IDENTIFICATION, CHARACTERIZATION, AND DEVELOPMENTAL EXPRESSION OF A NOVEL ALPHA-2-]8-KDN-TRANSFERASE WHICH TERMINATES ELONGATION OF ALPHA-2-]8-LINKED OLIGO-POLYSIALIC ACID CHAIN SYNTHESIS IN TROUT EGG POLYSIALOGLYCOPROTEINS

A novel glycosyltransferase which catalyses transfer of deaminated neu raminic acid, KDN (2-keto-3-deoxy-D-glycero-D-galacto-nononic acid) fr om CMP-KDN to the non-reducing termini of oligo-polysialyl chains of p olysialoglycoprotein (PSGP), was discovered in the ovary of rainbow tr out (Oncorhynchus mykiss). The KDN-transferase activity was optimal at neutral pH, and stimulated 2 to 2.5-fold by 2-5 mM Mg2+ or Mn2+. Expr ession of KDN-transferase was developmentally regulated in parallel wi th expression of the alpha 2 --> 8-polysialyltransferase, which cataly ses synthesis of the oligo-polysialyl chains in PSGP. Incorporation of the KDN residues into the oligo-polysialyl chains prevented their fur ther elongation, resulting in 'capping' of the oligo-polysialyl chains . This is the first example of a glycosyltransferase that catalyses te rmination of alpha 2 --> 8-polysialylation in glycoproteins.