More Feedback Keeps It Going
Authors: Gough Nancy R. 
Published: September 2008

The orderly and unidirectional progress through the four phases of the eukaryotic cell cycle that allows one cell to become two is governed by multiple feedback loops (see Santos and Ferrell). Two positive feedback loops--one in the G1 phase (Skotheim
et al
.) and one in anaphase (Holt
et al
.)--have been revealed by single-cell analysis. Both groups studied the budding yeast
Saccharomyces cerevisiae
, in which cyclin-dependent kinases (CDKs) and their associated regulatory proteins, the cyclins (Cln), are at the heart of the cell cycle. Skotheim
et al
. monitored an unstable green fluorescent protein (GFP) under the control of the
CLN2
promoter to determine the rate at which the
CLN2
promoter was turned on in wild-type cells and in cells lacking
CLN2
and
CLN1
(
cln1Δcln2Δ
). In the previous models based on studies of synchronized populations of cells, regulation of Cdk1 by only the G1 cyclin Cln3 appeared to be required for progression through the G1 Start checkpoint, which initiates transcription of the G1/S regulon that includes the
CLN1
and
CLN2
genes. However, Skotheim
et al
. found that the time from the "birth" of a daughter cell to the activation of the
CLN2
promoter was much shorter and less variable in wild-type cells than in the
cln1Δcln2Δ
cells and that a higher proportion of
cln1Δcln2Δ
cells failed to bud. Translocation from the nucleus of the transcriptional inhibitor Whi5, which contributes to activation of the G1/S regulon and is a substrate of Cdk1, was delayed in
cln1Δcln2Δ
cells; this delay may contribute to the delays and variability observed for the expression of other genes of the G1/S regulon. Thus, although not essential for cell cycle entry, Cln1 and Cln2 appear to serve as part of a positive feedback loop that dictates the timing of progression through G1. Holt
et al
. investigated events near the end of the cell cycle, specifically the separation of sister chromatids during anaphase. Chromatids are held together by a protein called securin, which is targeted for degradation by the ubiquitin ligase APC (anaphase-promoting complex), which, once degraded, allows the protease separase to cleave cohesin, thereby separating the chromatids. Separase also activates the phosphatase Cdc14, which removes phosphates from Cdk1 substrates. Holt
et al
. identified a new Cdk1 phosphorylation site in the N-terminal domain of securin near the destruction box that is important for APC recognition (other Cdk1 phosphorylation sites were already known in the C-terminal domain). When securin was phosphorylated by Cdk1 in vitro, it was a poor substrate for APC, and dephosphorylation by Cdc14 restored the ubiquitination of securin by APC. By tagging two chromosomes with GFP-repressor fusion proteins at specific loci, the authors quantified chromosome segregation in single cells. In wild-type cells, there was a consistent 90-second delay between separation of chromosomes IV and V. In cells in which Cdc14 activity was reduced, Cdk1 activity was increased, or a securin that could not be phosphorylated by Cdk1 was present, the separation of the two chromosomes was delayed. Thus, securin, separase, and APC appear to function in a positive feedback loop that stimulates securin's degradation by activating the phosphatase Cdc14, which promotes securin's destruction. This cycle appears to contribute to the synchronicity of chromosome segregation during anaphase.

J. M. Skotheim, S. Di Talia, E. D. Siggia, F. R. Cross, Positive feedback of G1 cyclins ensures coherent cell cycle entry.
Nature
454
, 291-296 (2008).
[PubMed]

L. J. Holt, A. N. Krutchinsky, D. O. Morgan, Positive feedback sharpens the anaphase switch.
Nature
454
, 353-357 (2008).
[PubMed]

S. D. M. Santos, J. E. Ferrell, On the cell cycle and its switches.
Nature
454
, 288-289 (2008).
[PubMed]