Several studies of the growth of the budding yeast Saccharomyces cerevisiae
indicate that growth is constant throughout the cell cycle. Protein synthesis does not appear to vary during the cell cycle [18
]. Measurement of a protein synthesis reporter in single cells or measurement of growth by estimating cell volumes by microscopy of synchronized cultures supported this conclusion [19
]. Other studies reported growth rate changes during the cell cycle [21
]. By examining dry cell mass by interference microscopy, Murdock Mitchison reported a growth rate of 0.17pg/min during G1, followed by a double growth rate of 0.32pg/min after budding (, solid line). When measuring growth by volume using light microscopy for the same cells, Mitchison observed a slightly different pattern: cells grew at a given rate until before budding, then growth slowed down, then it increased at a faster rate (, dashed line). A recent examination of the growth rates of S. cerevisiae
cell division cycle (cdc) mutants that arrest at different points of the cell cycle cells using volume displacement (coulter counter) support this idea of varying growth rates during the cell cycle. Cells arrested at different cell cycle stages grow at different rates and the maximal cell size reached in the arrest varies greatly between cell cycle stages (). Cells arrested in G1 by the inactivation of the single CDK in budding yeast, Cdc28, grow the most [23**
]. High growth rates were also observed in anaphase-arrested cells. Cells that arrest at the time of cell cycle entry or in metaphase exhibited lower growth rates. Microscopic measurements of wild type single cells also suggested a slowing of growth at the time of budding, indicating that at least some of the differences in growth rate observed in cell cycle arrests are not artifacts of the cell cycle arrests [23**
Growth during the cell cycle in S. cerevisiae
One mechanism whereby cell cycle events affect growth is through changes in the actin cytoskeleton. In order to grow in size, cells must fuse new lipid vesicles with the cell membrane. The vesicles are transported to sites of fusion on actin cables by Myosin V [24
]. The actin cytoskeleton undergoes dramatic changes during the cell cycle (). In G1 phase, actin cables are evenly distributed throughout the cytoplasm, resulting in uniform vesicle deposition, isotropic growth and spherical cell morphology (). As cells enter the cell cycle, the actin cytoskeleton becomes polarized through the action of the Cln cyclin G1 CDKs () and vesicle deposition occurs apically at the site of bud emergence (). After initial bud emergence, growth remains limited to the developing daughter cell, but becomes isotropic to create a spherical bud () [24
Studies of cdc mutants and of cells treated with the mating pheromone α-factor, which polarizes the actin cytoskeleton and causes mating projection formation, showed that polarization of the actin cytoskeleton decreases growth. Mutants that arrest with high G1-CDK activity (cdc34
mutants) or form mating projection showed decreased growth and protein synthesis rates compared to other cell cycle arrests. Preventing actin polarization after pheromone treatment suppressed the growth inhibitory effects of pheromone [23**
]. Inactivation of actin organizers or inactivation of cyclin-CDK activity also improved the growth of cells that arrest with a hyper-polarized actin cytoskeleton such as cdc34
The actin cytoskeleton not only affects growth but also appears to influence cellular density. Murdock Mitchison predicted that cell density peaks around the time of budding [21
]. Several studies provided experimental proof for this prediction [21
]. How do changes in the actin cytoskeleton affect cell growth and density and are these two events connected? A simple hypothesis is that at the time of budding, the cell surface increases at a lower rate but initially protein synthesis continues at the same rate. This results in a temporary (<30min) uncoupling of cell surface growth and protein synthesis and hence increased cell density. The basis for this observation could be the properties of apical growth: there is limited space at the bud tip and only few vesicles are incorporated into the membrane to contribute to cell surface growth (). Initially protein synthesis continues unabated causing a transient increase in cell density. Sometimes thereafter feedback mechanisms are activated that down-regulate protein synthesis in response to actin hyperpolarization.
How actin polarization leads to the down-regulation of protein synthesis is not understood, but the analysis of mutants defective in secretion could provide a framework for how to think about this signaling mechanism. In cells defective for secretion, unincorporated vesicles accumulate [30
]. Secretion mutants also show decreased protein synthesis rates [31
]. The cell wall integrity (Pkc1) pathway, which senses cell wall stress [32
], down-regulates protein synthesis in these secretion mutants [33
]. Polarized growth may create a similar situation as is observed in secretion mutants. The area of membrane where vesicles can be deposited is limited (), which may lead to the accumulation of unincorporated vesicles. This could cause activation of the Pkc1 pathway and hence a down-regulation of protein synthesis.
Other cell cycle events could also regulate growth in budding yeast. Cells arrested in metaphase, grow relatively little compared to G1 arrested cells, although the actin cytoskeleton is not highly polarized in this arrest. Regulators of anaphase entry, i.e. the ubiquitin ligase APC, could accelerate growth as cells exit from mitosis and resume rapid growth. Like in S. pombe, duplication of the DNA content could also accelerate growth. However, given the poorly defined nature of G2 in S. cerevisiae, detecting this growth rate change may be difficult.