Listeria Motility Explained

Listeria monocytogenes is a fascinating food-borne pathogen known for its ability to move within and between host cells using a remarkable mechanism involving the host cell's own actin cytoskeleton. This movement is characterized by the formation of a "comet tail" structure made of actin filaments at the rear of the bacterium.

To achieve this movement, Listeria must recruit and polymerize host cell actin efficiently. This process relies on specific proteins present on the surface of the bacterium and their interaction with host cellular machinery.

Actin Polymerization Mechanism

The core mechanism by which Listeria moves involves:

• Initiation of actin filament formation (nucleation) at the bacterial surface.
• Elongation of these filaments, pushing the bacterium forward.
• Branching of actin filaments to create a dense, dynamic network.

Let's evaluate the given statements regarding how Listeria assembles host-cell actin:

• Statement A: Listeria has on its surface a protein called ActA.

  This statement is correct. The protein ActA (Actin Assembly-Inducing Protein A) is a key virulence factor located on the surface of Listeria monocytogenes. ActA is essential for the bacterium's ability to polymerize actin. It acts as a mimic of host cell proteins that regulate actin dynamics.

• Statement B: Listeria can activate Arp 2/3 complex.

  This statement is also correct. The host cell's Actin-Related Protein 2/3 (Arp 2/3) complex is a major nucleator of branched actin filaments. ActA on the Listeria surface binds to and activates the host cell's Arp 2/3 complex. Activated Arp 2/3 complex then binds to the side of existing actin filaments and nucleates new filaments, creating a branched network that pushes the bacterium forward.

• Statement C: Listeria has on its surface γ-tubulin.

  This statement is incorrect. γ-tubulin is a component of the centrosome and is primarily involved in nucleating microtubules, which are part of a different cytoskeletal system. It is not involved in the actin-based motility of Listeria.

• Statement D: Listeria has on its surface myosin II motor.

  This statement is incorrect in explaining how Listeria *assembles* actin for its comet tail motility. Myosin II is an actin-based motor protein that generates force, often involved in contraction or movement along actin filaments. While host cell myosin might interact with the actin tail, the presence of myosin II on the bacterial surface is not the mechanism by which Listeria *polymerizes* or *assembles* the actin filaments at its rear end for propulsion.

Conclusion on Listeria Actin Assembly

Based on the roles of these proteins in cellular cytoskeleton dynamics, Listeria's ability to assemble host-cell actin at its rear end to form a comet tail structure is directly dependent on the presence of ActA on its surface and ActA's ability to activate the host cell's Arp 2/3 complex. These two factors work together to initiate and propagate the branched actin polymerization that drives the bacterium's movement.

Therefore, statements A and B correctly describe the reasons why Listeria can assemble host-cell actin for motility.