A role for phospholipid polyunsaturation in modulating membrane protein function.
Visual transduction is one of the best characterized G protein--coupled signalling systems. In addition, about 50% of the disk membrane phospholipid acyl chains are 22:6n-3, making this system ideal for determining the role of polyunsaturation in modulating membrane-signalling systems. The extent of formation of metarhodopsin II (MII), the G protein--activating photointermediate of rhodopsin, was studied in phospholipid vesicles composed of a variety of phosphatidylcholines, differing in their acyl chain composition at the sn-2 position. The amount of MII formed increased progressively with the level of acyl chain unsaturation at the sn-2 position. The effect of added cholesterol was to reduce the amount of MII formed. The acyl chain packing free volume of the rhodopsin containing lipid vesicles was characterized by a fractional volume parameter fv derived from measurements of the time-resolved fluorescence anisotropy decay of the hydrophobic membrane probe 1,6-diphenyl-1,3,5-hexatriene. The relationship among sn-2 acyl chain unsaturation, cholesterol content, and MII formation is explained on the basis of variation in fv with bilayer lipid composition and a novel model for the packing of phospholipids containing polyenoic acyl chains, such as 22:6n-3.
Litman
B J
BJ
Laboratory of Membrane Biochemistry and Biophysics, National Institute on Alcoholism and Alcohol Abuse, National Institutes of Health, Rockville, Maryland 20852, USA.
Mitchell
D C
DC
D005224:Fats, Unsaturated / Q000737:chemistry*; D005454:Fluorescence Polarization; D008565:Membrane Proteins / Q000502:physiology*; D010743:Phospholipids / Q000737:chemistry*; D012243:Rhodopsin / Q000031:analogs & derivatives* / Q000096:biosynthesis
D005224:Fats, Unsaturated; D008565:Membrane Proteins; D010743:Phospholipids; C013110:metarhodopsins; D012243:Rhodopsin