Delineating a Ca2+ binding pocket within the venus flytrap module of the human calcium-sensing receptor

Area of Science:

• Biochemistry
• Molecular Biology
• Structural Biology

Background:

• The Ca2+-sensing receptor (CaSR) is a class III G-protein-coupled receptor (GPCR) with a large extracellular Venus flytrap module (VFTM) responsible for ligand binding.
• Understanding Ca2+ recognition by the CaSR VFTM is crucial for deciphering calcium homeostasis.

Purpose of the Study:

• To elucidate the molecular determinants of Ca2+ recognition within the CaSR VFTM.
• To investigate the conservation of the Ca2+ binding site across class III GPCRs.

Main Methods:

• Homology modeling of the human CaSR VFTM based on the metabotropic glutamate receptor type 1 (mGluR1) structure.
• Phylogenetic analysis of 14 class III GPCR VFTMs.
• Site-directed mutagenesis and functional analysis of key residues.

Main Results:

• A distinct Ca2+ binding pocket was identified within the CaSR VFTM, adjacent to the predicted amino acid binding site.
• Critical Ca2+-contacting residues in this pocket are highly conserved among class III GPCRs.
• Mutations in Glu-297 within this pocket are linked to human diseases affecting calcium homeostasis (autosomal dominant hypocalcemia and familial hypocalciuric hypercalcemia).

Conclusions:

• A specific Ca2+ binding pocket in the CaSR VFTM has been defined.
• This pocket and its Ca2+ sensing mechanism may be conserved in other class III GPCRs.
• Provides a molecular basis for extracellular Ca2+ sensing by the CaSR and related receptors.