LOCALIZATION OF CA2-STIMULATED ATPASE IN THE COCCOLITH-PRODUCING COMPARTMENT OF CELLS OF PLEUROCHRYSIS SP (PRYMNESIOPHYCEAE)()

Cell homogenates of Pleurochrysis sp. (CCMP299) were fractionated by m eans of sucrose gradients. Ca2+-stimulated ATPase (EC 3.6.1.3., ATP ph osphohydrolase) was associated primarily with the plasma membrane, Gol gi, and high density (1.21 g.m(-3)) membranous structures. Ca2+-stimul ated ATPase was highly enriched in the latter. Based on treatments wit h Triton X-100 and NBD ceramide, we conclude that the high-density str uctures were membrane-delimited organelles. These vesicle-like organel les contained complex polysaccharides, a high concentration of calcium , and, upon microscopic examination, structures resembling coccoliths. These findings ave consistent with observations on the known composit ion of coccoliths and the presumed mineralizing function of the subcel lular coccolith-producing compartment. The high-density vesicles were linked to the Golgi by means of colchicine-sensitive materials, presum ably microtubules. These data and prior ultrastructural observations b y other investigators indicating vectorial assembly and secretion sugg est that the subcellular movement of the newly formed coccoliths may b e directed and/or powered by colchicine-sensitive cytoskeletal element s. We interpret the data to mean that the high-density vesicles repres ent the coccolith-producing compartment previously observed by others in electron micrographs.