Rhodopsin palmitoylation in bovine rod outer segment disk membranes of different age/spatial location

S-acylation of Orai1 regulates store-operated Ca2+ entry

Store-operated Ca2+ entry is a central component of intracellular Ca2+ signaling pathways. The Ca2+ release-activated channel (CRAC) mediates store-operated Ca2+ entry in many different cell types. The CRAC channel is composed of the plasma membrane (PM)-localized Orai1 channel and endoplasmic reticulum (ER)-localized STIM1 Ca2+ sensor. Upon ER Ca2+ store depletion, Orai1 and STIM1 form complexes at ER-PM junctions, leading to the formation of activated CRAC channels. Although the importance of CRAC channels is well described, the underlying mechanisms that regulate the recruitment of Orai1 to ER-PM junctions are not fully understood. Here, we describe the rapid and transient S-acylation of Orai1. Using biochemical approaches, we show that Orai1 is rapidly S-acylated at cysteine 143 upon ER Ca2+ store depletion. Importantly, S-acylation of cysteine 143 is required for Orai1-mediated Ca2+ entry and recruitment to STIM1 puncta. We conclude that store depletion-induced S-acylation of Orai1 is necessary for recruitment to ER-PM junctions, subsequent binding to STIM1 and channel activation.

Palmitoylation stabilizes unliganded rod opsin

S-palmitoylation is a conserved feature in many G protein-coupled receptors (GPCRs) involved in a broad array of signaling processes. The prototypical GPCR, rhodopsin, is S-palmitoylated on two adjacent C-terminal Cys residues at its cytoplasmic surface. Surprisingly, absence of palmitoylation has only a modest effect on in vitro or in vivo signaling. Here, we report that palmitoylation-deficient (Palm(-/-)) mice carrying two Cys to Thr and Ser mutations in the opsin gene displayed profound light-induced retinal degeneration that first involved rod and then cone cells. After brief bright light exposure, their retinas exhibited two types of deposits containing nucleic acid and invasive phagocytic macrophages. When Palm(-/-) mice were crossed with Lrat(-/-) mice lacking lecithin:retinol acyl transferase to eliminate retinoid binding to opsin and thereby rendering the eye insensitive to light, rapid retinal degeneration occurred even in 3- to 4-week-old animals. This rapid degeneration suggests that nonpalmitoylated rod opsin is unstable. Treatment of 2-week-old Palm(-/-)Lrat(-/-) mice with an artificial chromophore precursor prevented this retinopathy. In contrast, elimination of signaling to G protein in Palm(-/-)Gnat1(-/-) mice had no effect, indicating that instability of unpalmitoylated opsin lacking chromophore rather than aberrant signal transduction resulted in retinal pathology. Together, these observations provide evidence for a structural role of rhodopsin S-palmitoylation that may apply to other GPCRs as well.

Quantitation of the effect of hydroxylamine on rhodopsin palmitylation

Rhodopsin (the photosensitive rod visual pigment) has been a model for photobiologic studies of the opsins as well as a structural model for G-protein-coupled receptors. The two palmitate groups attached to cysteines 322 and 323 are thought to serve as membrane anchors for the rhodopsin C-terminus, but the absence of the palmitates does not alter membrane localization. However, removal of the palmitates affects rhodopsin function. Therefore, it is important to quantitate the stability of rhodopsin palmitates to hydroxylamine, which is a widely utilized reagent in biochemical preparations of the apoprotein. We have developed a mass spectrometric method to quantitate the resulting opsin palmitylation. Our data show that both of the bovine rhodopsin palmitates are labile to hydroxylamine, with significant depalmitylation occurring at concentrations of >or=100 mM, with an EC(50) of 220 mM L(-1). The palmitate at position 322 is the more stable to hydroxylamine. Samples prepared in the presence of >50 mM should therefore be considered to be at least partially depalmitylated and the results interpreted accordingly.

Palmitylation of cone opsins

Palmitylation is a widespread modification in G-protein-coupled receptors and often a dynamic process. In rhodopsins, palmitylation is static on C322/C323. Red/green (M/LWS) cone opsins have no cysteines at corresponding positions and no palmitylation. Blue (SWS2) cone opsins have a single corresponding cysteine and mass spectrometric analysis showed partial palmitylation of salamander SWS2 cone opsin. Ultraviolet (SWS1) cone opsins have one corresponding cysteine, but only unpalmitylated opsin was observed for mouse and salamander. The results show that the static palmitylation found on rhodopsin is not found on cone opsins and suggest the possibility of an unidentified role for opsin palmitylation in cones.