Light Flips the Lipid Switch

Cells attach a variety of lipids to proteins in various ways, but only one of these lipid modifications--the tethering of palmitate or a related lipid to a protein via a cysteine thiol side chain--is reversible. It's that distinction that has made researchers wonder whether cells use palmitoylation as a regulatory "switch," just as cells use phosphorylation to turn protein function off and on. The first such palmitoyl switch has now been shown to play a crucial role in how we see.

Vision begins with rhodopsin, a protein in the eye that contains a light-absorbing pigment called 11-cis-retinal. Visible light causes this chromophore--which is derived from vitamin A--to isomerize to all-trans-retinal, a change that triggers a cascade of molecular events that lead to vision. In order for vision to continue, however, the 11-cis-retinal chromophore must be resynthesized.

A series of enzyme-catalyzed steps are required to transform all-trans-retinal back into the crucial rhodopsin chromophore. Flux through the cycle must be carefully controlled, however, because 11-cis-retinal is highly reactive and energetically expensive to produce. In addition, all-trans-retinal can react with membrane lipids to form highly toxic molecules that can cause severe damage to the eye and even blindness.

The dangers and costs associated with allowing either of these reactive aldehydes to build up forces the eye to adjust the rate of 11-cis-retinal resynthesis to the amount of available light: The cycle must be slowed down when it's dark to avoid wasteful synthesis of 11-cis-retinal and sped up when it's light to minimize accumulation of toxic all-trans-retinal.

Just how ambient light might control the rate at which the 11-cis-retinal chromophore is resynthesized in the eye "is a nagging question in vision," according to Robert R. Rando, a professor of biological chemistry and molecular pharmacology at Harvard Medical School. Recent work from Rando, postdocs Linlong Xue and Deviprasad R. Gollapalli, and coworkers suggests that palmitoylation of a key vision protein called RPE65 acts as a switch controlling this process [Cell, 117, 761 (2004)].

Although palmitoylation has been shown to be one of the most frequent modifications made to proteins, so far only limited function has been attributed to the modification. Researchers have shown that the principle role of tagging a protein with palmitate--a saturated 16-carbon chain capped with a carboxylate group--is to direct the modified protein to the membrane: The waxy, dangling palmitate groups prefer to stick themselves in a nearby lipid bilayer. In a few cases, palmitoylation has been shown to coax the modified protein to interact with a hydrophobic protein partner. But despite predictions that this reversible and widespread modification might play a regulatory role in cells, examples of palmitoyl switches haven't turned up.

UNTIL NOW. Palmitoylating RPE65 at three specific cysteine residues clearly results in the protein relocating to the membrane, Rando notes. "But it also alters RPE65's ligand-binding specificity," he adds, noting that the unmodified protein binds vitamin A (all-trans-retinol), whereas the palmitoylated version prefers all-trans-retinyl esters. This is the first example of palmitoylation controlling a protein's ligand-binding preference, he says.

Although Rando is quick to point out that these studies were carried out in vitro, he suggests it's easy to envision how this palmitoyl switch might work in vivo. Because RPE65 must carry the all-trans-retinyl esters to the next enzyme in the pathway, depalmitoylation of RPE65 inhibits resynthesis of the rhodopsin chromophore. Thus, palmitoylation of RPE65 is a switch that regulates resynthesis of the rhodopsin chromophore by altering RPE65's ligand-binding preference.

To their surprise, Rando and his colleagues also uncovered a second new role for palmitoylated proteins. The team finds that the palmitoylated version of RPE65 also functions as a palmitoyl donor--a first for a palmitoylated protein, Rando says. In the vision cycle, palmitoylated RPE65 provides the enzyme lecithin retinal acyl transferase (LRAT) with the palmitate it needs to convert vitamin A into the corresponding all-trans-retinyl ester, a step along the road to chromophore resynthesis.

RPE65's role as a palmitate donor may also play a key role in how the eye adjusts chromophore resynthesis to the amount of ambient light, Rando suggests. In the light, when rhodopsin isomerization leads to vitamin A buildup, LRAT transfers palmitate from palmitoylated RPE65 to vitamin A, making all-trans-retinyl esters and spurring resynthesis of 11-cis-retinal.

But in the dark, when the 11-cis-retinal rhodopsin chromophore is not needed and its 11-cis-retinol precursor begins to build up, LRAT prefers to transfer palmitate from palmitoylated RPE65 to 11-cis-retinol. This transfer not only blocks conversion of 11-cis-retinol into the chromophore, but also depletes the supply of palmitoylated RPE65 available to help LRAT convert vitamin A into all-trans-retinyl esters.

Although his lab is still working to prove that the RPE65 palmitoyl switch operates in the eye, Rando is confident that this is not the only palmitoyl switch out there. He notes that LRAT--the transferase enzyme that flips the RPE65 palmitoyl switch--is the founding member of an expanding group of proteins, most of which don't have a known function. Until now, searches for human palmitoyl transferases have come up empty-handed, causing some scientists to go so far as to suggest that the modification takes place without enzymatic catalysis. Rando suggests that LRAT's relatives, which are involved in processes from tumor growth to development, should be considered possible palmitoyl transferase candidates. "This is probably the first of many palmitoyl switches," Rando predicts.