Abstract
The Type III Secretion Systems (T3SS) needle complex is a conserved syringe-shaped protein translocation nanomachine with a mass of about 3.5 MDa essential for the survival and virulence of many Gram-negative bacterial pathogens. This system is composed of a membrane-embedded basal body and an extracellular needle that deliver effector proteins into host cells. High-resolution structures of the T3SS from different organisms and infection stages are needed to understand the underlying molecular mechanisms of effector translocation. Here, we present the cryo-electron microscopy structure of the isolated Shigella T3SS needle complex. The inner membrane (IM) region of the basal body adopts 24-fold rotational symmetry and forms a channel system that connects the bacterial periplasm with the export apparatus cage. The secretin oligomer adopts a heterogeneous architecture with 16- and 15-fold cyclic symmetry in the periplasmic N-terminal connector and C-terminal outer membrane ring, respectively. Two out of three IM subunits bind the secretin connector via a β-sheet augmentation. The cryo-EM map also reveals the helical architecture of the export apparatus core, the inner rod, the needle and their intervening interfaces.
Highlights
Bacterial diarrheal diseases cause million deaths in children under age five worldwide [1]
The type 3-secretion system (T3SS) needle complex is a syringe-shaped nanomachine consisting of two membrane-embedded ring systems that sheath a central export apparatus and a hollow needle-like structure through which the virulence factors are transported
We present here the structure of the Shigella T3SS needle complex obtained by high-end electron microscopy
Summary
Bacterial diarrheal diseases cause million deaths in children under age five worldwide [1]. The basal body is assembled by the stepwise incorporation of multiple copies of inner- (MxiG and MxiJ in Shigella) and outer- (MxiD in Shigella, which belongs to the family of the secretins) membrane proteins to form their respective rings in the corresponding membranes and periplasm [5,7]. These rings embrace the so-called inner rod and export apparatus core. Several studies suggest that efficient protein translocation requires ATP hydrolysis and a proton motive force [16,17,18,19], but the underlying molecular mechanisms are yet to be discovered
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