PNNL

Abstract

Human apolipoprotein E (apoE) is a 299-residue exchangeable apolipoprotein that was initially recognized as a major determinant in lipoprotein metabolism and cardiovascular diseases. Recent evidence has indicated that apoE also plays critical roles in several other important biological processes not directly related to its lipid transport function, including Alzheimer’s disease, cognitive function, immunoregulation, cell signaling, and possibly even infectious diseases. ApoE contains two structural/functional domains: A N-terminal domain spanning residues 1-191 that is responsible for apoE’s LDL receptor binding activity and a C-terminal domain (residues 216-199) that is responsible for lipoprotein-binding (1). The x-ray crystal structure of the lipid-free apoE N-terminal domain was solved by Wilson et al in 1991 which represented the only high-resolution structure of this protein. This structure showed an unusually elongated (65 Å) four-helix bundle (2) that was organized in such 2 a way that its hydrophobic faces were directed towards the protein interior, whereas the hydrophilic faces were oriented towards the solvent. The major receptor-binding region, residues 130-150, was located on the fourth helix. The amphipathic a-helices were connected by short loops, giving rise to a compact, globular structure. However, this structure only contained residues 23-165. Recent studies have shown that residues beyond residues 23-165 are also very important to the apoE LDL receptorbinding activity. For example, a mutation at position R172 reduces the receptor binding activity of apoE to only ~2% (3). In addition, an E3K mutant significantly increased the apoE receptor binding activity as well (4). While the x-ray crystal structure of the apoE N-terminal domain provided detailed structural information for most region of this domain, this structure does not provide an explanation of the above experimental results regarding the structural contribution to apoE’s LDL receptor binding activity by these residues that are located in the region which is not seen in the x-ray crystal structure.