The Role of Purine Interactions in Biogenic Crystal Shape Determination

Widespread through phyla, purine crystals are intracellular inclusions serving a myriad of organismal functions. In zebrafish, iridophores concentrate purines in membrane-bound organelles, the iridosomes, for controlled crystallization. These crystals assemble into large, flat, and thin hexagons following unknown mechanisms that evolve against thermodynamically favorable interactions. Here, we investigate the initial development of zebrafish iridosomal crystals. By performing in vivo confocal reflection imaging, cryoFIB-SEM, and establishing novel 2D and 3D analysis pipelines, we show that these crystals grow four times faster along the b-crystallographic axis, leading to their characteristic hexagonal shape. By analyzing zebrafish with impaired guanine production, while conducting crystal growth simulations, we find that crystal shape is directed by bond type, number, and interaction strength between purines. Mechanistically, the macroscopic shape of zebrafish crystals is controlled by the relative concentration of purines present in the iridosome. This process impacts crystal growth along the b-axis, by disrupting the crystal's in-plane hydrogen bond structure, without alteration of the other axes. Our work uncovers a layer of biogenic crystal growth regulation occurring in vertebrate biocrystallization processes.