Structural characterisation of two forms of procyclic acidic repetitive protein expressed by procyclic forms of Trypanosoma brucei

A procyclic acidic repetitive protein (PARP) fraction was purified from long-term cultures of Trypanosoma brucei procyclic forms by a solvent-extraction and reverse phase chromatography procedure. The PARP fraction yielded small quantities of a single N-linked oligosaccharide with the structure Mana1-6(Man a1-3)Mana1-6(Mana1-3)Manß1-4GlcNAcß1-4GlcNAc(Man₅GlcNAc₂). Fractionation of PARP on Con A-Sepharose revealed that the majority (80 to 90%) of the PARP fraction did not bind to Con A and was composed of the parpAa gene product that contains repeats of -Glu-Pro-Pro-Thr- (GPEET-PARP) and that lacks an N-glycosylation site. This form of PARP has not been previously identified at the protein-level. The minor Con-A-binding fraction was shown to be rich in the previously described form of PARP, encoded by the parpAß and/or parpBa genes, that contains a -Glu-Pro- repeat domain (EP-PARP) and an N-glycosylation site. Analysis of longer and shorter-term cultures suggested that procyclic tells initially express predominantly EP-PARP that is gradually replaced by GPEET-PARP. Both forms of PARP were shown to contain indistinguishable glycosylphosphatidylinositol (GPI) membrane anchors, where the conserved GPI core structure is substituted by heterogeneous sialylated branched polylactosamine-like structures that are predicted to form a dense surface glycocalyx above which the polyanionic -Glu-Pro-Pro-Thr- and -Glu-Pro- repeat domains are displayed. The phosphatidylinositol (PI) component of the GPI anchor was shown to be a mixture of 2-O-acyl-myo-inositol-1-HPO₄-(sn-1-stearoyl-2-lyso-glycerol) and 2-O-acyl-myo-inositol-1-HPO₄-(sn-1-octadecyl-2-lyso-glycerol), where the acyl chain substituting the inositol ring showed considerable heterogeneity. Mass spectrometric and light scattering experiments both suggested an average mass of approximately 15 kDa for GPEET-PARP, with individual glycoforms ranging from about 12 kDa to 20 kDa, that is consistent with its amino acid and carbohydrate composition. A measured translational diffusion coefficient of 3.9 x 10⁷ cm² s^-1 indicates that this molecule has a highly elongated shape. The possible functions of these unusual glycoproteins are discussed. (C) 1997 Academic Press Limited.