Cyclins

Cyclins

Cyclins are a family of proteins that play a crucial role in the regulation of the cell cycle. They are named for their cyclical levels of expression, which rise and fall in coordination with the cell cycle phases. Cyclins function as regulatory subunits that activate cyclin-dependent kinases (CDKs), which are essential for the progression of cells through the cell cycle.

Structure and Function

Cyclins are characterized by the presence of a conserved region known as the "cyclin box," which is responsible for binding to CDKs. The binding of a cyclin to a CDK activates the kinase activity of the CDK, allowing it to phosphorylate target proteins that drive the cell cycle forward.

There are several classes of cyclins, each associated with specific phases of the cell cycle:

• G1/S Cyclins: These cyclins, such as Cyclin D and Cyclin E, are involved in the transition from the G1 phase to the S phase, where DNA replication occurs.
• S Cyclins: Cyclin A is an example of an S cyclin, which is necessary for the initiation and progression of DNA synthesis.
• M Cyclins: Cyclin B is a key player in the transition from the G2 phase to mitosis (M phase), facilitating the processes required for cell division.

Regulation of Cyclin Levels

The levels of cyclins are tightly regulated by a combination of transcriptional control, protein degradation, and cellular localization. Cyclins are synthesized at specific points in the cell cycle and are degraded by the ubiquitin-proteasome system once they have fulfilled their role.

The degradation of cyclins is mediated by the anaphase-promoting complex (APC), a ubiquitin ligase that targets cyclins for destruction, ensuring that the cell cycle progresses in an orderly manner.

Cyclins and Cancer

Dysregulation of cyclin expression or function can lead to uncontrolled cell proliferation, a hallmark of cancer. Overexpression of certain cyclins, such as Cyclin D1, has been implicated in the development of various cancers, including breast and prostate cancer.

Mutations in cyclin genes or their regulatory pathways can disrupt the normal cell cycle control, leading to oncogenesis. As a result, cyclins and CDKs are considered potential targets for cancer therapy, with several CDK inhibitors currently being investigated in clinical trials.

See Also

• Cell cycle
• Cyclin-dependent kinase
• Ubiquitin-proteasome system
• Anaphase-promoting complex

References

• Alberts, B., Johnson, A., Lewis, J., Raff, M., Roberts, K., & Walter, P. (2002). Molecular Biology of the Cell. 4th edition. New York: Garland Science.
• Morgan, D. O. (2007). The Cell Cycle: Principles of Control. Oxford University Press.