Ring Formation in the Dimorphic Yeast Candida

Stationary phase cultures of Candida albicans inoculated into fresh medium at 37°C synchronously form buds at pH 4.5 and mycelia at pH 6.5. During bud formation, a filament ring forms just under the plasma membrane at the mother cell-bud junction at roughly the time of evagination. A filament ring also forms in myce}ium-forming cells, but it appears later than in a budding cell and it is positioned along the elongating mycelium, on the average 2 ~tm from the mother cell-mycelium junction. Sections of filament rings in early and late budding cells and in mycelia appear similar. Each contains approximately 11 to 12 filaments equidistant from one another and closely associated with the plasma membrane. In both budding and mycelium-forming cells, the filament ring disappears when the primary septum grows inward. The close temporal and spatial association of the filament ring and the subsequent chitin-containing septum suggests a role for the filament ring in septum formation. In addition, a close temporal correlation is demonstrated between filament ring f6rmation and the time at which cells become committed to bud formation at pH 4.5 and mycelium formation at pH 6.5. The temporal and spatial differences in filament ring formation between the two growth forms also suggest a simple model for the positioning of the filament ring. In the budding yeast Saccharomyces cerevisiae, septation invariably occurs at the junction between mother cell and bud (1, 2). In this process, a chitin-containing ring grows inward, pinching off the plasma membrane between mother cell and bud to form the primary septum (3). This is followed by the deposition of secondary septal layers on both sides of the primary septum (3), and subsequent cell separation (4). Although the components involved in the regulation of chitin synthetase activity have been investigated in some detail (3, 5), it is still unclear why the chitin ring and subsequent septum form exactly at the mother cell-bud junction even though the enzyme chitin synthetase appears to be distributed throughout the plasma membrane (6). A clue to septum localization may lie in the ordered ring of 10-nm filaments which appears just under the plasma membrane at the junction of mother cell and evagination and which disappears sometime during the formation of the primary septum (7). Although the chemical nature of this ring is unknown, its location at the junction suggests that it may be related to septum formation, and the absence of organized filaments in particular cell division cycle mutants correlates with defective cytokinesis (8). However, several possible functions of the filament ring have not been completely ruled out, including positioning of the evagination, maintenance of an open channel at the mother cell-bud junction, involvement in nuclear migration and division, and constriction at the junction. The yeast Candida albicans affords us with a very useful system for investigating the information and function of the filament ring. This dimorphic yeast is capable of growing either in a budding form, in a fashion similar to Saccharomyces (9), or in an elongate, mycelial form (10), depending upon environmental conditions and the growth history of the cells (11, 12, 13). In both growth forms, a chitin-containing septum is formed between the mother cell and daughter compartment (14, 15, 16), but the timing and position of septum formation differ between the two growth forms (14). During synchronous bud formation, the septum begins to form at approximately the same time as initial evagination and is invariably positioned at the junction of mother cell and bud (14), just as in Saccharomyces (1, 2, 3). In contrast, during synchronous mycelium formation, the septum begins to form roughly 20 to 30 min after evagination and is positioned along the mycelium, on the average, 2 ttm from the mother cell-mycelial junction (14). During the establishment of either growth form, the onset of septum formation correlates closely with the time at which a shift to the alternate pH does not affect the phenotype prescribed by the original pH, the point of phenotypic committment (17). These temporal events are diagrammed in Fig. I. THE JOURNAL OF CELL BIOLOGY VOLUME 96 FEBRUARY 1983 486-493 486 © The Rockefel ler Univers i ty Press 0021-9525/83/02/0486/08 $1.00 on A uust 8, 2017 jcb.rress.org D ow nladed fom