A Palmitoylated Light Switch

Abstract

Vision depends on the activation of rhodopsin by light: The covalently bound rhodopsin antagonist 11- cis -retinal absorbs light, which triggers its irreversible photoisomerization to the all- trans agonist form and initiates a conformational change that allows rhodopsin to stimulate transducin. All- trans -retinal dissociates from opsin and is reduced to all- trans -retinol (vitamin A); lecithin retinol acyl transferase (LRAT) esterifies vitamin A to create all- trans -retinyl esters (mostly palmitate), which bind to a high-molecular-weight membrane-associated form of the retinal pigment epithelium protein RPE65 (mRPE65). mRPE65 mobilizes all- trans -retinyl esters, enabling their conversion into 11- cis -retinol and thereby allowing 11- cis -retinal to be regenerated and vision to continue. 11- cis -Retinol is regenerated more rapidly in the light; however, the mechanisms by which light regulates this visual cycle are poorly understood. Xue et al. used a fluorometric assay to show that soluble RPE65 (sRPE65) displayed complementary retinoid binding specificity to mRPE65, binding vitamin A with much higher affinity than all- trans -retinyl-palmitate and facilitating its conversion to all- trans -retinyl-palmitate. mRPE65 expressed in insect cells was labeled when exposed to 3 H-palmitate, and mass spectroscopic analysis indicated that mRPE65 was triply palmitoylated whereas sRPE65 was not palmitoylated. LRAT reversibly transferred a palmitoyl moiety from mRPE65 to vitamin A, generating sRPE65 and all- trans -retinyl palmitate. Moreover, 11- cis -retinol served as a substrate for mRPE65- and LRAT-mediated palmitoylation as well, so that 11- cis -retinol added to retinal pigment epithelium membranes promoted mRPE65 conversion to sRPE65 and thereby inhibited its own biosynthesis. Thus, the authors propose that palmitoylation of RPE65 provides a molecular switch regulating 11- cis -retinal synthesis, which can be flipped by the light-dependent increase in the supply of all- trans -retinyl esters and the ensuing conversion of sRPE65 to mRPE65. L. Xue, D. R. Gollapalli, P. Maiti, W. J. Jahng, R. R. Rando, A palmitoylation switch mechanism in the regulation of the visual cycle. Cell 117 , 761-771 (2004). [Online Journal]