Polynucleotide binding to macrophage scavenger receptors depends on the formation of base-quartet-stabilized four-stranded helices

Abstract

Macrophage scavenger receptors exhibit unusually broad, but circumscribed, polyanionic ligand-binding specificity. For example, the polyribonucleotides poly(I) and poly(G) are ligands but poly(A) and poly(C) are not. To further investigate the molecular basis of this polynucleotide-binding specificity, we tested the capacity of various oligodeoxyribonucleic acids to inhibit the scavenger receptor-mediated degradation of 125I-labeled acetylated low density lipoprotein by Chinese hamster ovary cells expressing the type I bovine scavenger receptor. A series of short oligodeoxyriboguanines (dGn, where 5 < or = n < or = 37) were effective inhibitors. The dG6, dG12, and dA5G37 members of this series were shown by circular dichroism and UV spectroscopy to be assembled into four-stranded helices stabilized by G-quartets. [32P]dA5G37 bound directly to scavenger receptors. Partial or complete denaturation of the quadruplex structures of these oligonucleotides by boiling destroyed their inhibitory activity. Receptor activity was also inhibited by d(T4G4)4, a telomere-like oligonucleotide which forms an intramolecular quadruplex. In addition, conversion of the four-stranded potassium salt of poly(I) to the single-stranded lithium salt dramatically reduced its inhibitory activity. Addition of KCl to the Li+ salt resulted in the reformation of poly(I)'s quadruplex structure and restoration of its inhibitory activity. A variety of single-stranded and double-stranded oligo- and polydeoxyribonucleotides (e.g. dA37, HaeIII restriction fragments of phi X174) exhibited very little or no inhibitory activity. Thus, a base-quartet-stabilized four-stranded helix appears to be a necessary structural determinant for polynucleotide binding to and inhibition of scavenger receptors. This conformational requirement accounts for the previously unexplained polyribonucleotide-binding specificity of scavenger receptors. The spatial distribution of the negatively charged phosphates in polynucleotide quadruplexes may form a charged surface which is complementary to the positively charged surface of the collagenous ligand-binding domain of the scavenger receptor.