The Brucella abortus Cyclic β-1,2-Glucan Virulence Factor Is Substituted with O-Ester-Linked Succinyl Residues

Abstract

Brucella periplasmic cyclic ␤-1,2-glucan plays an important role during bacterium-host interaction. Nuclear magnetic resonance spectrometry analysis, thin-layer chromatography, and DEAE-Sephadex chromatography were used to characterize Brucella abortus cyclic glucan. In the present study, we report that a fraction of B. abortus cyclic ␤-1,2-glucan is substituted with succinyl residues, which confer anionic character on the cyclic ␤-1,2-glucan. The oligosaccharide backbone is substituted at C-6 positions with an average of two succinyl residues per glucan molecule. This O-ester-linked succinyl residue is the only substituent of Brucella cyclic glucan. A B. abortus open reading frame (BAB1_1718) homologous to Rhodobacter sphaeroides glucan succinyltransferase (OpgC) was identified as the gene encoding the enzyme responsible for cyclic glucan modification. This gene was named cgm for cyclic glucan modifier and is highly conserved in Brucella melitensis and Brucella suis. Nucleotide sequencing revealed that B. abortus cgm consists of a 1,182-bp open reading frame coding for a predicted membrane protein of 393 amino acid residues (42.7 kDa) 39% identical to Rhodobacter sphaeroides succinyltransferase. cgm null mutants in B. abortus strains 2308 and S19 produced neutral glucans without succinyl residues, confirming the identity of this protein as the cyclic-glucan succinyltransferase enzyme. In this study, we demonstrate that succinyl substituents of cyclic ␤-1,2-glucan of B. abortus are necessary for hypo-osmotic adaptation. On the other hand, intracellular multiplication and mouse spleen colonization are not affected in cgm mutants, indicating that cyclic-␤-1,2-glucan succinylation is not required for virulence and suggesting that no lowosmotic stress conditions must be overcome during infection.Brucella spp. are gram-negative, facultative intracellular bacteria that cause a chronic zoonotic disease known worldwide as brucellosis. Brucella abortus, the etiological agent of bovine brucellosis, causes abortion and infertility in cattle and undulant fever in humans (17). The human disease is also caused by Brucella melitensis, Brucella suis, and Brucella canis. Brucella spp. are intracellular pathogens that invade and proliferate within host cells; virulence is associated with the ability to multiply inside professional and nonprofessional phagocytic cells (40).Cyclic glucans appear to be important as intrinsic components of gram-negative bacterial cell envelopes (7). Beyond their common features, glucans show an unexpected structural diversity (4). The role of cyclic glucan in gram-negative bacterial cell envelopes is still controversial. Cyclic-glucan mutants have a highly pleiotropic phenotype, suggesting an overall alteration of their cell envelope properties. Mutants deficient in cyclic-glucan production were found to be attenuated or avirulent compared to their wild-type parent strains, suggesting that the cyclic glucan is an important virulence attribute for the bacteria in their interaction with eukaryotic host...

A glycosyltransferase with a length-controlling activity as a mechanism to regulate the size of polysaccharides

“…Moreover, we have previously observed that a B. abortus cgm mutant unable to succinylate glucans produced only neutral C␤G with the same size distribution as the wild-type strain (2).These evidences ruled out the possibility that mutants in the C-terminal region were affected in C␤G modification.…”

“…Removal of substituents from the wild-type anionic C␤G fraction recovered from the TLC plates by mild alkaline treatment yielded neutral glucans with the same DP of those of the nonsubstituted fraction, thus indicating that succinylation was not affected by ring size (2). Moreover, we have previously observed that a B. abortus cgm mutant unable to succinylate glucans produced only neutral C␤G with the same size distribution as the wild-type strain (2).These evidences ruled out the possibility that mutants in the C-terminal region were affected in C␤G modification.…”

“…Common features of these oligosaccharides are the presence of glucose as the sole sugar constituent and the regulation of their synthesis by the osmolarity of the growth media. Osmoregulated periplasmic glucans may be cyclic, branched cyclic, or branched linear and, depending on the species, may be substituted with a variety of nonglycosidic residues (1,2). Agrobacterium, Rhizobium, Sinorhizobium, and Brucella species synthesize osmoregulated periplasmic glucans of family II.…”

“…Agrobacterium, Rhizobium, Sinorhizobium, and Brucella species synthesize osmoregulated periplasmic glucans of family II. Glucans of this family have 17-25 glucose residue cyclic ␤-1,2-backbones substituted with sn-1-phosphoglycerol, succinic acid, methylmalonic acid, or a combination of them (1)(2)(3).…”

Periplasmic glucans isolated from Proteobacteria

“…In addition, the backbone of the PGs is substituted at the C-6 positions with an average of two succinyl resides per molecule. Moreover, the succinyl substituents of cyclic β-(1,2)-glucans in B. abortus are known to be necessary for hypo-osmotic adaptation (28). In Family IV, novel PGs substituted with non-sugar residues http://bmbreports.org BMB reports have recently been found in X. campestris, R. solanacearum and R. sphaeroides.…”