Topic 7.2 - CDK - Cyclin Regulation
The last topic introduced the idea that the cell is broken down into several stages, in which particular events
must occur. Like all things in bioloy, there was a time when the details of the cell cycle stages were not known
to scientists. The first experiments that showed that there was something controlling the progression of the cell
cycle involved injection of cytoplasm into the very large cells of frog (Xenopus laevis) oocytes.
In each case, the cytoplasm from one oocyte that was at one stage of the cell cycle, was injected into another
oocyte, as shown in the image below.
These results show that:
The active agent is in the cytoplasm of the cell
Control of DNA synthesis and mitosis is positive. That is, that the
active agent turns on the process in the recipient cell which is capable
to respond but had not yet gotten around to initiating the process
That cells can be advanced into the next stage by appropriate
Based on experiments such as these, scientists identified that there
was some cytoplasmic 'factor' that helped to promote the cell's
progression into mitosis. As a result, they called this the Mitosis
Promoting Factor (MPF).
Since the discovery of MPF, a great deal of work has been done to understand more specifically what
was regulating the progression of the cell into mitosis. We now know that the active agent is a protein
called Cyclin-Dependant Kinase (CDK) (Important Note: a kinase is a protein that is capable of adding
phosphate groups to other proteins). It regulates the activity of a large number of other proteins that are
required for cell cycle progression by phosphorylating them.
Proteins can be phosphorylated by adding phosphate groups to specific amin acid side chains that bear hydroxyl
groups (serine, threonine, tyrosine).
Phosphorylation can result in big changes in tertiary structure of proteins. What happens if a highly charged group is added to or
removed from a protein?
We have already seen one example of phosphorylation modifying the function of a protein... can you remember where? (Hint: Topic
3.1 and then again in topic 6.1)
In addition to CDK, MPF also contains second protein that interacts with CDK, known as cyclin. Cyclin binds
to cyclin-dependant kinase, and acts as a regulatory unit; CDK can ONLY perform its function as a kinase when
cyclin is bound. If cylcin is removed from CDK, then CDK is inactivated. This is also the source of its name...
These two proteins combine to produce a single enzyme, the cyclin-CDK complex.
While CDK concentrations remain
constant throughout the cell cycle, cyclin
levels build up during the cell cycle and
the crash to zero at the end of mitosis
(How might that happen?).
Figure 18-5 (both editions) Cyclin
concentration vs. CDK-cyclin enzyme
activity. CDK has is ONLY functional when it is bound to cyclin, thus the degradation of cyclin at the end of mitosis
effectively shuts off CDK activity and allows the cell to move out of M-phase and into G . 1
In addition to this, the CDK-cyclin complex can be phosphorylated in several places (by other kinases), which
will also have a role in activating/ deactivating the complex. Cyclin-CDK's are very tightly controlled as the
consequences of a cell moving into the next stage of the cell cycle before its ready could be fatal.
The activity of the cyclin-CDK complex is controlled by two independent processes (shown in the figure
The binding/proteolysis of cyclin, and
The phosphorylation of CDK
MPF contains the activated cyclin-CDK complex that promotes entry into mitosis, which we call M-CDK-
cyclin. It is important to remember that there are several cyclins and CDKs involved in regulating the cell cycle
at several stages, as shown in the table below. It is these complexes that control the checkpoints that we
discussed in Topic 8.1. In this course we will focus primarily on those involved in entry into M-phase, with
some discussion of S-phase cyclin-CDKs as well.
Figure 18-8 (3rd ed. 18-10).
Relative concentrations of S- and M-
cyclins throughout the cell cycle.
Phosphorylation controls CDK activity
Phosphorylation of specific amino acid residues (usually serines, threonines or tyrosines) can be used to activate
or inhibit protein activity. In the case of CDK, it is possible that BOTH activating AND inhibitory phosphates
are added and/ or removed, depending on the cyclin-CDK complex that we are discussing. The specifics of all
of the ways that cyclin-CDK gets phosphorylated is beyond the scope of this course, other than knowing about
the first inhibitory phosphate, as described below: The activation of a cyclin-CDK complex happens as follows:
1. Cyclin levels begin to build up well before the checkpoint. For example, M-cyclin synthesis begins at
the end of S-phase and builds up until the onset of mitosis.
2. CDK combines with cyclin. The interaction between the subunits changes the configuration of the active
site of the CDK and makes it possible for the enzyme to become active. Interaction with cyclin is
necessary, but not sufficient for CDK enzyme activity.
3. Inhibitory phosphorylation. The phosphates are added by specific protein kinases onto specific tyrosine
residues in the CDK primary sequence. Its worthwhile to note that this may not be the only
phosphorylation that takes place. Some CDKs also need activating phosphates added. Some of the
images in your textbook show this as well. Here is a more detailed figure on events in budding yeast.
4. When the cell is prepared to pass the checkpoint, removal of the inhibiting phosphate occurs by action of
a specific phosphatase. At this point CDK-cyclin becomes enzymatically active.
M-CDK controls entry into mitosis, and re-entry into G1
In yeast, the two key regulator proteins for M-cyclin-CDK activation are wee1 and cdc25. The antagonistic
relationship of wee1 and cdc25 was discovered using genetic experiments in which the dosage of the genes was