What is the Difference Between Cyclins and Cyclin Dependent Kinases

The key difference between cyclins and cyclin dependent kinases is that cyclins are regulatory proteins that have no enzymatic function in the cell cycle, while cyclin-dependent kinases are catalytic proteins that have an enzymatic function in the cell cycle.

The cell cycle is a very important event that takes place in the cell. It is a series of events that causes a cell to divide into two daughter cells. The eukaryotic cell cycle consists of four different phases: G1, S, G2, and M. G1, S, and G2 are collectively known as interphase. In G1, the cell continues to grow. DNA replication takes place in the S phase. In the G2 phase, cells will continue to grow further. Finally, in the M phase, cell division proceeds. Moreover, two regulatory molecules known as cyclins and cyclin-dependent kinases participate in the progression of the cell through the cell cycle. Therefore, cyclins and cyclin dependent kinases are two key classes of proteins that participate in cell cycle regulation.

CONTENTS

1. Overview and Key Difference
2. What are Cyclins  
3. What are Cyclin Dependent Kinases
4. Similarities – Cyclins and Cyclin Dependent Kinases
5. Cyclins vs Cyclin Dependent Kinases in Tabular Form
6. Summary – Cyclins vs Cyclin Dependent Kinases

What are Cyclins?

Cyclins are a key class of regulatory proteins involved in the cell cycle. They have no enzymatic function. Many of the genes that encode cyclins are conserved among all eukaryotic species. H. L Hartwell, R.T Hunt, and P.M Nurse won the Nobel Prize in 2001 for their discovery of cyclins and cyclin dependent kinases. However, cyclins were originally found in 1982 by R. Timothy Hunt while studying the cell cycle of sea urchins.

Cyclins have no catalytic function. When it is bound to cyclin dependent kinases, it forms a complex called maturation promoting factor. This complex activates other proteins through phosphorylation. These phosphorylated proteins are responsible for specific events that occur during the cell cycle, such as microtubule formation and chromatin remodelling. All these events help a cell to progress through the cell cycle. Based on the behaviour of the cell cycle, the cyclins can be divided into four classes: G1 cyclins, G1/S cyclins, S cyclins and M cyclins.

What are Cyclin Dependent Kinases?

Cyclin dependent kinases are a class of catalytic protein kinases that play an important role in the cell cycle. They are also involved in regulating transcription, mRNA processing, and the differentiation of nerve cells. These proteins are present in all eukaryotes. Their function is evolutionary conserved. Cyclin dependent kinases are small proteins. Their molecular weight ranges from 34 kDa to 40 kDa. They are normally inactive without cyclins.

A cyclin dependent kinase binds to a regulatory protein called cyclin. Without cyclin, cyclin dependent kinase has little activity. The cyclin-cylin dependent kinase complex is called the maturation promoting factor. This complex is highly important for the cell’s progression through the cell cycle. Moreover, cyclin dependent kinases phosphorylate their substrates of serines and threonines. Therefore, they are also called serine-threonine kinases. Furthermore, cyclin dependent kinases are constitutively expressed in cells.

What are the Similarities Between Cyclins and Cyclin Dependent Kinases?

• Cyclins and cyclin dependent kinases are two key classes of proteins in cell cycle regulation.
• They are proteins made up of amino acids.
• They are components of the maturation promoting factor.
• Both cyclins and cyclin dependent kinases are forming an activated heterodimer.
• L Hartwell, R.T Hunt, and P.M Nurse won the Nobel Prize for the discovery of cyclins and cyclin dependent kinases in 2001.

What is the Difference Between Cyclins and Cyclin Dependent Kinases?

Cyclins are regulatory proteins involved in the cell cycle that have no enzymatic function, while cyclin dependent kinases are catalytic proteins involved in the cell cycle that have an enzymatic function. So, this is the key difference between cyclins and cyclin dependent kinases. Furthermore, cyclins are expressed at specific stages of the cell cycle in response to various molecular signals. On the other hand, cyclin dependent kinases are expressed constitutively throughout the cell cycle.

The below infographic lists the differences between cyclins and cyclin dependent kinases in tabular form for side by side comparison.

Summary – Cyclins vs Cyclin Dependent Kinases

Cyclins and cyclin dependent kinases are two key classes of regulatory molecules in cell cycle regulation. Cyclins do not have an enzymatic function, while cyclin dependent kinases have an enzymatic function. Thus, this is the summary of the difference between cyclins and cyclin dependent kinases.

Reference:

1. “Cyclins.” An Overview | ScienceDirect Topics.
2. “Cyclin-Dependent Kinase.” An Overview | ScienceDirect Topics.

Image Courtesy:

1. “0332 Cell Cycle With Cyclins and Checkpoints” By OpenStax – (CC BY 4.0) via Commons Wikimedia
2. “Protein CDK6 PDB 1bi7”  By Emw – Own work (CC BY-SA 3.0) via Commons Wikimedia