Conformational flexibility in the apolipoprotein E amino-terminal domain structure determined from three new crystal forms: Implications for lipid binding

An amino-terminal fragment of human apolipoprotein E3 (residues 1-165) has been expressed and crystallized in three different crystal forms under simi lar crystallization conditions. One crystal form has nearly identical cell dimensions to the previously reported orthorhombic (P2(1)2(1)2(1)) crystal form of the amino-terminal 22 kDa fragment of apoliporprotein E (residues 1 -191). A second orthorhombic crystal form (P2(1)2(1)2(1) With cell dimensio ns differing from the first form) and a trigonal (P3(1)21) crystal form wer e also characterized. The structures of the first orthorhombic and the trig onal from were determined by seleno-methionine multiwavelength anomalous di spersion, and the structure of the second orthorhombic form was determined by molecular replacement using the structure from the trigonal form as a se arch model. A combination of modern experimental and computational techniqu es provided high-quality electron-density maps. which revealed new features of the apolipoprotein E structure, including an unambiguously traced loop connecting helices 2 and 3 in the four-helix bundle and a number of multico nformation side chains. The three crystal Terms contain a common intermolec ular, antiparallel packing arrangement. The electrostatic complimentarity o bserved in this anti-parallel packing resembles the interaction of apolipop rotein E with the monoclonal antibody 2E8 and the low density lipoprotein r eceptor. Superposition of the model structures from all three crystal forms reveals flexibility and pronounced kinks in helices near one end of the fo ur-helix bundle. This mobility at one end of the molecule provides new insi ghts into the structural changes in apolipoprotein E that occur with lipid association.