title: Identification of the 11-cis-specific retinyl-ester synthase in retinal Müller cells as multifunctional O-acyltransferase (MFAT).
abstract: Absorption of a photon by a rhodopsin or cone-opsin pigment isomerizes its 11-cis-retinaldehyde (11-cis-RAL) chromophore to all-trans-retinaldehyde (all-trans-RAL), which dissociates after a brief period of activation. Light sensitivity is restored to the resulting apo-opsin when it recombines with another 11-cis-RAL. Conversion of all-trans-RAL to 11-cis-RAL is carried out by an enzyme pathway called the visual cycle in cells of the retinal pigment epithelium. A second visual cycle is present in Müller cells of the retina. The retinol isomerase for this noncanonical pathway is dihydroceramide desaturase (DES1), which catalyzes equilibrium isomerization of retinol. Because 11-cis-retinol (11-cis-ROL) constitutes only a small fraction of total retinols in an equilibrium mixture, a subsequent step involving selective removal of 11-cis-ROL is required to drive synthesis of 11-cis-retinoids for production of visual chromophore. Selective esterification of 11-cis-ROL is one possibility. Crude homogenates of chicken retinas rapidly convert all-trans-ROL to 11-cis-retinyl esters (11-cis-REs) with minimal formation of other retinyl-ester isomers. This enzymatic activity implies the existence of an 11-cis-specific retinyl-ester synthase in Müller cells. Here, we evaluated multifunctional O-acyltransferase (MFAT) as a candidate for this 11-cis-RE-synthase. MFAT exhibited much higher catalytic efficiency as a synthase of 11-cis-REs versus other retinyl-ester isomers. Further, we show that MFAT is expressed in Müller cells. Finally, homogenates of cells coexpressing DES1 and MFAT catalyzed the conversion of all-trans-ROL to 11-cis-RP, similar to what we observed with chicken-retina homogenates. MFAT is therefore an excellent candidate for the retinyl-ester synthase that cooperates with DES1 to drive synthesis of 11-cis-retinoids by mass action.
authors.affiliation: Jules Stein Eye Institute and.
authors.forename: Joanna J
authors.initials: JJ
authors.lastname: Kaylor
authors.affiliation: Jules Stein Eye Institute and.
authors.forename: Jeremy D
authors.initials: JD
authors.lastname: Cook
authors.affiliation: Jules Stein Eye Institute and.
authors.forename: Jacob
authors.initials: J
authors.lastname: Makshanoff
authors.affiliation: Jules Stein Eye Institute and.
authors.forename: Nicholas
authors.initials: N
authors.lastname: Bischoff
authors.affiliation: Jules Stein Eye Institute and.
authors.forename: Jennifer
authors.initials: J
authors.lastname: Yong
authors.affiliation: Jules Stein Eye Institute andDepartment of Biological Chemistry, University of California, Los Angeles School of Medicine, Los Angeles, CA 90095 travis@jsei.ucla.edu.
authors.forename: Gabriel H
authors.initials: GH
authors.lastname: Travis
mesh_terms: D000123:Acetyltransferases / Q000378:metabolism*; D000818:Animals; D002384:Catalysis; D002417:Cattle; D002645:Chickens; D055358:Cone Opsins / Q000378:metabolism; D063928:Ependymoglial Cells / Q000201:enzymology*; D004952:Esters / Q000737:chemistry; D005227:Fatty Acids / Q000737:chemistry; D020869:Gene Expression Profiling; D015971:Gene Expression Regulation, Enzymologic; D057809:HEK293 Cells; D006801:Humans; D007700:Kinetics; D051379:Mice; D064251:Multifunctional Enzymes / Q000378:metabolism*; D055355:Opsins / Q000378:metabolism; D012160:Retina / Q000378:metabolism; D051142:Retinol O-Fatty-Acyltransferase / Q000378:metabolism*
chemical_list: D055358:Cone Opsins; D004952:Esters; D005227:Fatty Acids; D064251:Multifunctional Enzymes; D055355:Opsins; D000123:Acetyltransferases; D051142:Retinol O-Fatty-Acyltransferase