Small‐Angle X‐Ray Scattering Analysis of Pneumococcal Lipoteichoic Acid Phase Structure

Abstract

X-Ray-scattering analysis was performed on micelles of lipoteichoic acid from Streptococcus pneumoniae. This lipoteichoic acid differs from poly(g1ycerophosphate) and poly(glycosylg1ycerophosphate) lipoteichoic acids by a unique positively charged diacylglyceroglycolipid anchor and a complex structure of the hydrophilic chain which is composed of zwitterionic tetrasaccharide ribitol phosphate repeats each carrying two zwitterionic phosphocholine substituents. The size distribution of the lipoteichoic acid micelles was sufficiently homogenous to determine their size and some related molecular parameters by small-angle scattering analysis. Nearly independent on the ionic strength of the aqueous dispersion, an average micelle contained about 150 lipoteichoic acid molecules arranged in a spherical assembly with a diameter of about 23 nm, whereby the hydrophilic region occupied an outer shell of about 8.5 nm thick- A comparison with the micelle of poly(g1ycerophosphate) lipoteichoic acid of Staphylococcus aureus suggests that the supramolecular structure is largely independent of the structure of the hydrophilic chain and solely dictated by the small cross-sectional area of the diacylglycerol moiety common to all lipoteichoic acids and lipoglycans of gram-positive bacteria.Keywords : lipoteichoic acid ; small-angle X-ray scattering ; micelle ; Streptococcus pneumoniae.Lipoteichoic acids and lipoglycans are lipid macroamphiphiles in the cytoplasmic membrane of numerous gram-positive bacteria. Lipoteichoic acids occur in one subdivision in which the DNA contains less than 5 5 % guanine + cytosine, whereas lipoglycans preferentially appear associated with the other subdivision in which the guanine + cytosine content of DNA is higher than 55% (Fischer, 1990(Fischer, , 1994Sutcliffe, 1994). Most widespread is the classical type of lipoteichoic acid which possesses a single unbranched 1,3-linked poly(g1ycerophosphate) chain attached by a phosphodiester bond to a diacylglyceroglycolipid (Fig. 1). The glycerophosphate residues are at position 2 partially substituted with D-alanine ester and in certain lipoteichoic acids additionally with glycosyl residues. Poly(g1yco-sylglycerophosphate) lipoteichoic acids constitute a second less widespread type in which a monosaccharide or disaccharide is intercalated between the glycerophosphate residues (Koch and Fischer, 1978 ;Fischer, 1994). The largest modification of the basic structure shows the lipoteichoic acid of Streptococcus pneurnoniae (Behr et al., 1992). The glycerophosphate moieties are replaced by ribitol phosphate residues (Fig. 1). Intercalated between them is a tetrasaccharide which contains D-ghCOSe, the positively charged 2-acetamido-4-amino-2,4,6-trideoxy-~-galactose, and two N-acetyl-D-galactosaminy] residues each of which is substituted at 0 6 with a zwitterionic phosphocholine residue. Recently we studied the supramolecular structure of lipoteichoic acids in aqueous dispersion by X-ray diffraction analysis and found that variously substituted poly(g1ycerophospha...

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