The conformation of the alpha-helical coiled coil domain of macrophage scavenger receptor is pH dependent

Macrophage scavenger receptor is a trimerized membrane protein that binds ligands and undergoes internalization by endocytosis. The receptor releases the ligands in the endosome, and then is recycled. The mechanisms of the ligand release and the recycling of the receptor have not been clearly determined. We analyzed the structure of the alpha-helical coiled coil domain considered to be responsible for acid-mediated ligand dissociation, by chemical cross-linking, sedimentation equilibrium, Western blot, and circular dichroism analyses. This domain has 22 heptad repeats, which are characteristic of the sequence of an alpha-helical coiled coil structure, with a discontinuity in the middle. We prepared three peptides, corresponding to the entire alpha-helical coiled coil domain (alpha), its N-terminal half (alpha-N), and its C-terminal half (alpha-C), by expression of each gene in Escherichia coli. The alpha and alpha-N peptides show triple-stranded alpha-helical coiled coil structures, but in contrast, the alpha-C peptide shows a random structure. When connected to the N-terminus by a chemical ligation method, the alpha-C peptide also shows an alpha-helical coiled coil structure, but only at an acidic pH. These results suggest that the N-terminus of the alpha-helical coiled coil domain is responsible for the formation of a stable trimer and the C-terminus exhibits the pH-dependent conformational change that might be involved in the ligand release by the macrophage scavenger receptor.