A glycosyltransferase with a length-controlling activity as a mechanism to regulate the size of polysaccharides.

CIOCCHINI, ANDRÉS E.; GUIDOLIN, L. SOLEDAD; CASABUONO, ADRIANA C.; COUTO, ALICIA S.; IÑÓN DE IANNINO, NORA; UGALDE, RODOLFO A.

PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA

Stanford University's Highwire Press

Año: 2007 vol. 104 p. 16492 - 16497

0027-8424

Cyclic beta-1,2-glucans are osmolite homopolysaccharides with a cyclic beta-1,2-glucan backbone of 17 to 25 glucose residues present in the periplasmic space of several bacteria. Initiation, elongation and cyclization, the three distinctive reactions required for building the cyclic structure, are catalyzed by the same protein, the cyclic beta-1,2-glucan synthase (Cgs). The initiation activity catalyzes the transference of the first glucose from uridine diphosphate(UDP)-glucose to a yet unidentified amino acid residue in the same protein. Elongation proceeds by the successive addition of glucose residues from UDP-glucose to the non-reducing end of the protein-linked beta-1,2-oligosaccharide intermediate. Finally, the protein-linked intermediate is cyclized and the cyclic glucan released from the protein. These reactions do not explain, however, the mechanism by which the number of glucose residues in the cyclic structure is controlled. We now report that control of the degree of polymerization (DP) is carried out by a beta-1,2-glucan phosphorylase present at the Cgs C-terminal domain. This last activity catalyzes the phosphorolysis of the beta-1,2-glucosidic bond at the non-reducing end of the linear protein-linked intermediate releasing glucose 1-phosphate. The DP is thus regulated by this ?length-controlling? phosphorylase activity. To our knowledge, this is the first description of a control of the DP of homopolysaccharides.