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Discovery of a wide variety of α-1,6-cyclized β-1,2-glucan synthases: a new entrance for host-microbe interactions.

Motouchi, S.; Kim, I.; Ishiguro, A.; Niu, K.; Komba, S.; Kashima, T.; Kitano, Y.; Nakai, H.; Nakajima, M.

2025-04-21 biochemistry
10.1101/2025.04.21.649730 bioRxiv
Show abstract

-1,6-Cyclized {beta}-1,2-glucan (C{beta}G) composed of 16 glucose units is essential for pathogenicity of Xanthomonas species that are phytopathogens as an immune avoidance factor. C{beta}G synthase from Xanthomonas was found in glycoside hydrolase family 186 (GH186), a phylogenetically new enzyme group that was identified recently. Structural analyses of the enzyme suggested extensive diversity of reaction products in GH186. In this study, we found that 12 homologs from proteobacteria covering major groups in GH186 synthesized C{beta}Gs with various specific chain lengths. Cyclic {beta}-1,2-gluco-tridecaose and octadecaose produced by Ralstonia pseudosolanacearum, a major phytopathogen, and Cereibacter sphaeroides, a photosynthetic bacterium, respectively, were identified, and new types of C{beta}Gs (cyclic {beta}-1,2-gluco-dodecaose, henicosaose and nonacosaose) were produced by the homologs from a root nodule bacterium (Bradyrhizobium diazoefficiens), soil bacteria and marine bacteria. Structural analyses suggested that the small region of two or three residues in Loop X is the principal determinant of DP ranges of the products. Combination with co-determinants achieves specificity to the particular DPs. This study clearly indicated potential ability for various proteobacteria to synthesize particular C{beta}Gs.

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