Intracrystalline Proteins from a Sea Urchin and a Mollusk: A Comparison

Abstract

Control over mineral formation by organisms is achieved in a variety of ways. The unique feature in almost all of these processes is the intimate contact between the mineral and the assembly of organic macromolecules secreted by specialized cells at the site of crystallization [1,2]. This results in the formation of composite phases of mineral and organic material displaying physical and mechanical properties characteristic of the mixed phase, that are also presumably optimally adapted to fulfill specific mechanical functions. The crystals of the composite often display a unique orientation, or set of orientations of at least one of their crystallographic axes, as well as a unique morphology. We consider these properties as indications of involvement at the molecular level of the organic components in the processes of nucleation and growth of the crystals [3]. Less well understood, although recognized for a long time [4], is the ability of organisms to introduce organic molecules and macromolecules into the crystal itself, thus possibly altering the nature of the mineral phase. Here we compare the manner in which such processes occur in an echinoderm and a mollusk, and derive information on the mechanisms of intercalation and on the possible function of crystal-occluded proteins.