Effects of UV radiation on membrane proteins, lipids and long-distance translocation of fatty acids: Implications for signaling

An important consequence of stratospheric ozone depletion is increased transmission of solar ultraviolet (UV) radiation to the Earth's lower atmosphere and surface. UV radiation is known to affect a wide range of biological and chemical processes, and thus is deleterious to all living organisms. Plants are particularly vulnerable to UV radiation as a result of their dependence upon sunlight for photosynthesis. UV radiation has been demonstrated to retard plant growth, decrease biomass accumulation, inhibit photosynthesis and induce changes in gene expression patterns, which in concert have profound ecological and agricultural implications.; In the present study, the effects of sublethal doses of UV radiation on the plasma membrane H+-ATPase, membrane lipids and long-distance translocation of fatty acids have been investigated in Brassica napus L. plants. H+-ATPase was utilized as an index of UV stress response. Sublethal doses of UV radiation resulted in activation of the enzyme, and the sensitivity of the protein to UV increased markedly with the onset of organ senescence, possibly attributable to higher levels of free fatty acids. Indeed, sublethal UV radiation also activated membrane lipid metabolism resulting in an increase in free fatty acid content within membrane bilayers. Of particular interest is the finding that this UV treatment did not result in an overall decrease in lipid concentration in irradiated tissues, but rather caused de-esterification of polar lipids and a concurrent accumulation of free fatty acids.; Results of the present study also provide evidence for a mechanism that allows rapid removal of damaged H+-ATPase protein and lipid metabolites from the bilayer. It is evident that membrane lipid and H +-ATPase catabolites, once formed, phase-separate within the plane of the bilayer and form distinct domains which are subsequently removed from the membrane by microvesiculation or blebbing of lipid-protein particles. These distinct domains were isolated from the plasma membrane by immunoprecipitation using antibodies specific to H+-ATPase catabolites rather than the native form of the polypeptide. Detailed lipid and fatty acid analysis of immunopurified plasma membrane vesicles corresponding to the phase-separated domains revealed significant enrichment in lipid catabolites including diacylglycerol, free fatty acids and steryl/wax esters, and a concurrent depletion in polar lipid content.; In addition, a novel class of lipid-containing particles was isolated from the phloem sap of canola. These particles are spherical, and of variable size. The lipid composition of the particles is markedly different from that of microsomal membranes and cytosol isolated from both leaves and stems. Specifically, the phloem particles contain predominantly unesterified fatty acids and are enriched in medium-chain fatty acids, in particular, lauric, myristic and pentadecanoic acids. This unique lipid profile distinguishes phloem particles from other plant lipid bodies. Furthermore, the phloem content of unusual fatty acids increases significantly in plants exposed to sublethal levels of UV radiation. The greatest effect was observed in respect of lauric acid, which increased from 3.4% to 24.1% of the total phloem fatty acid pool. The presence of lipid particles in phloem sap suggests that long-distance transport of lipid occurs in plants, and their enrichment in unusual medium chain fatty acids implies involvement in signaling events. This notion is supported by the rapid increase in phloem lauric acid observed in response to sublethal UV stress. Indeed, translocation of lauric acid through the phloem may be an element of UV-induced signal transduction.