A mechanistic role of Helix 8 in GPCRs: Computational modeling of the dopamine D2 receptor interaction with the GIPC1-PDZ-domain.

Helix-8 (Hx8) is a structurally conserved amphipathic helical motif in class-A GPCRs, adjacent to the C-terminal sequence that is responsible for PDZ-domain-recognition. The Hx8 segment in the dopamine D2 receptor (D2R) constitutes the C-terminal segment and we investigate its role in the function of D2R by studying the interaction with the PDZ-containing GIPC1 using homology models based on the X-ray structures of very closely related analogs: the D3R for the D2R model, and the PDZ domain of GIPC2 for GIPC1-PDZ. The mechanism of this interaction was investigated with all-atom unbiased molecular dynamics (MD) simulations that reveal the role of the membrane in maintaining the helical fold of Hx8, and with biased MD simulations to elucidate the energy drive for the interaction with the GIPC1-PDZ. We found that it becomes more favorable energetically for Hx8 to adopt the extended conformation observed in all PDZ-ligand complexes when it moves away from the membrane, and that C-terminus palmitoylation of D2R enhanced membrane penetration by the Hx8 backbone. De-palmitoylation enables Hx8 to move out into the aqueous environment for interaction with the PDZ domain. All-atom unbiased MD simulations of the full D2R-GIPC1-PDZ complex in sphingolipid/cholesterol membranes show that the D2R carboxyl C-terminus samples the region of the conserved GFGL motif located on the carboxylate-binding loop of the GIPC1-PDZ, and the entire complex distances itself from the membrane interface. Together, these results outline a likely mechanism of Hx8 involvement in the interaction of the GPCR with PDZ-domains in the course of signaling.