The key difference between G-actin and F-actin is that G-actin is a form of actin with a globular configuration. At the same time, F-actin is a form of actin with a filamentous configuration.
Actin is the most abundant protein in most eukaryotic cells. It is highly conserved and forms microfilaments in the cytoskeleton. It participates in more protein and protein interactions than any other protein. G-actin and F-actin are two forms of actin. G-actin monomer aggregates to form the F-actin filamentous structure. Both these forms are essential for important cellular functions like the mobility and contraction of cells during cell division.
CONTENTS
1. Overview and Key Difference
2. What is G-Actin
3. What is F-Actin
4. Similarities – G-Actin and F-Actin
5. G-Actin vs. F-Actin in Tabular Form
6. Summary – G-Actin vs. F-Actin
What is G-Actin?
G-Actin is a free monomer and globular form of actin. It is approximately 46 kDa in size. Monomeric G-actin is a globular protein present in all cells. It undergoes polymerization in three stages to form F-actin microfilaments. There is one high-affinity calcium-binding site per each G-actin monomer, which stabilizes the globular configuration of the G-actin. Moreover, G-actin also has one ATP binding site per monomer. Commonly, actin filament polymerization occurs through three phases: nucleation phase, elongation phase, and steady-state phase.
Figure 01: G-Actin
During the nucleation phase of actin polymerization, the formation of a stable ‘actin nucleus takes place. This actin nucleus usually has three actin monomers in the complex. On the other hand, in the elongation phase, actin monomers (G-actin) are rapidly added to the filament at the barbed end (positive end). This reaction is often facilitated by elongation factors such as formin. Furthermore, for this reaction to occur, the barbed end of the filament must be exposed. In the steady state, the monomer disassembly and polymerization are in a balanced state or equilibrium state.
What is F-Actin?
F-actin is a filamentous polymer composed of several G-actin monomers. The F-actin filaments consist of two helical aggregates of G-actin. These G-actin aggregates are twisted around each other with 13.5 subunits per turn. F-actin filaments are typically microfilaments seen under electron microscopy. These F-actin filaments can be observed after various immunofluorescence staining procedures using actin filament-binding compounds or antibodies.
Figure 02: F-Actin
Furthermore, F-actin is usually insoluble in nature and only occurs in high ionic concentrations. In low ionic concentrations, it converts back to a G-actin monomer. The primary function of F-actin is to form the cytoskeleton and contractile apparatus of the muscle cells.
What are the Similarities Between G-Actin and F-Actin?
- G-actin and F-actin are two different structural forms of actin.
- They are proteins in nature.
- G-actin monomer aggregates to form the F-actin filamentous structure.
- Both these forms are essential for important cellular functions like the mobility and contraction of cells during cell division.
What is the Difference Between G-Actin and F-Actin?
G-actin is a form of actin with a globular configuration, while F-actin is a form of actin with a filamentous configuration. Thus, this is the key difference between G-actin and F-actin. Furthermore, G-actin has a structure of two lobes separated by a cleft, while F-actin has a structure of a double-helical filament.
The infographic below presents the differences between G-actin and F-actin in tabular form for side-by-side comparison.
Summary – G-Actin vs. F-Actin
Actin is a multi-functional protein involved in forming microfilaments in the cytoskeleton. It also forms thin filaments in muscle fibrils. Actin is found in essentially all eukaryotic cells. Actin is the highly abundant intracellular protein in all eukaryotic cells. It has a vital role in muscle contraction and cell movements. There are two structural forms of actin: G-actin and F-actin. G-actin is a globular monomer, while F-actin is a filamentous polymer. So, this summarizes the difference between G-actin and F-actin.
Reference:
1. “G Actin – An Overview.” ScienceDirect Topics.
2. Dominguez, Roberto, and Kenneth C Holmes. “Actin Structure and Function.” Annual Review of Biophysics.
Image Courtesy:
1. “G-actin subdomains” By Thomas Splettstoesser – Own work (CC BY-SA 4.0) via Commons Wikimedia
2. “F-actin filaments in cardiomyocytes” By Ps1415 – Own work (CC BY-SA 4.0) via Commons Wikimedia