Required for translocation of effector proteins via the type III secretion system SPI-1, which is essential for an efficient bacterial internalization. Probably acts by modulating the secretion of SipA, SipB, and SipC (By similarity). Secreted via the type III secretion system 1 (SPI-1 TTSS). Localizes to the tip of the external secretion needle that is part of the TTSS apparatus (By similarity). Transcriptionally regulated by SicA and InvF. Also regulated by InvE (By similarity). The N-terminal domain is an intra-molecular chaperone that prevents premature oligomerization of the residues on the coiled-coil region that are involved in interactions with the needle and/or itself. The residues in the C-terminal domain probably form oligomeric structures at the tip of the needle that are responsible for the regulation of secretion of other effectors (By similarity). Belongs to the invasin protein D family.