From CellBiology

Cyclin B/CDK1

Figure 2. Stages of Mitosis.
Figure 1. Different phases of the cell cycle.


The cell cycle is dictated by a family of cyclin-dependent kinases (CDKs) and their regulatory subunits, cyclins. During G1 of the cell cycle there are at least three cyclins in operation, some of which are cyclin C, cyclin D and cyclin E, with another two functioning during the G2/M phase, these are cyclin A and cyclin B. This page will concentrate specifically on the mitotic cyclin, cyclin B.

CDK1 is the main cyclin-dependent kinase involved for the transition from the G2 phase to the mitotic phase and is regulated by the cyclin B subunit.

Cyclin B binds to CDK1 forming a CycB/CDK1 complex. These mitotic cyclin-CDK complexes act as master regulators for almost every subcellular organelle and macromolecular structures that experience structural alterations during mitosis. These protein kinases phosphorylate a broad array of select protein substrates that initiate the early events of mitosis and mediate alterations to cytoskeletal and nuclear structures required for the segregation of chromosomes.


Figure 3. Cyclin B1.

Common to all members of the cyclin family is a region of protein sequence known as the cyclin box. The cyclin box is required for the interaction between the cyclin and their corresponding CDK partner.

The sequence domains of cyclin B1 include the destruction box which is necessary for the breakdown of cyclin B1 during the metaphase/anaphase transition. This is followed by the cyclin's cytoplasmic retention sequence (CRS) and the nuclear export signal (NES) both of which are crucial for the localisation of cyclin B1 during mitosis with the sequence ending with the cyclic box for CDK1 interaction.

Cyclin B

Figure 4. Cyclin B/Cdk1 complex continues mitotic processes until cyclin b levels are reduced

In all eukaryotes, cyclin B is the key regulatory protein for the control of mitosis. It regulates the subcellular localisation, substrate specificity and activation of CDK1. Conversely the degradation of cyclin B levels are required for mitotic exit and cell division, these cyclin levels are reduced by anaphase-promoting complexes (APC/C).

There are two main B-type cyclins that operate during mitosis, cyclin B1 and cyclin B2. During prophase, cyclin B1 has been found to accumulate at the nucleus while at prometaphase they are located at the centrosomes, spindle microtubules and the condensed chromatin. These localisations correspond to known Cdk1 substrates which include nuclear lamins and RCC1, a chromatin-associated protein. This is in comparison to cyclin B2 which remains associated with the disassembled Golgi throughout mitosis.

Cyclin B1

Cyclin B1 encodes a regulatory protein involved in mitosis. This protein then complexes with p34 (CDC2) forming a serine/threonine kinase holoenzyme complex known as the maturation-promoting factor (MPF). It is also involved in chromosomal alignment during mitosis and is co-localised with the microtubules of the mitotic spindle. During interphase concentration levels of cyclin B1 are higher within the cytoplasm but is able to transport to the nucleus also cyclin B1 interacts with active RALBP1 along with CDC2 creating an endocytotic complex.

Cyclin B2

Cyclin B2 is another member of the cyclin B-type family. Similarly to cyclin B1, cyclin B2 also associates with p34 (CDC2) and is primarily localised around the Golgi region. Cyclin B2 has also been found to bind to transforming growth factor beta RII suggesting the B2-CDC2 complex has a major role in growth factor mediated cell cycle control. Cyclin B2 however is not required for cell proliferation even though it binds to CDK1. In prophase cyclin B2 does not relocate to the nucleus like cyclin B1 but rather becomes uniformly distributed around the cell.

Current Research

Figure 5. a. Mitotic exit reversal without segregating chromosomes b. Mitotic exit reversal after chromatid separation c. Wild-type cyclin B1 do not undergo reversal of mitotic exit

Recent research has found that cyclin B1 is localized to unattached kinetochores and are also involved in chromosomal alignment in mitosis. Also studies have found that Cdk1 at the kinetochores are active and that cyclin B-cdk1 complexes vary during mitosis. The overexpression of human cyclin B1 has also been found in numerous cases of cancer and has been associated with tumor aggressiveness.

Another study has determined, through the use of spinning disk confocal microscopy, the spatial distribution of cyclin B1 to the mitotic apparatus; and were able to conclude that there were distinct mechanisms to target cyclin B1 to kinetochores, chromatin and centrosomes during mitosis.

Through the use of transgenic mice, some deficient in cyclin B1 others in cyclin B2, research has also revealed that cyclin B1 is an essential gene for growth as homozygous B1-null pups were unable to be born. In comparison, homozygous B2-null pups were found to develop normally without displaying any obvious abnormalities. Furthermore, cyclin B1 was found both on the intracellular membranes and freely in the cytoplasm unlike cyclin B2 which was strictly membrane associated. Observations from this experiment therefore suggest that cyclin B1 has the capability to target essential p34 (CDC2) kinase substrates of cyclin B2.

Another study has shown the effects of cyclin B if it is not degraded. The study found that by blocking the degradation of cyclin b mitotic exit was able to be reversed, resulting in the cell reverting back into the M phase. The reversal was characterized by chromosome recondensation, assembly of microtubules into a mitotic spindle, reopening of the cleavage furrow, and realignment of chromosomes at the metaphase plate. The research demonstrates that proteasome-dependent degradation of cyclin B is essential for the M phase to G1 phase transition to occur.


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