Title : Core
protein structure and sequence determine the
site and presence of heparan sulfate and chondroitin sulfate on
syndecan-1
Abstract :
- Most proteoglycans bear either chondroitin sulfate or heparan sulfate chains linked to serine residues at Ser-Gly attachment sites on the core protein
- However, only a fraction of proteins with Ser-Gly sites exhibit glycosaminoglycan chains
- A variable proportion of these sites may be glycanated, and an unknown mechanism distinguishes whether these sites are for chondroitin sulfate or heparan sulfate
- To evaluate the core protein features that determine whether and where chondroitin sulfate or heparan sulfate will be linked, we have studied mouse syndecan-1 , a transmembrane proteoglycan that is invariably glycanated and can contain both chondroitin sulfate and heparan sulfate chains
- The extracellular domain of the syndecan-1 core protein contains five Ser-Gly sites , three clustered near its N terminus and two adjacent to the transmembrane domain near its C terminus
- We have established the distribution of glycosaminoglycans on these attachment clusters
- In contrast to the C-terminal cluster, the N-terminal cluster was always glycanated, suggesting that this domain of the core protein contains sequences responsible for the invariable attachment of glycosaminoglycan chains
- Solely chondroitin sulfate was found on the C-terminal cluster
- This cluster contains the sequences EGSGE and ETSGE, both estimated to be on the protein surface in a hydrophilic environment
- Heparan sulfate was found solely on the N-terminal cluster, which also bears some chondroitin sulfate
- This cluster contains the sequences FSGSGTG and DGSGD, the former estimated to be in a hydrophobic pocket and the latter, similar to the sequence on the C-terminal cluster, in an exposed hydrophilic region
- This glycosaminoglycan distribution was identical on mouse syndecan-1 produced by either mouse epithelial (NMuMG) or hamster mesenchymal (CHO) cells, suggesting that site-specific attachment of glycosaminoglycans is independent of cell type
- These results implicate a cellular mechanism that distinguishes among the potential sites and attaches the correct glycosaminoglycan type unambiguously
- Thus, structural elements of the core protein other than the Ser-Gly attachment sites determine if a site will be glycanated and, if so, whether with chondroitin sulfate or heparan sulfate