What is the Difference Between G Actin and F Actin

The main difference between G actin and F actin is that G-actin is the soluble monomer while F-actin is the actin filament. Furthermore, G-actin is globular while F-actin is filamentous. Moreover, G-actin polymerizes to form F-actin. 

In brief, G-actin and F-actin are two types of structural forms of actin, which is a multifunctional protein, involved in the formation of microfilaments. Almost all types of eukaryotic cells contain the actin protein, which helps in muscular contraction, cell motility, cell division, etc. 

Key Areas Covered 

1. What is G Actin
     – Definition, Structure, Importance
2. What is F Actin
     – Definition, Structure, Importance
3. What are the Similarities Between G Actin and F Actin
     – Outline of Common Features
4. What is the Difference Between G Actin and F Actin
     – Comparison of Key Differences

Key Terms 

Actin Polymerization, Cytoskeleton, G Actin, F Actin, Microfilament, Muscular Contractions, Polarity

What is G Actin 

G-actin or globular actin is the free monomer of actin. It has two lobes separated by a cleft. This cleft is an ATPase fold, which serves as an enzymatic catalysis center where the hydrolysis of ATP occurs. This cleft can bind with both ATP and Mg2+. G-actin is only functional when the cleft binds with an ATP or ADP.  

Figure 1: G Actin Structure

G-actin readily polymerizes to form actin filaments or F-actin under physiological conditions. The initial step of actin polymerization is the nucleation or the formation of the first trimer of G-actins. Here, nucleation factors are responsible for the stimulation of the process. For instance, though the actin filament is a strong structure, it is still dynamic. This means G-actin monomers in the actin filament can dissociate from it as well. 

What is F Actin 

F-actin or filamentous actin is the polymeric form of the actin protein. Since it is a filamentous protein, it is not soluble. The structure of F-actin is a double-helical filament. Moreover, one end of F-actin filament contains an exposed ATP binding site and all the other actin monomeric units are pointed towards it. Therefore, actin filament has a polarity. The end with the active ATP binding site is called the (-) end while the opposite end is called the (+) end. 

Figure 2: Actin Polymerization

Moreover, F-actin occurs in the cytoskeleton in the form of microfilaments in all of the eukaryotic cells. Therefore, it participates in cellular processes like cell motility, cell division, cytokinesis, formation of cell junctions, organelle and vesicle movement, cell signalling, etc. It also forms the thin filament of the muscle cells; hence, it is responsible for the muscular contractions.  

Similarities Between G Actin and F Actin 

• G-actin and F-Actin are two structural forms of actin. 
• Furthermore, both serve as components in the formation of microfilament. 

Difference Between G Actin and F Actin 

Definition 

G-actin refers to the globular monomeric form of actin produced in solutions of low ionic concentration while F-actin refers to the fibrous actin polymerized in the form of a double-helix produced in the presence of a metal cation and ATP. Thus, this is the main difference between G actin and F actin.

Structure 

Their structure is also a major difference between G actin and F actin. G-actin is globular while F-actin is filamentous. 

Solubility 

Furthermore, G-actin is a soluble protein while F-actin is insoluble. 

Ionic Concentration 

G-actin occurs in low ionic concentrations while F-actin occurs in high ionic concentrations. Hence, this is another difference between G actin and F actin.

Role 

In addition, G-actin forms the actin filament while F-actin microfilament forms the cytoskeleton and the contractile apparatus of the muscle cells. 

Conclusion 

In conclusion, G-actin is the free, globular monomer of actin. It is soluble and polymerizes to form the F-actin, which is filamentous. F-actin filament forms both the cytoskeleton and the contractile apparatus of the muscle cells. Moreover, it is responsible for cell mobility and muscular contractions. Therefore, the main difference between G actin and F actin is their structure and role.

Reference:

1. Dominguez, Roberto and Kenneth C Holmes. “Actin structure and function” Annual review of biophysics vol. 40 (2011): 169-86. Available Here
2. Lodish H, Berk A, Zipursky SL, et al. Molecular Cell Biology. 4th edition. New York: W. H. Freeman; 2000. Section 18.2, The Dynamics of Actin Assembly. Available Here

Image Courtesy:

1. “G-actin subdomains” By Thomas Splettstoesser (www.scistyle.com) – Own work (CC BY-SA 4.0) via Commons Wikimedia  
2. “Thin filament formation” By Mikael Häggström. Häggström, Mikael (2014). “Medical gallery of Mikael Häggström 2014”. WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.008. ISSN 2002-4436. (Public Domain) via Commons Wikimedia  

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