Insights into the Activation Mechanism of HCA1, HCA2, and HCA3

Area of Science:

• Biochemistry
• Structural Biology
• Pharmacology

Background:

• Hydroxy-carboxylic acid receptors (HCAs) are activated by energy metabolism intermediates.
• HCA2 is a focus for research but has clinical limitations due to adverse effects.
• HCA1 is a promising target for lipolysis, but HCAs share activation mechanisms, necessitating further structural elucidation.

Purpose of the Study:

• To unveil the cryo-electron microscopy structures of HCA1, HCA2, and HCA3 signaling complexes.
• To comparatively analyze HCAs to identify key residues for ligand binding and activation.
• To elucidate the conserved yet unique activation mechanisms across the HCA family.

Main Methods:

• Cryo-electron microscopy (cryo-EM) to determine the structures of HCA-Gi signaling complexes.
• Comparative structural analysis of HCA1, HCA2, and HCA3 bound to various ligands.
• Chimeric complex construction and site-directed mutagenesis for functional analysis.

Main Results:

• The cryo-EM structures of 3,5-DHBA-HCA1-Gi, acifran/MK6892-HCA2-Gi, and acifran-HCA3-Gi complexes were determined.
• Key residues in HCA1 stabilizing the ligand-binding pocket were identified.
• Residues critical for HCA2 and HCA3 selectivity were pinpointed through chimeric and mutational analyses.

Conclusions:

• Structural insights into HCA ligand recognition and activation mechanisms were elucidated.
• Understanding HCA1's unique activation provides a basis for developing targeted lipolysis therapies.
• The study broadens comprehension of ligand specificity across the HCA receptor family.