Acylation of glucosaminyl phosphatidylinositol revisited - Palmitoyl-CoA dependent palmitoylation of the inositol residue of a synthetic dioctanoyl glucosaminyl phosphatidylinositol by hamster membranes permits efficient mannosylation of the glucosamine residue

Two critical steps in the assembly of yeast and mammalian glycosylphosphatidylinositol (GPI) anchor precursors are palmitoylation of the inositol residue and mannosylation of the glucosamine residue of the glucosaminyl phosphatidylinositol (GlcN alpha-PI) intermediate. Palmitoylation has been reported to be acyl-CoA dependent in yeast membranes (Costello, L. C., and Orlean, P. (1992) J. Biol. Chem. 267, 8599-8603) but strictly acyl-CoA independent in rodent membranes (Stevens, V. L., and Zhang, H. (1994) J. Biol. Chem. 269, 31397-31403), and thus poorly conserved. In addition, it was suggested that acylation must precede mannosylation in both yeast (Costello, L. C., and Orlean, P. (1992) J. Biol. Chem. 276, 8599-8603) and rodent (Urakaze, M., Kamitani, T., DeGasperi, R., Sugiyama, E., Chang, H.-M., Warren, C. D., and Yeh, E. T. H. (1992) J. Biol. Chem. 267, 6459-6462) cells because GlcN alpha-acyl-PI accumulates in vivo when mannosylation is blocked. However, GlcN alpha-acyl-PI accumulation would also be expected if mannosylation and acylation were independent of each other. These issues were addressed by the use of a synthetic dioctanoyl GlcN alpha-PI analogue (GlcN alpha-PI(C8)) as an in vitro substrate for GPI-synthesizing enzymes in Chinese hamster ovary cell membranes. GlcN alpha-PI(C8) was acylated in an manner requiring acyl-CoA. Thus, the process involving acyl-CoA reported for yeast has been conserved in mammals. Furthermore, both GlcN alpha-PI(C8) and GlcN alpha-acyl-PI(C8) could be mannosylated in vitro, but mannosylation of the latter was significantly more efficient. This provides direct support for the earlier suggestion that acylation precedes mannosylation in rodents cells. A similar result was also observed with the Saccharomyces cerevisiae mannosyltransferase. In contrast, it has been reported that mannosylation of endogenous GlcN alpha-PI by Trypansoma brucei membranes occurs without prior acylation. The same result was obtained with GlcN alpha-PI(C8), confirming that the mannosyltransferase of trypanosomes is divergent from those in yeasts and rodents.