Structure-Function Analysis of Inositol Hexakisphosphate-induced Autoprocessing of the Vibrio cholerae Multifunctional Autoprocessing RTX Toxin

Abstract

Vibrio cholerae secretes a large virulence-associated multifunctional autoprocessing RTX toxin (MARTX Vc ). Autoprocessing of this toxin by an embedded cysteine protease domain (CPD) is essential for this toxin to induce actin depolymerization in a broad range of cell types. A homologous CPD is also present in the large clostridial toxin TcdB and recent studies showed that inositol hexakisphosphate (Ins(1,2,3,4,5,6)P 6 or InsP 6 ) stimulated the autoprocessing of TcdB dependent upon the CPD (Egerer, M., Giesemann, T., Jank, T., Satchell, K. J., and Aktories, K. (2007) J. Biol. Chem. 282, 25314 -25321). In this work, the autoprocessing activity of the CPD within MARTX Vc is similarly found to be inducible by InsP 6 . The CPD is shown to bind InsP 6 (K d , 0.6 M), and InsP 6 is shown to stimulate intramolecular autoprocessing at both physiological concentrations and as low as 0.01 M. Processed CPD did not bind InsP 6 indicating that, subsequent to cleavage, the activated CPD may shift to an inactive conformation. To further pursue the mechanism of autoprocessing, conserved residues among 24 identified CPDs were mutagenized. In addition to cysteine and histidine residues that form the catalytic site, 2 lysine residues essential for InsP 6 binding and 5 lysine and arginine residues resulting in loss of activity at low InsP 6 concentrations were identified. Overall, our data support a model in which basic residues located across the CPD structure form an InsP 6 binding pocket and that the binding of InsP 6 stimulates processing by altering the CPD to an activated conformation. After processing, InsP 6 is shown to be recycled, while the cleaved CPD becomes incapable of further binding of InsP 6 .Vibrio cholerae is the etiologic agent of the acute intestinal infection cholera, that remains a world-wide problem with over 200,000 reported and an estimated 1 million actual cases each year (1, 2). To cause illness, V. cholerae colonizes the small intestine, where it secretes its major virulence factor, the ADPribosylating cholera toxin, which elicits massive fluid secretion resulting in the profuse diarrhea that is the hallmark of cholera infection. Nearly all O1, O139, and non-O1/non-O139 clinical isolates of V. cholerae produce another secreted toxin that is the founding member of a new family of bacterial protein toxins called the multifunctional autoprocessing repeats-in-toxins (MARTX) 2 toxins (3-7). In V. cholerae, this toxin has recently been shown to contribute to virulence in mice and is among three secreted factors associated with the ability of V. cholerae to establish an intestinal infection that persists beyond 24 h (8, 9). Hence, MARTX Vc is proposed to function during the earliest stages of human exposure to V. cholerae either to modify the intestinal tract allowing colonization to occur or to reduce the functionality of innate immune cells preventing clearance. The broad distribution of MARTX Vc among environmental isolates further suggests this toxin may have a role in extraintestinal survival (3, 5-7).MAR...

Structural and Molecular Mechanism for Autoprocessing of MARTX Toxin of Vibrio cholerae at Multiple Sites

“…While it is clear that InsP 6 activates the CPD and that autoprocessing is essential for MARTX function (7), the mechanism by which InsP 6 activates CPD is not well understood. Furthermore, only one processing site at Leu 3428 -Ala 3429 has been identified, although multiple processing events would be required to release each effector independently.…”

“…Furthermore, only one processing site at Leu 3428 -Ala 3429 has been identified, although multiple processing events would be required to release each effector independently. In fact, after autoprocessing at Leu 3428 -Ala 3429 , CPD is reported to adopt a conformation with reduced affinity for InsP 6 (7), raising questions as to how the protease might process MARTX at other sites.…”

“…This toxin causes disassembly of the actin cytoskeleton and enhances V. cholerae colonization of the small intestine, possibly by facilitating evasion of phagocytic cells (2, 3). The central region of the V. cholerae MARTX toxin contains four discrete domains: the actin cross-linking domain (ACD) that introduces lysine-glutamate cross-links between actin protomers (4, 5), the Rho-inactivating domain (RID) that disables small Rho GTPases (6), an ␣␤ hydrolase of unknown function (1), and an autoprocessing cysteine protease domain (CPD) (7,8).…”

“…The CPD is a 25-kDa domain found in all MARTX toxins located just before the start of the C-terminal repeats (7,8). This domain is activated for autoproteolysis upon binding inositol hexakisphosphate (InsP 6 ) (7), a molecule ubiquitously present in eukaryotic cell cytosol (9 -11), but absent in extracellular spaces and bacteria.…”

“…This toxin causes disassembly of the actin cytoskeleton and enhances V. cholerae colonization of the small intestine, possibly by facilitating evasion of phagocytic cells (2, 3). The central region of the V. cholerae MARTX toxin contains four discrete domains: the actin cross-linking domain (ACD) that introduces lysine-glutamate cross-links between actin protomers (4, 5), the Rho-inactivating domain (RID) that disables small Rho GTPases (6), an ␣␤ hydrolase of unknown function (1), and an autoprocessing cysteine protease domain (CPD) (7,8).The CPD is a 25-kDa domain found in all MARTX toxins located just before the start of the C-terminal repeats (7,8). This domain is activated for autoproteolysis upon binding inositol hexakisphosphate (InsP 6 ) (7), a molecule ubiquitously present in eukaryotic cell cytosol (9 -11), but absent in extracellular spaces and bacteria.…”