Cyclines

Cyclins are proteins that occur in all eukaryotes. They play an essential role in cell cycle progression through the activation of various cyclin-dependent kinases (Cdks), an associated phosphorylation of various substrates, but also through their interactions with Cdk inhibitors (CKIs).

Although the levels of cyclin-dependent kinases remain constant throughout the cell cycle, their activity peaks at the G2/M boundary to drive entry into mitosis. Cyclins, on the other hand, are only produced immediately before the corresponding cardinal point of the cell cycle and are coupled to ubiquitin shortly thereafter, after which they are degraded.

Although Cdc2 levels remain constant throughout the cell cycle, Cdc2 kinase activity peaks at the G2/M boundary to drive entry into mitosis. Cyclins are only produced immediately before the corresponding cardinal point of the cell cycle and are coupled to ubiquitin and degraded shortly thereafter.

By binding to the cyclins, the Cdc2-serine/threonine kinase also gains the ability to migrate from the cytoplasm to the nucleus. The cyclins therefore control the activity and function of the cdc2 kinase function.

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