how does the explosive polymerization of G-actin propels L. monocytogenes through the cytoplasm?
The explosive polymerization of G-actin propels L. monocytogenes through the cytoplasm by generating a force that pushes the bacterium forward. To understand this process, let me first explain a few key concepts.
G-actin (globular actin) refers to the monomeric form of actin, a protein found in eukaryotic cells, including the cytoplasm. Actin is involved in various cellular processes such as cell movement, division, and maintaining cell shape.
The polymerization of G-actin into F-actin (filamentous actin) is a dynamic process that occurs in the cytoplasm. F-actin forms long, helical filaments composed of multiple G-actin subunits bound together.
Now, coming to L. monocytogenes, it is a pathogenic bacterium that can invade and move within host cells, including the cytoplasm. It achieves this movement by harnessing the polymerization of actin filaments.
Here's how the explosive polymerization of G-actin propels L. monocytogenes:
1. Attachment: L. monocytogenes attaches to the surface of the host cell and forms a specialized structure called an actin comet tail at the posterior end of the bacterium.
2. Activation: L. monocytogenes secretes a protein known as ActA, which interacts with host cell proteins. ActA triggers the nucleation of actin filaments by binding to the Arp2/3 complex, a cellular protein complex that helps initiate actin polymerization.
3. Polymerization: The interaction between ActA and the Arp2/3 complex promotes the rapid polymerization of G-actin into F-actin filaments. Actin polymerization occurs primarily at the rear of the bacterium, pushing against the cell membrane.
4. Propulsion: As the actin filaments elongate, they generate pushing forces against the host cell's cytoskeleton. These forces, combined with the release of energy stored in the actin polymerization process, propel L. monocytogenes through the cytoplasm in a comet-like manner. The bacterium uses this pushing force to move in the direction opposite to the comet tail.
5. Continuous cycle: L. monocytogenes continually repeats this process, resulting in a continuous propulsion and movement within the host cell's cytoplasm.
In summary, the explosive polymerization of G-actin into F-actin, stimulated by the bacterial protein ActA, creates a propulsive force within the host cell's cytoplasm that drives L. monocytogenes forward.