Novel fold of VirA, a type III secretion system effector protein from Shigella flexneri

Abstract

VirA, a secreted effector protein from Shigella sp., has been shown to be necessary for its virulence. It was also reported that VirA might be related to papain-like cysteine proteases and cleave a-tubulin, thus facilitating intracellular spreading. We have now determined the crystal structure of VirA at 3.0 Å resolution. The shape of the molecule resembles the letter ''V,'' with the residues in the N-terminal third of the 45-kDa molecule (some of which are disordered) forming one clearly identifiable domain, and the remainder of the molecule completing the V-like structure. The fold of VirA is unique and does not resemble that of any known protein, including papain, although its N-terminal domain is topologically similar to cysteine protease inhibitors such as stefin B. Analysis of the sequence conservation between VirA and its Escherichia coli homologs EspG and EspG2 did not result in identification of any putative protease-like active site, leaving open a possibility that the biological function of VirA in Shigella virulence may not involve direct proteolytic activity.

EseG, an Effector of the Type III Secretion System of Edwardsiella tarda , Triggers Microtubule Destabilization

“…The Shigella effector VirA also interacts with ␣-and ␤-tubulin heterodimers but induces microtubule destabilization through an ␣-tubulin-specific cysteine protease-like activity (51,52,53). Interestingly, VirA is not a protease, nor does it directly sever or destabilize microtubules (11,15).…”

Actin Cytoskeleton Manipulation by Effector Proteins Secreted by DiarrheagenicEscherichia coliPathotypes

“…VirA degrades α -tubulin, thus creating a “tunnel” in the dense intracellular microtubule network. However, a large body of evidence, including structural studies, has now shown that VirA is not a cysteine protease and does not directly destabilize microtubules [60, 61]. Intracellular motility, replication, and survival further depend on the T3SS substrate IcsB, that protects the bacteria from being recognized and entrapped by the host cell autophagy machinery [62].…”

Protein Flexibility and Coiled-Coil Propensity: New Insights Into Type III and Other Bacterial Secretion Systems

“…The 129 residue N-terminal domain has two functions: the first 70 residues contain the CBD domain for chaperone SycH, while the full 129 N-terminal domain binds to phosphotyrosine-containing proteins and adopts an overall globular fold (Khandelwal et al, 2002). The N-terminal region of VirA is partially disordered (Davis et al, 2008). Apart from the N-terminal disorder, a propensity for overall disorder is predicted for T3SS effectors, which may reflect an increased structural flexibility.…”