Cell Cycle

The mammalian cell cycle control system is regulated by a group of protein kinases called cyclin-dependent kinases (CDKs). These proteins catalyze the attachment of phosphate groups to specific serine or threonine amino acids in a target protein. The phosphate groups alter the target protein's properties, such as its interaction with other proteins. (The alteration of protein activity by the attachment of phosphate groups occurs frequently in cells.)

CDKs are called "cyclin-dependent" because their activity requires their association with activating subunits called cyclins. While the number of CDKs in a cell remains constant during the cell cycle, the levels of cyclins Figure 1. oscillate. There are G₁ cyclins, S-phase cyclins, and G₂/M cyclins, each of which interact differently with CDK subunits to regulate the various phases of the cell cycle. CDKs can also associate with inhibitory subunits called CDK inhibitors (CKIs). In response to signals that work against proliferation, such as growth factor deprivation, DNA damage, cell-cell contact inhibition and lack of cell adhesion, CKIs cause the cell cycle to halt.

By the end of 2001, many structurally related cyclins (A1, A2, B1, B2, B3, B4, B5, C, D1, D2, D3, E1, E2, F, G1, G2, H, I, L, and T) and nine CDKs (CDK1 to CDK9) were identified in mammalian cells. Complexes of cyclin D and CDK4, as well as complexes of cyclin D and CDK6, operate during the G₁ phase. Complexes of cyclin A and CDK2, as well as complexes of cyclin E and CDK2, act during the transition from the G₁ to the S phase. Complexes of cyclin A and CDK1, as well as cyclin B and CDK1, function during the transition from the G₂ to the M phase.

Active complexes of cyclins and CDKs exert their biological effects by phosphorylating proteins. During the G₁ phase, a major target of cyclin/CDK complexes is the retinoblastoma protein (pRb). pRb is a growth-suppressing protein whose activity is controlled by whether or not it is phosphorylated.

When pRb is in the dephosphorylated form, during the G₀ phase and early in the G₁ phase, it is active. pRb exerts its growth-suppressing effects by binding to many cellular proteins, including the transcription factors of the E2F family (Figure 1). E2F transcription factors regulate the expression of numerous genes that are expressed during G₁, or at the transition from the G₁ to the S phase, to initiate DNA replication.

pRb that is bound to an E2F transcription factor inhibits the transcription factor's activity. Following phosphorylation by cyclin/CDK complexes, pRb dissociates from E2F, allowing the transcription factor to bind DNA sequences and activate the expression of genes necessary for the cell to enter the S phase. Cyclin D1/CDK4 complexes phosphorylation of pRb during the middle of the G₁ phase. They allow for subsequent phosphorylation of pRb by additional cyclin/CDK complexes that act later in the cell cycle.

Two families of CKIs have been identified, based on their amino acid sequence similarity and the specificity of their interactions with CDKs. One of the families of CKIs, the INK family, includes four proteins (p15, p16 p18 and p20). These CKIs exclusively bind complexes of cyclin D and CDK4, as well as complexes of cyclin D and CDK6, to block cells that are in the G₁ phase of the cell cycle. The other family of CKIs, the Cip/Kip family, consists of three proteins (p21, p27, and p57). These inhibitors bind to all complexes of cyclins and CDKs that function during the G₁ phase and during the transition from the G₁ to the S phase. They act preferentially, however, to block the activity of complexes containing CDK2.