CRYSTAL PROTEIN INTERACTIONS STUDIED BY OVERGROWTH OF CALCITE ON BIOGENIC SKELETAL ELEMENTS

A key parameter in the biological control of crystal formation is the interaction of a group of acidic macromolecules with the mineral phase. Here we study protein-calcite interactions using epitaxial overgrowth of synthetic calcite crystals under conditions in which local release of occluded macromolecules from the biogenic substrate occurs. The macromolecules subsequently interact with the newly formed overgrown crystals, resulting in modified calcite morphology. This novel method provides a means of mapping crystal-protein interactions under conditions that minimally affect the conformational states of the acidic macromolecules. We show that proteins released from calcitic sponge spicules and mollusc prisms specifically interact with {001} and {01l} faces of calcite, whereas proteins released from echinoderm skeletal elements only interact with {01l} faces. The extent to which the overgrown crystals are affected by the proteins varies even in the same organism and within the same element, depending on the site and crystallographic orientation of the skeletal elements.