Essential for bacterial spreading by eliciting polar deposition of filamentous actin (actin-based motility). Inside the host cell mediates nucleation and polymerization of actin molecules on the bacterial surface, which provides the propulsive force for intracellular movement and intercellular dissemination of the bacterium. During invasion of mammalian cells, triggers autophagy by binding to APG5L. Interaction with IcsB leads to escape from the autophagic host defense system. Also binds ATP and displays weak ATPase activity. Forms a ternary complex with host WASL (neural Wiskott-Aldrich syndrome protein) and ARP2/3. IcsA enhances the affinity of WASL for ARP2/3, thus assembling a tight complex which has maximal activity in actin assembly. Also binds directly to host vinculin, which may serve as an actin filament recruiter. The alpha domain is localized as a cap over the old pole of the bacterial surface. The cleaved C-terminal fragment (autotransporter domain) is localized in the outer membrane. Transcriptionally regulated by VirF. The signal peptide, cleaved at the inner membrane, guides the autotransporter protein to the periplasmic space. Then, insertion of the C-terminal translocator domain (or beta domain) in the outer membrane forms a hydrophilic pore for the translocation of the passenger domain (or alpha domain) to the bacterial cell surface, where IcsA is cleaved by IcsP (SopA) and released into the extracellular medium. This cleavage is important for the maintenance of the sharply polarized distribution of IcsA but is not a prerequisite for intracellular spreading. The glycine-rich domain interacts with the WASL CRIB domain. Phosphorylated by host cAMP-dependent protein kinase, which may represent a host defense mechanism during the invasion process. Secretion across inner membrane is dependent on the SecA and SecYEG apparatus. However, the polar localization comes before secretion and is independent of SecA. Deletion experiments show that amino acids 58-103, 505-537 and 507-729 are required for polar localization and that amino acids 102-779 are responsible for interaction with APG5L and IcsB.