The remarkable mechanical strength of cellulose reflects the arrangement of multiple b-1,4-linked glucan chains in a paracrystalline\nfibril. During plant cellulose biosynthesis, a multimeric cellulose synthesis complex (CSC) moves within the plane\nof the plasma membrane as many glucan chains are synthesized from the same end and in close proximity. Many questions\nremain about the mechanism of cellulose fibril assembly, for example must multiple catalytic subunits within one CSC\npolymerize cellulose at the same rate? How does the cellulose fibril bend to align horizontally with the cell wall? Here we\nused mathematical modeling to investigate the interactions between glucan chains immediately after extrusion on the\nplasma membrane surface. Molecular dynamics simulations on groups of six glucans, each originating from a position\napproximating its extrusion site, revealed initial formation of an uncrystallized aggregate of chains from which a protofibril\narose spontaneously through a ratchet mechanism involving hydrogen bonds and van der Waals interactions between\nglucose monomers. Consistent with the predictions from the model, freeze-fracture transmission electron microscopy using\nimproved methods revealed a hemispherical accumulation of material at points of origination of apparent cellulose fibrils\non the external surface of the plasma membrane where rosette-type CSCs were also observed. Together the data support\nthe possibility that a zone of uncrystallized chains on the plasma membrane surface buffers the predicted variable rates of\ncellulose polymerization from multiple catalytic subunits within the CSC and acts as a flexible hinge allowing the horizontal\nalignment of the crystalline cellulose fibrils relative to the cell wall.
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