Cyclin D

cyclin D1
Crystal structure of human cyclin D1 (blue/green) in complex with cyclin-dependent kinase 4 (yellow/red).
Identifiers
Symbol	CCND1
Alt. symbols	BCL1, D11S287E, PRAD1
Entrez	595
HUGO	1582
OMIM	168461
RefSeq	NM_053056
UniProt	P24385
Other data
Locus	Chr. 11 q13

Cyclin D is a member of the cyclin protein family that is involved in regulating cell cycle progression. The synthesis of cyclin D is initiated during G1 and drives the G1/S phase transition. Cyclin D protein is anywhere from 155 (in zebra mussel) to 477 (in Drosophila) amino acids in length.

Once the cells reach a critical cell size (and if no mating partner is present in yeast) and if growth factors and mitogens (for multicellular organism) or nutrients (for unicellular organism) are present, cells enter the cell cycle. In general, all stages of the cell cycle are chronologically separated in humans and are triggered by cyclin-Cdk complexes which are periodically expressed and partially redundant in function. Cyclins are eukaryotic proteins that form holoenzymes with cyclin-dependent protein kinases (Cdk), which they activate. The abundance of cyclins is generally regulated by protein synthesis and degradation through an APC/C dependent pathway.

Cyclin D is one of the major cyclins produced in terms of its functional importance. It interacts with four Cdks: Cdk2, 4, 5, and 6. In proliferating cells, cyclin D-Cdk4/6 complex accumulation is of great importance for cell cycle progression. Namely, cyclin D-Cdk4/6 complex partially phosphorylates retinoblastoma tumor suppressor protein (Rb), whose inhibition can induce expression of some genes (for example: cyclin E) important for S phase progression.

Mice, Drosophila and many other organisms only have one cyclin D protein. In humans, in addition to the mouse homologue, two more cyclin D proteins have been identified. These human proteins, called cyclin D1, cyclin D2, and cyclin D3 are expressed in most proliferating cells and the relative amounts expressed differ in various cell types.