The main structural determinants responsible for the high constitutive activity of US28.
The large outward position of TM6 due to the absence of ionic lock between residues 3.50 and 6.30 maintains US28 in an active conformation. The hydrogen bond network between TM3, TM5, and TM7 involving Y7.53, Y5.58, I3.43 and R3.50 induces an inward position of TM7. This inward position of TM7 is common in active GPCR structures. The ionic interaction between R139 in ICL2 and E3.45 instead of D3.49 destabilises the inactive state of US28. The shorter ICL2 preventing α-helix formation in this region also contributes to the absence of interaction between R139 and D3.49.