Structure−Function Relationships in Side Chain Lactam Cross-Linked Peptide Models of a Conserved N-Terminal Domain of Apolipoprotein E

• Published in: Biochemistry (Easton) 1998-09, Vol.37 (38), p.13222-13229
• Main Authors: Benzinger, Tammie L. S, Braddock, Demetrios T, Dominguez, Samuel R, Burkoth, Timothy S, Miller-Auer, Hélène, Subramanian, Ram M, Fless, Gunther M, Jones, David N. M, Lynn, David G, Meredith, Stephen C
• Format: Article
• Language: English
• Subjects: Amino Acid Sequence; Animals; Apolipoproteins E - chemistry; Apolipoproteins E - metabolism; Cell Line; Circular Dichroism; Conserved Sequence; Embryo, Mammalian; Fibroblasts; Iodine Radioisotopes; Lactams - chemistry; Lactams - metabolism; Mice; Models, Molecular; Molecular Sequence Data; Nuclear Magnetic Resonance, Biomolecular; Peptide Fragments - chemistry; Peptide Fragments - metabolism; Protein Structure, Secondary; Receptors, LDL - metabolism; Structure-Activity Relationship
• ISSN: 0006-2960; 1520-4995
• DOI: 10.1021/bi980482f

Bioactive peptides have multiple conformations in solution but adopt well-defined conformations at lipid surfaces and in interactions with receptors. We have used side chain lactam cross-links to stabilize secondary structures in the following peptide models of a conserved N-terminal domain of apolipoprotein E (cross-link periodicity in parentheses):  I, H2N-GQTLSEQVQEELLSSQVTQELRAG-COOH (none); III, (i to i + 3); IV, (i to i + 4); IVa, (i to i + 4) (lactams above the sequence, potential salt bridges below the sequence). We previously demonstrated [Luo et al. (1994) Biochemistry 33, 12367−12377; Braddock et al. (1996) Biochemistry 35, 13975−13984] that peptide III, containing lactam cross-links between the i and i + 3 side chains, enhances specific binding of LDL via a receptor other than the LDL-receptor. Peptide III in solution consists of two short α helices connected by a non α helical segment. Here we examine the hypothesis that the domain modeled by peptide III is one antipode of a conformational switch. To model another antipode of the switch, we introduced two strategic modifications into peptide III to examine structure−function relationships in this domain:  (1) the spacing of the lactam cross-links was changed (i to i + 4 in peptides IV and IVa) and (2) peptides IV and IVa contain the two alternative sequences at a site of a possible end-capping interaction in peptide III. The structure of peptide IV, determined by 2D-NMR, is α helical across its entire length. Despite the remarkable degree of structural order, peptide IV is biologically inactive. In contrast, peptides III and possibly IVa contain a central interruption of the α helix, which appears necessary for biological activity. These and other studies support the hypothesis that this domain is a conformational switch which, to the extent that it models apolipoprotein E itself, may modulate interactions between apo E and its various receptors.