Sequential Logic of Polarity Determination during the Haploid-to-Diploid Transition in Saccharomyces cerevisiae

Abstract

cIn many organisms, the geometry of encounter of haploid germ cells is arbitrary. In Saccharomyces cerevisiae, the resulting zygotes have been seen to bud asymmetrically in several directions as they produce diploid progeny. What mechanisms account for the choice of direction, and do the mechanisms directing polarity change over time? Distinct subgroups of cortical "landmark" proteins guide budding by haploid versus diploid cells, both of which require the Bud1/Rsr1 GTPase to link landmarks to actin. We observed that as mating pairs of haploid cells form zygotes, bud site specification progresses through three phases. The first phase follows disassembly and limited scattering of proteins that concentrated at the zone of cell contact, followed by their reassembly to produce a large medial bud. Bud1 is not required for medial placement of the initial bud. The second phase produces a contiguous bud(s) and depends on axial landmarks. As the titer of the Axl1 landmark diminishes, the third phase ultimately redirects budding toward terminal sites and is promoted by bipolar landmarks. Thus, following the initial random encounter that specifies medial budding, sequential spatial choices are orchestrated by the titer of a single cortical determinant that determines whether successive buds will be contiguous to their predecessors.

Cell biology of yeast zygotes, from genesis to budding

“…Specificity could also be ensured by a) delaying release of lytic enzymes until the ZOC compartment is sufficiently mature to restrict lateral diffusion of such enzymes, or b) releasing enzymes whose specificity corresponds to that of the intervening wall. Consistent with this latter possibility, the wall at the tip of mating projections and at the ZOC are newly-synthesized, and at least the wall at the tip of the projection has a distinct appearance and lectin-binding properties [11, 85–90]. There has, however, been no report of wall isolation and compositional analysis after exposure to mating pheromone.…”

“…The apical and basal domains are separated from each other by a circumferential belt of septins. Curiously, some apical proteins are tightly concentrated at the tip, while others appear composite or define a broader cap [11, 55–59] – Figure 4. Features of this polarization remain conspicuous in mature zygotes.…”

“…Judging from surveys and studies of individual proteins, the apical group also includes adhesion proteins, proteins important for actin orientation (e.g. Far1 and Cdc24), wall metabolism, calcium metabolism, the “cell integrity path,” and proteins that contribute to cell-cell fusion, as is discussed below [11, 24, 55],…”

“…In addition, many ABC cassette nutrient transporters are found around the entire perimeter of prezygotes [11]. There have been no investigations of the determinants that cause proteins to assume a basal distribution or to be uniformly distributed around the cortex.…”

“…At least during the first several hours, parental vacuoles do not fuse together and mature peroxisomes, although they intermix, also do not fuse with each other [9, 10]. Moreover, many proteins of the parental plasma membrane domains do not intermix rapidly, reflecting the low diffusional mobility of many cortical proteins in yeast [11, 12]. …”