Structure and composition of the Shigella flexneri 'needle complex', a part of its type III secreton

Type III secretion systems (ITSSs or secretons), essential virulence determ inants of many Gramnegative bacteria, serve to translocate proteins directl y from the bacteria into the host cytoplasm. Electron microscopy (EM) indic ates that the TTSSs of Shigella flexneri are composed of: (1) an external n eedle; (2) a transmembrane domain; and (3) a cytoplasmic bulb. EM analysis of purified and negatively stained parts 1, 2 and a portion of 3 of the TTS S, together termed the 'needle complex' (NC), produced an average image at 17 Angstrom resolution in which a base, an outer ring and a needle, inserte d through the ring into the base, could be discerned. This analysis and cry oEM images of NCs indicated that the needle and base contain a central 2-3 nm canal, Five major NC components, MxiD, MxiG, MxiJ, MxiH and Mxil, were i dentified by N-terminal sequencing, MxiG and MxiJ are predicted to be inner membrane proteins and presumably form the base. MxiD is predicted to be an outer membrane protein and to form the outer ring. MxiH and Mxil are small hydrophilic proteins, Mutants lacking either of these proteins formed need leless secretons and were unable to secrete Ipa proteins, As MxiH was prese nt in NCs in large molar excess, we propose that it is the major needle com ponent. Mxil may cap at the external needle tip.