Analysis of functional domains present in the N-terminus of the SipB protein.

SipB (593 aa), one of the Salmonella invasion proteins (Sips), is secreted via the Salmonella pathogenicity island 1 (SPI-1) type III secretion system (T3SS). Here, we report the delineation of several functional regions present in the SipB protein. Our data show that residues 3-8 of the SipB protein are essential for its secretion from the bacterial cell and that the SicA chaperone, which is important to ensure stability of SipB and SipC in the bacterial cytosol, binds to SipB somewhere between amino acids 80 and100 of the SipB N-terminal region. Interestingly, the N-terminal region (residues 1-160) of SipB (SipB160) cannot be secreted via the SPI-1 T3SS, but fusion of the C-terminal amphipathic region (residues 300-593) to SipB160 can restore secretion via this system.