As acid levels rise, a second "cluster" involving the ECL2 loop transforms from a short hairpin into a long, protruding beta-sheet. This structural "swing" fully stabilizes the receptor in its active state. 3. Why This Matters
The GPR4 receptor is a member of the G protein-coupled receptor (GPCR) family that acts as a "sensor" for extracellular acidity. This process is vital for physiological balance and is often deregulated in diseases like cancer and inflammation.
A specific residue (H269) becomes protonated first, initiating a partial change in the receptor's shape.