Glycosylinositol phospholipid (GPI)
membrane anchors are the sole means of membrane attachment of a large
number of cell surface proteins, including the variant surface
glycoproteins (VSGs) of the parasitic protozoan, Trypanosoma brucei.
Biosynthetic data suggest that GPI-anchored proteins are synthesized
with carboxy-terminal extensions that are immediately replaced by
GPI, suggesting the existence of preformed GPI species available for
transfer to the nascent protein in the ER. Candidate precursor
glycolipids having a linear sequence indistinguishable from the
conserved core structure found on all GPI anchors, have been
characterized in T brucei. In this paper we describe the transfer of
three GPI variants to endogenous VSG in vitro. GPI addition is not
reduced by inhibitors of protein synthesis and does not require ATP
or GTP, consistent with a transpeptidation mechanism.