Proteoglycan Structure & Assembly
Proteoglycans are glycoproteins that reside on the plasma membrane of virtually all animal cells and represent major components of extracellular matrices. They consist of a protein core and one or more glycosaminoglycan chains, which are composed of alternating hexosamine and a hexuronic acid units. Heparan sulfate and heparin assemble from N-acetylglucosamine (GlcNAc) and glucuronic acid (GlcA) units. During the polymerization reaction, some GlcNAc residues undergo N-deacetylation/N-sulfation and addition of sulfate groups at C6 and more rarely at C3. An epimerase can flip the stereochemistry at C5, converting GlcA units to L-iduronic 
acid (IdoA) and a sulfotransferase can add sulfate to C2 of GlcA or IdoA units, usually at sites adjacent to modified glucosamine residues. Chondroitin sulfate, another type of glycososaminoglycan, are composed of N-acetylgalactosamine (GalNAc) and GlcA. Sulfate groups can be added to C4 and C6 or the GalNAc units. In dermatan suflate, some of the GlcA residues are converted to IdoA and sulfated at C2 and in some organisms at C3. In heparan sulfate, the modifications occur across segments of variable size and to different extents, creating enormous structural heterogeneity. In chondroitin sulfate, the chains are more homogeneously modified. In both cases, the sulfated and IdoA-rich segments form binding sites for various ligands, including growth factors, proteases and their inhibitors, lipolytic enzymes and plasma apolipoproteins, and extracellular matrix proteins. The importance of these interactions is exemplified by the profound pathophysiological phenotypes in mice and humans bearing mutations in the core proteins or the biosynthetic enzymes responsible for assembly of the chains.
Ongoing projects in 
this area include creation of conditional mutants in mice in order to study the function of heparan sulfate and chondrotin sulfate proteoglycans in different physiological systems (see other pages on this website). We are also interested in the process of assembly, in particular how cells regualte the formation of ligand binding sites in the chains. Towards this end, we are examining two families enzymes, the GlcNAc N-deacetylase/N-sulfotrasnferases (Ndst) and the glucosaminyl 3-O-sulfotransferases(Hs3st) that modify heparan sulfate. We also have developed proteomic approaches that led to the discovery of a new class of chondroitin proteoglycan core proteins in C. elegans. These proteins appear to be unique to nematodes since vertebrates, Drosophila and Hydra do not contain orthologous genes. Mass spectrometry methods have also been developed to analyze the fine structure of the glycosaminoglycan chains as well. These highly sensitive techniques can be used to study how the enzymes regulate the pattern of sulfation of the chains and have already led to the discovery of unusual modifications in lower organisms.