Molecular directionality of beta-chitin biosynthesis

The molecular packing in beta-chitin unit cells was experimentally determin ed by a combination of unidirectional degradation by Bacillus circulans chi tinase Al and microdiffraction electron crystallography using highly crysta lline beta-chitin microfibrils from the protective tubes secreted by Lamell ibrachia satsuma. The mode of chain packing was found to be identical with that of the previously published crystal model for beta-chitin, despite a c ontroversial definition of the unit cell parameters. Here, a "parallel-down " packing was determined, where the reducing ends of chains point in an opp osite direction to the crystallographic c-axis. Microdiffraction analyses o f nascent beta-chitin microfibrils generated from diatom Thalassiosira sp. showed that the c-axis of the crystal was directed toward the diatoms, and therefore the reducing end of a growing chain pointed away from the locus o f biosynthesis. This mechanism agreed well with what we found recently in t he cellulose biosynthesis system, and provides strong evidence that the pol ymerization by the processive glycosyl transferase takes place at the non-r educing end of the growing polysaccharide chains. (C) 1999 Academic Press.