Component of the type III secretion (T3S) injectisome that translocates effector toxins into host cells, facilitating the establishment and dissemination of infection. Polymerizes into flexible and regularly twisted fibrils and plays an essential role in needle assembly. Homomultimer (through its C-terminal region). Deletion mutants display a reduced cytotoxicity toward macrophages. Belongs to the YscI/HrpB family.