Localized Secretion of Acid Phosphatase

Secretion of cell wall-bound acid phosphatase by Saccharomyces cerevisiae occurs along a restricted portion of the cell surface. Acid phosphatase activity produced during derepressed synthesis on a phosphate-limited growth medium is detected with an enzymespecific stain and is localized initially to the bud portion of a dividing cell . After two to three generations of phosphate-limited growth, most of the cells can be stained; if further phosphatase synthesis is repressed by growth in excess phosphate, dividing cells are produced in which the parent but not the bud can be stained . Budding growth is interrupted in a-mating-type cells by a pheromone (a-factor) secreted by the opposite mating type ; cell surface growth continues in the presence of a-factor and produces a characteristic cell tip. When acid phosphatase synthesis is initiated during a-factor treatment, only the cell tip can be stained; when phosphatase synthesis is repressed during afactor treatment, the cell body but not the tip can be stained . A mixture of derepressed a cells and phosphatase-negative a cells form zygotes in which mainly one parent cell surface can be stained . The cell cycle mutant, cdc 24 (Hartwell, L. H . 1971 . Exp. Cel l Res. 69:265-276), fails to bud and, instead, expands symmetrically as a sphere at a nonpermissive temperature (37 °C) . This mutant does not form a cell tip during a-factor treatment at 37 °C, and although acid phosphatase secretion occurs at this temperature, it is not localized. These results suggest that secretion reflects a polar mode of yeast cell-surface growth, and that this organization requires the cdc 24 gene product . Yeast cell-surface growth is restricted to the bud portion of a dividing cell. A bud develops on the parent cell and grows at a constant rate until cell division . Asymmetric surface growth is seen in the incorporation of glucan (19, 20) and mannan (14) into the cell wall . The assembly of surface determinants recognized by concanavalin A (31) or antibody (32) is also known to be bud-localized . However, the site of protein and lipid insertion into the plasmalemma is not known . Two mechanisms for yeast cell-surface growth have been proposed (13). One is that polymers such as glucan, mannan, and chitin are assembled at the plasmalemma and directly inserted into the wall . This theory is supported by the fact that partially purified plasmalemma fractions are enriched in mannosyltransferase (8) and chitin synthetase activities (11). An alternative proposal is that mannan and other secretory molecules are assembled on intracellular membranes and transported to the cell surface by membrane-bounded vesicles, TLa JOURNAL OF CELL BIOLOGY VOLUME 86 JULY 1980 123-128 © The Rockefeller University Press " 0021-9525/80/07/0123/06 $1 .00 where secretion is achieved by exocytosis . The latter model is supported by autoradiographic studies, which demonstrate an internal site for the initial incorporation of ['H)mannose, followed by enrichment of label along the bud surface (14) . Mannan synthesis is coupled to protein synthesis (l2), and the secreted enzymes acid phosphatase and invertase are representative cell wall mannoproteins (1, 21) . A number of intracellular organelles including the vacuole, nuclear membrane, endoplasmic reticulum, a Golgi-like structure, and a group of small bud-localized vesicles, are labeled when Gomori stain is applied to acid phosphatase-secreting cells (34) . Freeze-fracture and thin-section electron microscopic evidence (24) suggests that the bud-localized vesicles fuse with the plasmalemma . These findings support the exocytosis theory of cell-surface growth and suggest that assembly of the plasmalemma as well as of the cell wall is accomplished by this process . 1n this report we describe a technique for the localization of 123 on F ebuary 0, 2013 jcb.rress.org D ow nladed fom Published July 1, 1980