Cell division cycle regulation: Cyclin-Dependent Kinase (CDKs)

Cell division cycle regulation

The sequence of the events of in a cell cycle need to be run in a sequential manner. To maintain the order, a series of protein complexes are required for successfully driving the process.  The regulation of cell cycle is crucial to ensure proper segregation of genetic material for the normal development and maintenance of multicellular organisms. The failure of cell cycle leads to genetic instability and uncontrolled cell division. The complex macromolecular events of the cell cycle are regulated by a series of cellular proteins. The key regulatory protein of cell division cycle is a heteromeric complex consisting of Cyclin and Cyclin-Dependent protein Kinase (CDK), a family of serine-threonine protein kinases that are activated at specific points of the cell cycle. Cyclin is a regulatory subunit and CDK is catalytic subunit.

Cyclin-Dependent Kinase (CDKs)

CDKs are important master regulators of the cell cycle. CDK is a serine/threonine protein kinase that adds the phosphate group to target protein substrates. Phosphorylation brings changes in the enzymatic activity of the substrate to interact with other proteins.

How CDKs Work

CDKs are inactive without cyclins. CDKs are activated by binding to its regulatory subunit cyclins. The name “Cyclin-Dependent” refers to this absolute requirement of cyclins for CDK activity. CDKs need to hydrolize ATP for energy in order to perform phosphorylation. They have an ATP binding cleft whose ability to bind ATP is regulated by two mechanisms. First, CDKs have a ‘flexible T loop’ which contains a threonine (T) residue which normally blocks the ATP binding cleft, but not when the T is phosphorylated. Second, cyclins bind CDKs and induce a conformational change that also helps to expose the ATP binding cleft. Therefore a fully active CDK is one which is both phosphorylated at the T on the T loop and is bound to a cyclin. CDKs phosphorylate proteins on serine (S) or threonine (T) residues. The specificity of CDKs for their substrates is defined by the S/T-P-X-K/R sequence, where S/T is the phosphorylation site, P is proline, X is any amino acid, and the sequence ends with lysine (K) or arginine (R). This motif ensures CDKs accurately target and modify proteins, crucial for regulating cell cycle and other functions.

Major CDKs and Their Cyclin Partners

Yeast have just one CDK (Cdk1), while ‘metazoans’ (animals) like us have nine, of which five remain active during the cell cycle. The target proteins for CDK phosphorylation are dictated by the cyclin associated with the CDK. Different cyclins are associated with different phases of the cell cycle. During the G1 phase (CDK4 and CDK 6), S phase (CDK2), G2 and M phases (CDK1) are active for phosphorylating their target protein substrates.

Different cyclins are required at different stages of the cell cycle. Cyclin-CDK complex (cyclin D1, D2, D3 bind with CDK4 and CDK6) is necessary for entry of the cell in G1. Cyclin E also associates with CDK2 to regulate progression from G into S phase. Cyclin A and CDK2 complex is required during S phase. In late G and early M, cycling- CDK1 promotes the entry into M. Mitosis is further regulated by cyclin B – CDK1. Throughout the cell cycle, the levels of cyclins increase and decrease, thus, periodically activate different CDKs. The cyclin-CDK complex is formed as a result of the interaction between the cyclin box, a region conserved across cyclins and a region of the CDK, called the PSTAIRE.

Cyclin Dependent Kinases (CDKs) in Yeast

In yeast, cyclin-dependent kinases (CDKs) play central roles in regulating the cell cycle, just like in higher eukaryotes. However, yeasts (both fission and budding) use a single main CDK to control multiple phases of the cell cycle by associating with different cyclins.

Schizosaccharomyces pombe possess a single cyclin and single CDK (cdc2) that drive the entire cell cycle.  Cdc2 (Cell Division Cycle 2) is a cyclin-dependent kinase, it is the yeast homolog of CDK1, and plays a critical role in controlling the cell cycle.

In Saccharomyces cerevisiae, a single CDK protein binds to different cyclins such as G1-cyclins, G1/S-cyclins, S-cyclins and M-cyclins and drives the entire cell cycle by changing the cyclin at different stages.  Cdc28 is the master cyclin-dependent kinase (CDK) in Saccharomyces cerevisiae (budding yeast). Each CDK-cyclin complex acts as a molecular switch that triggers a specific cell cycle event.