Crystallization of phosphatidylinositol-specific phospholipase C from Bacillus cereus

Bacillus cereus: Epidemiology, Virulence Factors, and Host-Pathogen Interactions

The toxin-producing bacterium Bacillus cereus is an important and neglected human pathogen and a common cause of food poisoning. Several toxins have been implicated in disease, including the pore-forming toxins hemolysin BL (HBL) and nonhemolytic enterotoxin (NHE). Recent work revealed that HBL binds to the mammalian surface receptors LITAF and CDIP1 and that both HBL and NHE induce potassium efflux and activate the NLRP3 inflammasome, leading to pyroptosis. These mammalian receptors, in part, contribute to inflammation and pathology. Other putative virulence factors of B. cereus include cytotoxin K, cereulide, metalloproteases, sphingomyelinase, and phospholipases. In this review, we highlight the latest progress in our understanding of B. cereus biology, epidemiology, and pathogenesis, and discuss potential new directions for research in this field.

Probing the roles of active site residues in phosphatidylinositol-specific phospholipase C from Bacillus cereus by site-directed mutagenesis

The role of amino acid residues located in the active site pocket of phosphatidylinositol-specific phospholipase C (PI-PLC) from Bacillus cereus[Heinz, D. W., Ryan, M., Bullock, T., & Griffith, O. H. (1995) EMBO J. 14, 3855-3863] was investigated by site-directed mutagenesis, kinetics, and crystal structure analysis. Twelve residues involved in catalysis and substrate binding (His32, Arg69, His82, Gly83, Lys115, Glu117, Arg163, Trp178, Asp180, Asp198, Tyr200, and Asp274) were individually replaced by 1-3 other amino acids, resulting in a total number of 21 mutants. Replacements in the mutants H32A, H32L, R69A, R69E, R69K, H82A, H82L, E117K, R163I, D198A, D198E, D198S, Y200S, and D274S caused essentially complete inactivation of the enzyme. The remaining mutants (G83S, K115E, R163K, W178Y, D180S, Y200F, and D274N) exhibited reduced activities up to 57% when compared with wild-type PI-PLC. Crystal structures determined at a resolution ranging from 2.0 to 2.7 A for six mutants (H32A, H32L, R163K, D198E, D274N, and D274S) showed that significant changes were confined to the site of the respective mutation without perturbation of the rest of the structure. Only in mutant D198E do the side chains of two neighboring arginine residues move across the inositol binding pocket toward the newly introduced glutamic acid. An analysis of these structure-function relationships provides new insight into the catalytic mechanism, and suggests a molecular explanation of some of the substrate stereospecificity and inhibitor binding data available for this enzyme.

Crystal structure of phosphatidylinositol-specific phospholipase C from Bacillus cereus in complex with glucosaminyl(alpha 1-->6)-D-myo-inositol, an essential fragment of GPI anchors

Numerous proteins on the external surface of the plasma membrane are anchored by glycosylated derivatives of phosphatidylinositol (GPI), rather than by hydrophobic amino acids embedded in the phospholipid bilayer. These GPI anchors are cleaved by phosphatidylinositol-specific phospholipases C (PI-PLCs) to release a water-soluble protein with an exposed glycosylinositol moiety and diacylglycerol, which remains in the membrane. We have previously determined the crystal structure of Bacillus cereus PI-PLC, the enzyme which is widely used to release GPI-anchored proteins from membranes, as free enzyme and also in complex with myo-inositol [Heinz, D.W., Ryan, M. Bullock, T.L., & Griffith, O. H. (1995) EMBO J. 14, 3855-3863]. Here we report the refined 2.2 A crystal structure of this enzyme complexed with a segment of the core of all GPI anchors, glucosaminyl(alpha 1-->6)-D-myo-inositol [GlcN-(alpha 1-->6)Ins ]. The myo-inositol moiety of GlcN(alpha 1-->6)Ins is well-defined and occupies essentially the same position in the active site as does free myo-inositol, which provides convincing evidence that the enzyme utilizes the same catalytic mechanism for cleavage of PI and GPI anchors. The myo-inositol moiety makes several specific hydrogen bonding interactions with active site residues. In contrast, the glucosamine moiety lies exposed to solvent at the entrance of the active site with minimal specific protein contacts. The glucosamine moiety is also less well-defined, suggesting enhanced conformational flexibility. On the basis of the positioning of GlcN(alpha 1-->6)Ins in the active site, it is predicted that the remainder of the GPI-glycan makes little or no specific interactions with B. cereus PI-PLC. This explains why B. cereus PI-PLC can cleave GPI anchors having variable glycan structures.