Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

878 Chapter 15: Cell Signaling
regulated Per
phosphorylation
and degradation
Tim gene
Per gene
NUCLEUS
CYTOSOL
mRNAs
dissociation of
heterodimer
mRNAs
Tim degraded in
response to light
Tim protein
heterodimer
formation
Per protein
regulated Per
phosphorylation and degradation
Figure 15–18C and D). In Drosophila and many other animals, including humans,
the heart of the circadian clock is a delayed negative feedback loop based on transcription
regulators: accumulation of certain gene products switches off the transcription
of their own genes, but with a delay, so that the cell oscillates between a
MBoC6 m7.73/15.66
state in which the products are present and transcription is switched off, and one
in which the products are absent and transcription is switched on (Figure 15–66).
The negative feedback underlying circadian rhythms does not have to be based on
transcription regulators. In some cell types, the circadian clock is constructed of
proteins that govern their own activities through post-translational mechanisms,
as we discuss next.
Figure 15–66 Simplified outline of the
mechanism of the circadian clock in
Drosophila cells. A central feature of
the clock is the periodic accumulation
and decay of two transcription regulatory
proteins, Tim (short for timeless, based
on the phenotype of a gene mutation)
and Per (short for period). The mRNAs
encoding these proteins rise gradually
during the day and are translated in the
cytosol, where the two proteins associate
to form a heterodimer. After a time delay,
the heterodimer dissociates and Tim and
Per are transported into the nucleus, where
Per represses the Tim and Per genes,
resulting in negative feedback that causes
the levels of Tim and Per to fall. In addition
to this transcriptional feedback, the clock
depends on numerous other proteins. For
example, the controlled degradation of Per
indicated in the diagram imposes delays
in the accumulation of Tim and Per, which
are crucial to the functioning of the clock.
Steps at which specific delays are imposed
are shown in red.
Entrainment (or resetting) of the clock
occurs in response to new light–dark
cycles. Although most Drosophila cells
do not have true photoreceptors, light is
sensed by intracellular flavoproteins, also
called cryptochromes. In the presence of
light, these proteins associate with the
Tim protein and cause its degradation,
thereby resetting the clock. (Adapted from
J.C. Dunlap, Science 311:184–186, 2006.)
Three Proteins in a Test Tube Can Reconstitute a Cyanobacterial
Circadian Clock
The best understood circadian clock is found in the photosynthetic cyanobacterium,
Synechococcus elongatus. The core oscillator in this organism is remarkably
simple, being composed of just three proteins—KaiA, KaiB, and KaiC. The central
player is KaiC, a multifunctional enzyme that catalyzes its own phosphorylation
and dephosphorylation in a 24-hour cycle: it gradually phosphorylates itself
sequentially at two sites during the day and dephosphorylates itself during the
night. This timing depends on interactions with the two other Kai proteins: KaiA
binds to unphosphorylated KaiC and stimulates KaiC autophosphosphorylation,
first at one site and then, with a delay, at the other. The second phosphorylation
promotes the binding of the third protein, KaiB, which blocks the stimulatory
effect of KaiA and thereby allows KaiC to dephosphorylate itself, bringing KaiC
back to its dephosphorylated state. This clock depends on a negative feedback
loop: KaiC drives its own phosphorylation until, after a delay, it recruits an inhibitor,
KaiB, that stimulates KaiC to dephosphorylate itself. Amazingly, when the
three Kai proteins are purified and incubated in a test tube with ATP, KaiC phosphorylation
and dephosphorylation occur with roughly 24-hour timing over a
period of several days (Figure 15–67).
Circadian oscillations in KaiC phosphorylation lead to parallel rhythms in the
expression of large numbers of genes involved in controlling metabolic activities
and cell division (see Figure 15–67). As a result, many aspects of cell behavior are
synchronized with the circadian cycle.
Even in continuous darkness, cyanobacterial cells generate free-running oscillations
of KaiC phosphorylation with roughly 24-hour periods. As in other circadian
clocks, the cyanobacterial clock is entrained by the environmental light/dark