A peptide model of one antipode of a switch domain in apolipoprotein e: structure-function relationships

We have shown previously that a peptide model (Peptide III) of a conserved, anionic X-terminal domain of apolipoprotein K (residues 41-60} dramatically increases LDL binding and internalization by a pathway independent of the LDL receptor and LDL-R related protein. Two-dimensional NMR spec troscopy showed this peptide to have a helix-turn-helix structure (Biochem 33:12367, 1994), which was stabilized by two lactam crosslinks between i and i+3 residues. In the present work, we test the hypothesis that this region is a switch domain, i.e., capable of adopting two or more stable structures. We synthesized Peptide IV, a conservative model of this region, but with the two lactam crosslinks between i and i+4 residues. Two-dimensional NMR spec troscopy showed that the peptide is a single, continuous /alpha helix, i.e.. similar to Peptide III, but lacking the central turn. Despite a highly ordered. I alpha helical structure, the peptide lacked or inhibited the biological activity shown by Peptide III. Thermal denaturation studies showed that the lactam crosslinks stabilized the helix compared to Peptide I which lacked crosslinks. The stabilization was about 1.2 kcal/mole, and had both / DeltaS and /Deltatt contributions. These findings demonstrate that extremely minor variations in this region have major structural and functional conséquences, and are con sistent with the proposal that this region of apo E is a switch domain. The conforrnational switching may be a mechanism for switching a biological activity on and off.