NMR structure of a concatemer of the first and second ligand-binding modules of the human low-density lipoprotein receptor

The ligand-binding domain of the human low-density lipoprotein receptor con sists of seven modules, each of 40-45 residues. In the presence of calcium, these modules adopt a common polypeptide fold with three conserved disulfi de bonds. A concatemer of the first and second modules (LB1-2) folds effici ently in the presence of calcium ions, forming the same disulfide connectiv ities as in the isolated modules. The three-dimensional structure of LB1-2 has now been solved using two-dimensional 1H NMR spectroscopy and restraine d molecular dynamics calculations. No intermodule nuclear Overhauser effect s were observed, indicating the absence of persistent interaction between t hem. The near random-coil NH and H alpha chemical shifts and the low phi an d psi angle order parameters of the four-residue linker suggest that it has considerable flexibility. The family of LB1-2 structures superimposed well over LB1 or LB2, but not over both modules simultaneously. LB1 and LB2 hav e a similar pattern of calcium ligands, but the orientations of the indole rings of the tryprophan residues W23 and W66 differ, with the latter limiti ng solvent access to the calcium ion. From these studies, it appears that a lthough most of the modules in the ligand-binding region of the receptor ar e joined by short segments, these linkers may impart considerable flexibili ty on this region.