gross similarities and possess proteins that fall into four functional categories: matrix formers, nucleation assisters, communicators, and remodelers. In many cases the mineral-associated matrix former and nucleation assister sequences in both skeletal systems are unique and possess interactive conserved globular domains, intrinsic disorder, post-translational modifications, sequence redundancy, and amyloid-like aggregation-prone sequences. Together, these molecular features create a protein-based environment that facilitates mineral formation and organization and argue in favor of conserved features that evolve from the mollusk shell to bone. Interestingly, the mollusk shell secretome appears to be more complex compared to that of bone tissue, in that there are numerous protein subcategories that are required for the nucleation and organization of inner (nacre) and outer (prismatic) calcium carbonate regions of the shell. This may reflect the organizational and material requirements of an exoskeletal protective system.