The main difference between monomeric and trimeric G protein is that monomeric G protein is the active form of the G protein and contains only the alpha subunit while trimeric G protein is the inactive form of G protein and contains alpha, beta, and gamma subunits.
In brief, monomeric and trimeric G proteins are two forms of G protein involved in transmitting signals in the G protein-coupled receptors.
Key Areas Covered
1. What is a G Protein
– Definition, Structure, Function
2. What is Monomeric G Protein
– Definition, Structure, Function
3. What is Trimeric G Protein
– Definition, Structure, Function
4. Similarities Between Monomeric and Trimeric G Protein
– Outline of Common Features
5. Difference Between Monomeric and Trimeric G Protein
– Comparison of Key Differences
Key Terms
G protein, G Protein-Coupled Receptors (GPCRs), Monomeric G protein, Trimeric G Protein
What is G Protein
Generally, G proteins are guanine nucleotide-binding proteins that are molecular switches, taking part in transmitting signals of a variety of stimuli from outside to the inside of the cell. They are usually trimeric proteins in their inactive form and the three protein subunits of the inactive G protein are alpha, beta, and gamma. Of these, the alpha subunit of the inactive form of G protein usually binds to GDP. Whereas, the active form of G protein is a monomeric protein, containing only the alpha subunit. Meanwhile, the alpha subunit of the active G protein binds to GTP.
Moreover, G protein-coupled receptors (GPCRs) are responsible for the activation of G proteins. In general, these receptors contain seven transmembrane domains (7TM). Signaling molecules such as neurotransmitters bind to GPCRs to activate G proteins. Whereas, the inactive G protein binds to the cytosolic face of the plasma membrane. Active G proteins contain only the alpha subunit bound to GTP, thus, dissociating from the beta and gamma subunits.
What is Monomeric G Protein
Monomeric G protein is the form of G protein that contains the alpha subunit of the trimeric G protein. The active form of G protein, which is the monomeric G protein containing only the alpha subunit, binds with GTP.
Figure 1: G protein Structure
Furthermore, signaling molecules such as neurotransmitters bind to the GPCRs, activating G proteins bound to the GPCRs in the cytosolic face of the plasma membrane. More importantly, the activation of the G protein involves the dissociation of the alpha subunit from the beta-gamma complex of the G protein. Normally, the activated G protein or the monomeric G protein further activates cascades of signaling pathways. These signaling pathways include metabolic enzymes, ion channels, transporter proteins, and other parts of cell machinery, controlling transcription, motility, contractility, and secretion.
What is Trimeric G Protein
Trimeric G protein is the inactive form of G protein, containing three protein subunits: alpha subunit, beta subunit, and gamma subunits. Therefore, the trimeric G protein is a heterotrimeric protein. It is attached to GPCRs, which are transmembrane proteins at the cytosolic face of the plasma membrane. Due to the presence of three protein subunits in the trimeric G protein, it is a large G protein. Generally, the binding of GTP or GDP to the alpha subunit of the G protein determines the activation or inactivation of the G protein.
Figure 2: G protein Activation Pathway
Moreover, the binding of GTP to the alpha subunit of the G protein results in the activation of G proteins. This in turn dissociates the alpha subunit from the rest of the G protein. The dissociated alpha subunit with the GTP is called the monomeric G protein, which next binds to a membrane-bound effector protein.
Similarities Between Monomeric and Trimeric G Protein
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- Monomeric and trimeric G proteins are two forms of G protein.
- The G-protein is important in the G protein-coupled receptors in the signal transduction pathway.
- G proteins occur in the cytosolic face of the plasma membrane.
- Usually, the G protein is a relay protein between the receptors and their target signaling proteins.
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Difference Between Monomeric and Trimeric G Protein
Definition
Monomeric G protein refers to a smaller G protein, consisting only of the alpha subunit of the trimeric G protein. In comparison, trimeric G protein refers to a heterotrimeric G protein that attaches to the 7TM receptors at the cytosolic face of the plasma membrane.
Subunits
Monomeric G protein contains only the alpha subunit while trimeric G protein contains alpha, beta, and gamma subunits.
Size
Monomeric G protein is a small G protein while trimeric G protein is a large G protein.
GTP/GDP
Furthermore, monomeric G protein binds to GTP while trimeric G protein binds to GDP.
Activation
Monomeric G protein is the active form of G protein while trimeric G protein is the inactive form of G protein.
Function
The function of monomeric G proteins is to serve as relay proteins between the receptors and their target signaling proteins while the main function of trimeric G proteins is to bind 7TM at the cytosolic face of the plasma membrane.
Conclusion
In brief, monomeric and trimeric G proteins are the two forms of G protein, which is the relay molecule of the signal transduction pathways. Monomeric G protein contains the alpha subunit and it is small. It is also the active form of the G protein and its main function is to serve as the relay molecule between the receptor and the target protein. In contrast, the trimeric G protein is the inactive form of the G protein and contains alpha, beta, and gamma subunits. It occurs attached to the cytosolic face of the plasma membrane. Therefore, the main difference between monomeric and trimeric G protein is their structure and function.
References:
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- “3.2 Trimeric G Proteins.” Cell Signalling.
Image Courtesy:
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- “G protein1” By Bensaccount – Own work (Public Domain) via Commons Wikimedia
- “GPCR cycle” By Repapetilto – Own Work (CC BY-SA 3.0) via Commons Wikimedia
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