The lipid-associated conformation of the low density lipoprotein receptor binding domain of human apolipoprotein E

Apolipoprotein E (apoE) is a 34-kDa exchangeable apolipoprotein that regula tes metabolism of plasma lipoproteins by functioning as a ligand for member s of the LDL receptor family, The receptor-binding region localizes to the vicinity of residues 130-150 within its independently folded 22-kDa N-termi nal domain. In the absence of lipid, this domain exists as a receptor-inact ive, globular four-helix bundle. Receptor recognition properties of this do main are manifest upon lipid association, which is accompanied by a conform ational change in the protein. Fluorescence resonance energy transfer has b een used to monitor helix repositioning, which accompanies lipid associatio n of the apoE N-terminal domain. Site-directed mutagenesis was used to repl ace naturally occurring Trp residues with phenylalanine, creating a Trp-nul l apoE3 N-terminal domain (residues 1-183), Subsequently, tyrosine residues in helix 2, helix 3, or helix 4 were converted to Trp, generating single T rp mutant proteins. The lone cysteine at position 112 was covalently modifi ed with N-iodoacetyl-N'-(5-Sulfo-1-naphthyl)ethylenediamine, which serves a s an energy acceptor from excited tryptophan residues. Fluorescence resonan ce energy transfer analysis of apoE N-terminal domain variants in phospholi pid disc complexes suggests that the helix bundle opens to adopt a partiall y extended conformation. A model is presented that depicts a tandem arrange ment of the receptor-binding region of the protein in the disc complex, cor responding to its low density lipoprotein receptor-active conformation.