Teichoic acid (TA), a crucial cell wall constituent of the pathobiont Streptococcus pneumoniae, is bound to peptidoglycan (wall teichoic acid, WTA) or to membrane glycolipids (lipoteichoic acid, LTA). Both TA polymers share a common precursor synthesis pathway, but differ in the final transfer of the TA chain to either peptidoglycan or a glycolipid. Here, we show that LTA exhibits a different linkage conformation compared to WTA, and identify TacL (previously known as RafX) as a putative lipoteichoic acid ligase required for LTA assembly. Pneumococcal mutants deficient in TacL lack LTA and show attenuated virulence in mouse models of acute pneumonia and systemic infections, although they grow normally in culture. Hence, LTA is important for S. pneumoniae to establish systemic infections, and TacL represents a potential target for antimicrobial drug development.
Members of the Mitis group of streptococci possess teichoic acids (TAs) as integral components of their cell wall that are unique among Gram-positive bacteria. Both, lipoteichoic (LTA) and wall teichoic acid, are formed by the same biosynthetic pathway, are of high complexity and contain phosphorylcholine (P-Cho) residues. These residues serve as anchors for choline-binding proteins (CBPs), some of which have been identified as virulence factors of the human pathogen Streptococcus pneumoniae. We investigated the LTA structure of its close relative Streptococcus oralis. Our analysis revealed that S. oralis Uo5 LTA has an overall architecture similar to pneumococcal LTA (pnLTA) and can be considered as a subtype of type IV LTA. Its structural complexity is even higher than that of pnLTA and its composition differs in number and type of carbohydrate moieties, inter-residue connectivities and especially the P-Cho substitution pattern. Here, we report the occurrence of a saccharide moiety substituted with two P-Cho residues, which is unique as yet in bacterial derived surface carbohydrates. Finally, we could link the observed important structural variations between S. oralis and S. pneumoniae LTA to the divergent enzymatic repertoire for their TA biosynthesis.
The bacterial lung pathogen has a unique nutritional requirement for exogenous choline and attaches phosphorylcholine (-Cho) residues to the GalNAc moieties of its teichoic acids (TAs) in its cell wall. Two phosphorylcholine transferases, LicD1 and LicD2, mediate the attachment of -Cho to the O-6 positions of the two GalNAc residues present in each repeating unit of pneumococcal TAs (pnTAs), of which only LicD1 has been determined to be essential. At the molecular level, the specificity of the -Cho attachment to pnTAs by LicD1 and LicD2 remains still elusive. Here, using detailed structural analyses of pnTAs from a LicD2-deficient strain, we confirmed the specificity in the attachment of-Cho residues to pnTA. LicD1 solely transfers -Cho to α-d-GalNAc moieties, whereas LicD2 attaches -Cho to β-d-GalNAc. Further, we investigated the role of the pneumococcal phosphorylcholine esterase (Pce) in the modification of the -Cho substitution pattern of pnTAs. To clarify the specificity of Pce-mediated-Cho hydrolysis, we evaluated different concentrations and pH conditions for the treatment of pneumococcal lipoteichoic acid with purified Pce. We show that Pce can hydrolyze both -Cho residues of the terminal repeat of the pnTA chain and almost all-Cho residues bound to β-d-GalNAc However, hydrolysis was restricted to the terminal repeat. In summary, our findings indicate that LicD1 and LicD2 specifically transfer -Cho to α-d-GalNAc and β-d-GalNAc moieties, respectively, and that Pce removes distinct -Cho substituents from pnTAs.
In Table 1 the 1 H NMR resonances for glycerol (Gro) positions 2 and 3 are incorrect. In the corresponding Figure 5A the respective signals have been assigned correctly. The correct chemical shift of Gro2 in 1 H NMR is δ H 3.97-3.94*, for Gro3′ and Gro3 δ H 3.70-3.66* and δ H 3.62-3.58*, respectively.
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