Heteronuclear NMR assignments and secondary structure of the coiled coil trimerization domain from cartilage matrix protein in oxidized and reduced forms

The C-terminal oligomerization domain of chicken cartilage matrix protein is a trimeric coiled coil comprised of three identical 43-residue chains. NMR spectra of the protein show equivalent magnetic environments for each monomer, indicating a parallel coiled coil structure with complete threefold symmetry. Sequence-specific assignments for 1H-, 15N-, and 13C-NMR resonances have been obtained from 2D-1H NOESY and TOCSY spectra, and from 3D HNCA, 15N NOESY-HSQC, and HCCH-TOCSY spectra. A stretch of α- helix encompassing five heptad repeats (35 residues) has been identified from intra-chain HN-HN and HN-Hα NOE connectivities, 3J(HNHα) coupling constants, and chemical shift indices. The α-helix begins immediately downstream of inter-chain disulfide bonds between residues Cys 5 and Cys 7, and extends to near the C-terminus of the molecule. The threefold symmetry of the molecule is maintained when the inter-chain disulfide bonds that flank the N-terminus of the coiled coil are reduced. Residues Ile 21 through GIu 36 show conserved chemical shifts and NOE connectivities, as well as strong protection from solvent exchange in the oxidized and reduced forms of the protein. By contrast, residues Ile 10 through Val 17 show pronounced chemical shift differences between the oxidized and reduced protein. Strong chemical exchange NOEs between HN resonances and water indicate solvent exchange on time scales faster than 10 s, and suggests a dynamic fraying of the N- terminus of the coiled coil upon reduction of the disulfide bonds. Possible roles for the disulfide crosslinks of the oligomerization domain in the function of cartilage matrix protein are proposed.