Lipids Alter Rhodopsin Function via Ligand-like and Solvent-like Interactions

Abstract

Rhodopsin, a prototypical G protein-coupled receptor, is a membrane protein that can sense dim light. This highly effective photoreceptor is known to be sensitive to the composition of its lipidic environment, but the molecular mechanisms underlying this fine-tuned modulation of the receptor's function and structural stability are not fully understood. There are two competing hypotheses to explain how this occurs: 1) lipid modulation occurs via solvent-like interactions, where lipid composition controls membrane properties like hydrophobic thickness, which in turn modulate the protein's conformational equilibrium; or 2) protein-lipid interactions are ligand-like, with specific hot spots and long-lived binding events. By analyzing an ensemble of all-atom molecular dynamics simulations of five different states of rhodopsin, we show that a local ordering effect takes place in the membrane upon receptor activation. Likewise, docosahexaenoic acid acyl tails and phosphatidylethanolamine headgroups behave like weak ligands, preferentially binding to the receptor in inactive-like conformations and inducing subtle but significant structural changes.

Figure 1

Cross-sectional area of rhodopsin during activation. Average cross-sectional area profiles of rhodopsin states are calculated from six trajectories per protein state. Error bars indicate the mean ± SE, computed by treating each simulation as a single data point. Significantly different cross-sectional areas among protein states (p < 0.05) were determined with a t-test for each pairwise comparison and are annotated at the top by stars (dark state versus Meta I), open diamonds (dark state versus Meta II), and dots (Meta I versus Meta II). The inset corresponds to the cross-sectional areas of the −15 < z < 15 region, with the y axis scaled up. To see this figure in color, go online.

Figure 2

Activation-induced changes in the structure of the membrane. (a) Average distributions of molecular order parameters of STEA acyl chains as a function of distance from the protein centroid are shown. (b) Average distributions of STEA end-to-end distances as a function of distance from the protein centroid computed from atoms C1 and C18 of the acyl chain are shown. These averages were computed from the distributions of six trajectories per protein state. Error bars indicate the mean ± SE, calculated by treating each trajectory as a single data point. To see this figure in color, go online.

Figure 3

Residence times of DHA acyl chains inside rhodopsin. Top: Scheme illustrating the approach we employed to quantify DHA-binding events in our simulations is shown. Briefly, we took thin slices of the system along the membrane normal and used a Voronoi analysis to find the smallest convex hull containing all transmembrane helices in a slice. We counted a DHA tail as being inside the protein if one or more of its heavy atoms was inside any of these convex hulls. Bottom: Distributions of the residence times of DHA-binding events by protein state computed from the full trajectories (including the first 500 ns of the simulation). Binding events >1500 ns are grouped together. To see this figure in color, go online.

Figure 4

Protein residues with significantly different DHA occupancies in the dark state and Meta II. Amino acids that spent significantly different simulation times (on average) in contact with DHA in either the dark state or the Meta II trajectories are shown in sphere representation. Significance was judged as p < 0.05, using six trajectories per protein state as single measurements for the calculation. To see this figure in color, go online.

Figure 5

Direct DHA-rhodopsin interactions can alter the orientations of aromatic side chains and retinal methyl groups. (a) Time series of the minimum heavy-atom-to-heavy-atom distance between a DHA acyl chain and protein residue F212^5.47 in one of the dark-state trajectories. (b) Time series of the χ₁ torsion angle of F212^5.47 computed from the same dark-state trajectory as in (a). (c–e) Retinal methyl orientations as a function of simulation time computed from the same trajectory as in (a) and (b): (c) C5-methyl (C5-C18), (d) C9-methyl (C9-C19), and (e) C13-methyl (C13-C20). Right column: Time stills showing rhodopsin viewed from the intradiscal side of the membrane in cartoon representation (only TM segments are shown for clarity). K296^7.43 and retinal are shown in stick representation. An SDPE phospholipid is drawn in sphere representation. To see this figure in color, go online.

Figure 6

Retinal methyl orientations in dark-state rhodopsin. Average distributions of the orientations sampled by retinal methyl groups: C5-methyl (C5-C18), C9-methyl (C9-C19), and C13-methyl (C13-C20) are shown. cos θ = 1 means that the vector is parallel to the membrane normal (i.e., pointing toward the interdiscal/intracellular side); and cos θ = −1 indicates that it is anti-parallel to the membrane normal (i.e., pointing toward the intradiscal/extracellular side). Error bars indicate the mean ± SE of six trajectories. To see this figure in color, go online.

Figure 7

ICL3 samples multiple conformations in inactive-like rhodopsin. (a) Time course of secondary structure assignments of protein residues Q225^5.60–V250^6.33 in one of the dark-state trajectories is shown. ICL3 partially turns into a helix ∼1700 ns into the simulation. (b) Time course of the minimum distance between E232^5.67 and K248^6.31 computed from the same dark-state trajectory as in (a). (c) Fraction of contacts between E232^5.67 and K248^6.31 or PE headgroups computed from the same trajectory are shown. The fraction of contacts between PE and E232^5.67 increases concomitantly with the breakage of the E232^5.67–K248^6.31 salt bridge and ICL3 structuring. Bottom row: Conformation of ICL3 at different time points of the same trajectory, seen from the cytoplasmic side of the receptor. E232^5.67 and K248^6.31 are shown in stick representation. SDPE phospholipids located within 3.2 Å of E232^5.67 are shown in sphere representation. To see this figure in color, go online.