12. Glycosyl-phosphatidylinositol and Glycosyl-inositolphosphorylceramide
The discovery that glycosylated lipid molecules anchor proteins to cell membranes and that both
the lipid anchors and the proteins are involved in cellular response to environmental stimuli has
opened up an exciting area of research [104 – 106]. In plant cells, reports of the involvement of two
such anchors, a glycosyl – PtdIns GPI-anchored nitrate reductase in blue light-stimulated nitrate
uptake [107,108] and stimulation of glycosyl – ce- ramide-anchored alkaline phosphatase in low-
phosphate medium [18,109], suggest that they may play important roles in plants as well.
The structure of the lipid moiety that anchors enzymes to the hydrophobic membranes has been
investigated in a number of cells and structural information gleaned from these studies reveals
that the GPI anchors have conserved and variable structural moieties [18,104 – 106] Fig. 12. In the
conserved core structure, the 6-hydroxyl of the Ins moiety is glycosylated with a tetrasaccharide chain
containing one glucosamine and three mannose units. The third mannose is connected to a phos-
phorylated ethanolamine, the amine group of which forms an amide bond with the C-terminal
end of the protein. Variability in the lipid portion includes the presence of a 1-alkyl-2-acyl glycerol
or a ceramide unit in place of the diacylglycerol see below and variation in the fatty acid compo-
sition. Structural heterogeneity in the hydrophilic portion of the molecule includes esterification of a
hydroxyl at carbon 2 on the MI moiety with a long chain fatty acid, the presence of chiro-inosi-
tol, galactose molecules glycosidically linked to the core
mannose groups,
and two
or three
ethanolamine molecules. Biosynthesis of GPI an- chors involves the sequential glycosylation of Pt-
dIns in a step-wise manner and post translational modification of the protein with the preformed
GPI anchor [105].
Fig. 11. Scheme for biosynthesis of phosphatidylinositols with different head groups.
Fig. 12. Structures of A glycosyl-phosphatidylinositol and B glycosyl-inositolphosphorylceramide.
Employing methods developed for animal cells, Tischner and his colleagues showed that the inosi-
tol-containing lipid anchor of nitrate reductase includes a diacylglycerophosphatidyl-MI moiety
[107,108]. These methods include i release of the protein from membrane vesicles by the action of
PtdIns-specific phospholipase C, ii decrease in the hydrophobicity of the released protein, iii in
vivo
labeling of
the anchored
protein by
[
3
H]ethanolamine, and iv cross reactivity with monoclonal antibody raised against the GPI-an-
chor of
Trypanosoma GPI-anchored
protein [107,108]. However, the lipid anchor of alkaline
phosphatase in Spirodela exhibits chemical reactiv- ity that is not consistent with the presence of a
GPI unit [18,109]. These characteristics include, incorporation of myo-[
3
H]inositol, [
3
H]ethano- lamine, [
3
H]myristic and [
3
H]palmitic acid into lipid-bound enzyme, resistance to cleavage of
protein by PtdIns-specific phospholipase C, and hydrolysis of fatty acid units under strong acid
and alkaline conditions but not under mild alkali. On the basis of these chemical characteristics, the
authors have tentatively identified that the lipid moiety is a ceramide [18,109]. Thus the parent
lipid must be a glycosylated inositolphosphorylce- ramide, also called glycophosphosphingolipid [19]
Fig. 12. The presence of glycophosphosphin- golipid, namely lipids containing the inositolphos-
phorylceramide group, in seeds from cotton, peanut, corn and soybeans has been known since
the 1960s as a result of the pioneering work by Carter et al. [19]. However, our knowledge of the
biosynthesis, localization, and biological roles of sphingolipids is limited. In light of the continually
expanding role of lipids in signal transduction and the realization that sphingolipids mediate cell
growth and differentiation in animal cells, the recent discoveries [18,104] of the involvement of
inositol-containing glycosyl-lipids in nutrient as- similating processes has opened up an exciting
area for future research [110].
13. Concluding Remarks
Kategori