Sm25, a major schistosome tegumental glycoprotein, is dependent on palmitic acid for membrane attachment

Sm25, a major antigen in the surface tegument of the parasitic helminth Schistosoma mansoni, is a 25 kDa N-glycosylated glycoprotein which co-purifies with isolated surface membranes and behaves as an integral membrane protein in Triton X-114 (TX-114). The deduced amino acid sequence of Sm25 shows a short C-terminal hydrophobic domain between residues 163 and 180, containing six uncharged polar amino acids and followed by a Lys181-Ser192 dipeptide. We were interested in whether or not this marginal C-terminal amphiphilic domain is responsible for the association of Sm25 with the membrane or whether a post-translational modification such as the addition of glycosyl phosphatidyl inositol (GPI) represents the membrane anchor for this molecule. We find that treatment with phosphatidyl inositol-specific phospholipase C, which cleaves many GPI anchors, does not reveal Cross Reacting Determinant (CRD) on Sm25, nor affect the association of this protein with membranes, providing no support for the addition of GPI. However, Sm25 is palmitoylated via a thioester bond to the single Cys residue, at position 168, which lies within the C-terminal hydrophobic domain. Removal of palmitate by reduction results in a marked decrease in the hydrophobicity of Sm25, as demonstrated by its partitioning into the aqueous rather than detergent phase of TX-114 and its quantitative release from membrane preparations. The hydrophobicity of several membrane proteins in addition to Sm25 is also decreased by reduction, raising the possibility that fatty acylation by thioester linkage is an important mechanism used by schistosomes to stabilize protein-membrane interactions.