Structure-function relationships in side chain lactam cross-linked peptide models of a conserved N-terminal domain of apolipoprotein E.
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, [sequence; see text] (i to i + 3); IV,[sequence; see text] (i to i + 4); IVa, [sequence, see text] (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 alpha helices connected by a non alpha 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 alpha 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 alpha 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.
Benzinger
T L
TL
Department of Chemistry, The University of Chicago, Illinois 60637, USA.
Braddock
D T
DT
Dominguez
S R
SR
Burkoth
T S
TS
Miller-Auer
H
H
Subramanian
R M
RM
Fless
G M
GM
Jones
D N
DN
Lynn
D G
DG
Meredith
S C
SC
D000595:Amino Acid Sequence; D000818:Animals; D001057:Apolipoproteins E / Q000737:chemistry* / Q000378:metabolism; D002460:Cell Line; D002942:Circular Dichroism; D017124:Conserved Sequence; D004622:Embryo, Mammalian; D005347:Fibroblasts; D007457:Iodine Radioisotopes; D007769:Lactams / Q000737:chemistry* / Q000378:metabolism; D051379:Mice; D008958:Models, Molecular; D008969:Molecular Sequence Data; D019906:Nuclear Magnetic Resonance, Biomolecular; D010446:Peptide Fragments / Q000737:chemistry* / Q000378:metabolism; D017433:Protein Structure, Secondary; D011973:Receptors, LDL / Q000378:metabolism; D013329:Structure-Activity Relationship
D001057:Apolipoproteins E; D007457:Iodine Radioisotopes; D007769:Lactams; D010446:Peptide Fragments; D011973:Receptors, LDL