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

Abstract

The ligand-binding domain of the human low-density lipoprotein receptor consists of seven modules, each of 40–45 residues. In the presence of calcium, these modules adopt a common polypeptide fold with three conserved disulfide bonds. A concatemer of the first and second modules (LB_1–2) folds efficiently in the presence of calcium ions, forming the same disulfide connectivities as in the isolated modules. The three-dimensional structure of LB_1–2 has now been solved using two-dimensional ¹H NMR spectroscopy and restrained molecular dynamics calculations. No intermodule nuclear Overhauser effects were observed, indicating the absence of persistent interaction between them. The near random-coil NH and Hα chemical shifts and the low φ and ψ angle order parameters of the four-residue linker suggest that it has considerable flexibility. The family of LB_1–2 structures superimposed well over LB₁ or LB₂, but not over both modules simultaneously. LB₁ and LB₂ have a similar pattern of calcium ligands, but the orientations of the indole rings of the tryptophan residues W23 and W66 differ, with the latter limiting solvent access to the calcium ion. From these studies, it appears that although most of the modules in the ligand-binding region of the receptor are joined by short segments, these linkers may impart considerable flexibility on this region.