Elongation of glycosaminoglycan chains', such as heparan and chondroitin, is catalyzed by bi-functional glycosyltransferases, for which both 3-dimensional structures and reaction mechanisms remain unknown. The bacterial chondroitin polymerase K4CP catalyzes elongation of the chondroitin chain by alternatively transferring the GlcUA and GalNAc moiety from UDP-GlcUA and UDP-GalNAc to the non-reducing ends of the chondroitin chain. Here, we have determined the crystal structure of K4CP in the presence of UDP and UDP-GalNAc as well as with UDP and UDP-GlcUA. The structures consisted of two GT-A fold domains in which the two active sites were 60 A apart. UDP-GalNAc and UDP-GlcUA were found at the active sites of the N-terminal and C-terminal domains, respectively. The present K4CPstructures have provided the structural basis for further investigating the molecular mechanism of biosynthesis of chondroitin chain. (C) 2008 Elsevier Inc. All rights reserved.