Palmitoylation is a prerequisite for dimerization-dependent raftophilicity of rhodopsin

Abstract

The visual photopigment rhodopsin (Rh) is a prototypical G protein– coupled receptor (GPCR) responsible for initiation of the phototransduction cascade in rod photoreceptors. Similar to other GPCRs, Rh can form dimers or even higher oligomers and tends to have a supramolecular organization that is likely important in the dim light response. Rh also exhibits high affinity for lipid rafts (i.e. raftophilicity) upon light-dependent binding with the cognate G protein transducin (Gt), suggesting the presence of lipid raft-like domains in the retinal disk membrane and their importance in phototransduction. However, the relationship between Rh oligomerization and lipid rafts in the disk membrane remains to be explored. Given previous findings that Gt binds to dimeric Rh and that Rh is posttranslationally modified with two highly raftophilic palmitoyl moieties, we hypothesized that Rh becomes raftophilic upon dimerization. Here, using biochemical assays, we found that Rh*–Gt complexes in the detergent-resistant membrane are partially resistant to cholesterol depletion by methyl--cyclodextrin and that the Rh-to-Gt stoichiometry in this methyl--cyclodextrin–resistant complex is 2:1. Next, we found that IgG-mediated Rh–Rh cross-linking renders Rh highly raftophilic, supporting the premise that Rh becomes raftophilic upon dimerization. Rh depalmitoylation via reduction of thioester linkages blocked the translocation of IgG– cross-linked Rh to the detergent-resistant membrane, highlighting that the two palmitoyl moieties are important for the dimerization-dependent raftophilicity of Rh. These results indicate that palmitoylated GPCRs such as Rh can acquire raftophilicity upon G protein–stabilized dimerization and thereby organize receptor-cluster rafts by recruiting raftophilic lipids.