Mechanism of mixed-linkage glucan biosynthesis by barley cellulose synthase-like CslF6 (1,3;1,4)-β-glucan synthase
Date
2022
Authors
Purushotham, P.
Ho, R.
Yu, L.
Fincher, G.B.
Bulone, V.
Zimmer, J.
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Journal article
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Science Advances, 2022; 8(45):1-12
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Pallinti Purushotham, Ruoya Ho, Long Yu, Geoffrey B. Fincher, Vincent Bulone, Jochen Zimmer
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Abstract
Mixed-linkage (1,3;1,4)--glucans, which are widely distributed in cell walls of the grasses, are linear glucose polymers containing predominantly (1,4)--linked glucosyl units interspersed with single (1,3)--linked glucosyl units. Their distribution in cereal grains and unique structures are important determinants of dietary fibers that are beneficial to human health. We demonstrate that the barley cellulose synthase-like CslF6 enzyme is sufficient to synthesize a high–molecular weight (1,3;1,4)--glucan in vitro. Biochemical and cryo–electron microscopy analyses suggest that CslF6 functions as a monomer. A conserved “switch motif” at the entrance of the enzyme’s transmembrane channel is critical to generate (1,3)-linkages. There, a single-point mutation markedly reduces (1,3)-linkage formation, resulting in the synthesis of cellulosic polysaccharides. Our results suggest that CslF6 monitors the orientation of the nascent polysaccharide’s second or third glucosyl unit. Register-dependent interactions with these glucosyl residues reposition the polymer’s terminal glucosyl unit to form either a (1,3)- or (1,4)--linkage.
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© 2022 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to originalU.S. Government Works. Distributed under a Creative Commons Attribution License 4.0 (CC BY).