13C nuclear magnetic resonance suggests a flexible proteoglycan core protein

13C NMR was used to study the molecular dynamics of the chick limb bud proteoglycan core protein. Because only about 10% of the proteoglycan is protein, [2-13C]glycine and [3-13C]serine were incorporated into the core protein of the chick limb bud proteoglycan monomer using a chondrocyte culture system. The purified labeled monomer was studied in solution (50 mg/ml, 0.05 M sodium acetate/0.05 M sodium phosphate, pH 7.4) at 37 degrees C. Spin-lattice relaxation times, line widths, and nuclear Overhauser enhancements were measured for the labeled carbons in the protein and for the natural abundance carbons in the glycosaminoglycan chains. Analyses of these data show that correlation times for backbone reorientation of protein and polysaccharide chains in the intact monomer are predominantly in the range of 0.5-5.0 ns. These correlation times are 10(2)-10(3) times smaller than the minimum correlation time calculated for a rigid monomer, indicating that the core protein and polysaccharide backbones are segmentally flexible. Signal intensity data show that at least 80% of the protein backbone is flexible but do not exclude the possibility that 20% of the protein backbone has ordered structure. We observe both broad and narrow signal components in the spectrum of the intact monomer, showing that backbone motion is heterogeneous. The broad signal component is not observed after the monomer is digested with chondroitinase. This result and the strong concentration dependence of the 13C line width observed in solutions of chondroitin 4-sulfate suggest an assignment of the broad signal component to residues near the protein-polysaccharide linkage region. The difference in NMR parameters observed for free and substituted serine carbons confirms that motion of the substituted serine side chain is restricted.