The histidine interruption of an alpha-helical coiled coil allosterically mediates a pH-dependent ligand dissociation from macrophage scavenger receptors.

Macrophage scavenger receptors uptake modified low density lipoproteins (LDLs) through receptor-mediated endocytosis. In the alpha-helical coiled coil domain, 2 histidines (His168 and His260) disrupt leucine or isoleucine heptad repeats. Substitution of His168 or/and His260 to leucine had no effect on AcLDL binding activities. However, the His260-replaced receptors had their ligand degradation activities diminished. Cell surface ligand release experiments under acidic pH clarified that His260 mutants lost their ligand dissociation activities at 37 degrees C. Furthermore, immunoelectron microscopic experiments using anti-scavenger receptor antibody showed that the His260 replaced receptors were not able to release gold-labeled AcLDL in endosomes. Here we propose an allosterical ligand dissociation mechanism by His260 in macrophage scavenger receptors.