Structural organization of the needle complex of the type III secretion apparatus of Shigella flexneri

Abstract

The secretion apparatus known as the needle complex (NC) from the bacterium Shigella flexneri was studied by single particle electron microscopy. The isolated intact NC appears in projection to be composed of a basal body consisting of seven rings and a protruding needle appendage. A comparison of averaged projections of the intact NC and its fragments revealed the organization of the NC into several major subcomplexes. One of these lacks an inner membrane ring of the basal body but still presents the needle appendage attached to four upper rings. The position of the needle appendage within these rings is variable, suggesting that the dissociated component is necessary for stabilizing the needle appendage. Averaged images of the subcomplex lacking the inner membrane basal rings show a thicker extension at the base of the needle appendage, called the socket. This socket was also found to be present in images of the basal body fragment isolated from mutants lacking the mxiH and mxiI genes. This suggests that the socket is not composed of MxiH and MxiI subunits, which form the needle appendage. A symmetry analysis of the basal body top view projections indicated that a peripheral protein component of the inner membrane ring is present in a ring with 24 copies, in contrast to the Salmonella typhimurium NC. A model is presented in which the NC is only associated to the outer- and inner-membranes with its first and seventh ring, respectively.

Protein Export According to Schedule: Architecture, Assembly, and Regulation of Type III Secretion Systems from Plant- and Animal-Pathogenic Bacteria

“…and Shigella flexneri, respectively. Both systems consist of ring structures in the IM and OM that enclose a transport channel with an inner diameter of 2 to 3 nm (45,139,179,291,348,478) (Fig. 2 and 3).…”

“…The T3S needle from animal pathogens is significantly shorter than the pilus from plant pathogens and often has a determined length, which varies from 40 to approximately 80 nm in different pathogens (45,224,257,291,540). Needle assembly is probably initiated in the periplasm and occurs at the tip of the needle (444,478). Since purified needle proteins (members of the YscF family) build structures of several micrometers, the regulation of needle length likely depends on the context of the T3S system (444,447) (see below).…”

“…The inner rod of the T3S system from Salmonella spp. is connected by a socket-like structure to the IM rings and might be required for stable anchoring of the extracellular needle, which probably protrudes into the periplasm as revealed by single-particle EM (347,348,478,605). Experimental evidence for an inner rod structure was also reported for the T3S system from EPEC as well as for flagellar T3S systems (410), but the presence of this structure has not yet been confirmed.…”

The type III secretion injectisome, a complex nanomachine for intracellular ‘toxin’ delivery

“…gellar basal body. This structure, called the needle complex (NC) or injectisome basal body, consists of two pairs of rings that span the inner (IM) and outer (OM) bacterial membranes, joined together by a narrower cylinder and terminated by a needle, filament or pilus (Kubori et al, 1998;Blocker et al, 1999;Kimbrough and Miller, 2000;Daniell et al, 2001;Jin and He, 2001;Sekiya et al, 2001;MoritaIshihara et al, 2006;Sani et al, 2007;Hodgkinson et al, 2009).…”

“…Single particles of NCs from S. enterica SPI-1 (Marlovits et al, 2004(Marlovits et al, , 2006 and from S. flexneri (Blocker et al, 2001;Sani et al, 2007;Hodgkinson et al, 2009) were characterized by cryo-electron microscopy (cryo-EM). These reconstitutions show that an inner rod with a central channel of approximately 2-3 nm is anchored at the basis of the chamber by a socket-like structure and traverses the chamber of the NC.…”