User:Cellular Biochemistry II/Course 2010/Article 10

ActA is a protein used by Listeria monocytogenes to propel itself through a host cell. ActA interacts with the Arp2/3 complex and Actin to locally increase actin polymerization and generate an actin "comet tail".

Introduction
The facultative intracellular pathogenic bacterium L. monocytogenes initiates actin filament polymerization at its cell surface after it gains access to the cytosol of infected host cells. Actin polymerization is tightly coupled to intracellular bacterial motility, the physically cross-linked actin filaments lead to the formation of tail-like structures at one bacterial pole and it is this actin polymerization that provides the locomotive force in intracellular motility. Actin polymerization at the L. monocytogenes surface is mediated by bacterial and host cell factors. The only essential bacterial factor is ActA, a cell surface protein that recruits host cell factors that promote actin assembly. A critical host factor is the Arp 2/3 complex, which promotes actin assembly at the bacterial surface. Genetic analysis in yeast demonstrated that Arp 2/3 complex is essential for actin function and cell variety.

Structure
ActA is a 639-amino acid protein that is anchored in the bacterial membrane by the carboxy-terminal. The amino-terminal is exposed on the bacterial surface. ActA has a calculated molecular mass of 67kDa.

Function and mechanism
The ActA-Protein is asymmetrically distributed on the bacterial surface. It binds the Arp 2/3 complex, and the resulting activation of this complex initiates the novo actin polymerization at the bacterial surface. The polymerization is further promoted by VASP (Vasodilator stimulated phosphoprotein), which recruits profilin that is bound to active ATP monomers, and thereby increases the local concentration of actin monomers. So the VASP protein is dispensable for for inititaiation of actin assembly on the bacterial surface. On the other hand, the N-terminal of the ActA protein was found to be absolutely essential for actin recruitment.

Before encountering ActA, the Arp 2/3 complex only weakly enhances the kinetics of actin polymerization. Upon interacting with the NH2-terminal of ActA, the activity of the complex is stimulated and it nucleates actin assembly generating actin filaments.whose elongation propels the bacterium forward. Interaction with ActA may induce a conformational change in the Arp 2/3 complex. Alternatively, the complex may be activated by self-association facilitated by ActA, which is a dimer on bacterial surface. In either case, the activation of the Arp 2/3 complex needs the spatially localized ActA factor.

Isogenic mutants of L. monocytogenes lacking the surface-bound ActA polypeptide no longer interact with cytoskeletal elements and are, as a consequence, non-motile. The ActA polypeptide acts as a nucleator of the actin cytoskeleton and provides the first insights into the molecular nature of such controlling elements in microfilament organization.