DISTURBANCE OF THE SECRETORY PATHWAY IN MICRASTERIAS-DENTICULATA BY TUNICAMYCIN AND CYCLOPIAZONIC ACID

Both tunicamycin, an inhibitor of N-linked glycosylation of proteins, and cyclopiazonic acid, which inhibits the Ca2+-dependent ATPase in th e ER, influence the secretory pathway al the ER level and lead to a ce ssation of cell growth in Micrasterias. Electron microscopical investi gations reveal that the mode of action of the two inhibitors differs. While tunicamycin treatment results in a disintegration of the Golgi b odies into small vesicles, cyclopiazonic acid prevents products being supplied from the ER, resulting in the dilatation of ER cisternae and a reduction in the number of Golgi cisternae, combined with a loss of dictyosomal activity. The disturbed cell wall formation under tunicamy cin indicates that N-linked glycosylation of proteins is required for normal cell growth in Micrasterias. Moreover, our studies reveal that changes in cytoplasmic free calcium concentration, as a consequence of ATPase inhibition in the ER by cyclopiazonic acid, may inhibit wall m aterial secretion by interrupting the normal ER-dictyosome association .