ASGSB 1998 Annual Meeting Abstracts

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BIOMINERALIZATION IN COCCOLITHOPHORIDS. Mary E. Marsh. University of Texas Dental Branch, Houston, Texas, USA

Coccolithophores are phytoflagellates characterized by an extracellular covering of calcareous scales (coccoliths) formed intracellularly in Golgi-derived vesicles. In Pleurochrysis carterae the coccoliths consist of an oval organic base plate with a distal rim of interlocking calcite crystals, and a narrow flexible ribbon of organic material which tethers the mineral ring to the base plate. Each crystallite is surrounded by a thin organic coat consisting primarily of two acidic polysaccharides PS1 and PS2, which have a central role in the biomineralization process. The most abundant polyanion PS2 is extremely acidic (having four carboxyl groups per disaccharide repeat) and capable of sequestering massive quantities of calcium ions. PS1 and PS2 are synthesized in medial Golgi cisternae where they coaggregate with calcium ions to form 20-nm particles. The particles are secreted into the coccolith-forming vesicle where they associate with the rim of the unmineralized base before the onset of calcium carbonate deposition. After mineralization ceases, the PS1/PS2-containing particles are reorganized into the amorphous organic coat which surrounds the mature crystallites. Pulse-chase studies on wild-type cells and analysis of P. carterae mutants which do not express PS2 suggest that all calcium destined for coccolith formation is first complexed with PS1/PS2 for transport to mineralizing foci. Further analysis of mutants may lead to identification of elements controlling crystal nucleation and morphology.

Supported by the United States Office of Naval Research.

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