Lipid remodelling of glycosylphosphatidylinositol (GPI) glycoconjugates in procyclic-form trypanosomes: biosynthesis and processing of GPIs revisited

The African trypanosome, Trypanosoma brucei, has been used as a model to study the biosynthesis of GPI (glycosylphosphatidylinositol)anchors. In mammalian (bloodstream)-form parasites, diacyl-type GPI precursors are remodelled in their lipid moieties before attachment to variant surface glycoproteins. In contrast,the GPI precursors of insect (procyclic)-form parasites, consisting

of lyso-(acyl)PI (inositol-acylated acyl-lyso-phosphatidylinositol)species, remain unaltered before protein attachment. By using a combination of metabolic labelling, cell-free assays and complementary MS analyses, we show in the present study that GPIanchored glycoconjugates in T. congolense procyclic forms initially

receive tri-acylated GPI precursors, which are subsequently

de-acylated either at the glycerol backbone or on the inositol ring. Chemical and enzymatic treatments of [3H]myristate-labelled lipids in combination with ESI-MS/MS (electrospray ionizationtandem MS) and MALDI-QIT-TOF-MS3 (matrix-assisted laserdesorption ionization–quadrupole ion trap–time-of-flight MS)analyses indicate that the structure of the lipid moieties of steadystate GPI lipids from T. congolense procyclic forms consist of a mixture of lyso-(acyl)PI, diacyl-PI and diacyl-(acyl)PI species. Interestingly, some of these species are myristoylated at the sn-2 position. To our knowledge, this is the first demonstration of lipid remodelling at the level of protein- or polysaccharide-linked GPIanchors in procyclic-form trypanosomes.