Multiantennary group-specific polysaccharide of group B streptococcus

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Abstract
The group-specific antigen of group B Streptococcus is composed of four different oligosaccharide units of Mw 766 (III), 1277 (II), 1462 (IV), and 1788 (I). The major constituent sugars of the oligosaccharides are .alpha.-L-rhamnopyranose, .alpha.-D-galactopyranose, 2-acetamido-2-deoxy-.beta.-D-glucopyranose, and D-glucitol except that III does not contain .alpha.-D-galactopyranosyl or 2-acetamido-2-deoxy-.beta.-D-glucopyranosyl residues and IV contains no D-glucitol but has one additional .beta.-L-rhamnopyranosyl residue. The structures of II and III have been previously elucidated [Michon, F., Katzenellenbogen, E., Kasper, D. L., and Jennings, H. J. (1987) Biochemistry 26, 476-486]. In the group B antigen all the oligosaccharides are linked by one type of phosphodiester bond from O6 of the D-glucitol residue of one oligosaccharide to O6 of the .alpha.-D-galactopyranosyl residue of the next to form a complex and highly branched multiantennary structure. However, despite the heterogeneous nature of its component oligosaccharides, some order has been identified in the biosynthesis of the group B antigen from chemical and enzymatic sequence studies. Because III lacks an .alpha.-D-galactopyranosyl residue but has a D-glucitol residue, it is situated at the nonreducing terminus of all the branches of the group B antigen where it is always adjacent to a II moiety. Conversely, IV has an .alpha.-D-galactopyranosyl residue but has no D-glucitol and is therefore located at the reducing terminus of the group B antigen where it probably functions as a linker molecule between the group B polysaccharide and the cell wall peptidoglycan of the group B streptococcal organisms. Oligosaccharide I contains two .alpha.-D-galactopyranosyl residues and one D-glucitol residue and thus constitutes the branch point in the group B antigen, whereas II contains one of each of the above residues and therefore is situated in linear interchain positions. The group B antigen is highly branched and probably has a unique multiantennary structure.

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