Actin-based motility

Actin assembly-inducing protein
EVH1 domain-ActA peptide complex
Identifiers
Symbol	ActA
Entrez	2798121
UniProt	P33379

The Actin assembly-inducing protein (ActA) is a protein encoded and used by Listeria monocytogenes to propel itself through a mammalian host cell. ActA is a bacterial surface protein comprising a membrane-spanning region. In a mammalian cell the bacterial ActA interacts with the Arp2/3 complex and actin monomers to induce actin polymerization on the bacterial surface generating an actin comet tail. The gene encoding ActA is named actA or prtB.

As soon as L. monocytogenes bacteria are ingested by humans, they get internalized into intestinal epithelium cells and rapidly try to escape their internalization vacuole. In the cytosol they start to polymerize actin on their surface by the help of the ActA protein. It has been shown that ActA is not only necessary but also sufficient to induce motility of bacteria in the absence of other bacterial factors.

ActA was discovered by analysing lecithinase-negative Tn917-lac Listeria mutants because of the phenotype that they were unable to spread from cell to cell. These mutant bacteria still escaped from the phagosomes as efficiently as wild-type bacteria and multiplied within the infected cells but they were not surrounded by actin like wild-type bacteria. Further analysis showed, that Tn917-lac had inserted into actA, the second gene of an operon. The third gene of this operon, plcB, encodes the L. monocytogenes lecithinase. To determine whether actA itself, plcB or other co-transcribed downstream regions are involved in actin assembly, mutations in the appropriate genes were generated. All mutants except the actA mutants were similar to wild-type concerning association with F-actin and cell-cell spreading. Complementation with actA restored wild-type phenotype in the actA mutants.

ActA is a protein which acts as a mimic of Wiskott-Aldrich syndrome protein (WASP), a nucleation promoting factor (NPF) present in host cells. NPFs in the mammalian cell recruit and bind to the already existing actin-related-protein 2 and 3 complex (Arp2/3 complex) and induce an activating conformational change of the Arp2/3 complex. Due to this conformational change, NPFs initiate polymerization of a new actin filament at a 70° angle, which leads to the characteristic Y-branched actin structures in the leading edge of motile cells. ActA localizes to the old pole of the bacterium and spans both the bacterial cell membrane and the cell wall, lateral diffusion is inhibited; thus ActA localizes in a polarized and anchored manner on the bacterial surface. Consequently, actin polymerization only starts in this region on the surface of the bacterium. Expression of ActA is induced only after entering a mammalian host cell.