Regulation of protein trafficking by glycosylphosphatidylinositol valence in African trypanosomes

The structure, biosynthesis, and attachment of glycosylphosphatidylinositol (GPI) anchors were all first determined for the variant surface glycoprotein (VSG) of African trypanosomes, and all of the basic aspects of this work have been shown to be universal in eukaryotic organisms. However, the role of GPI anchors in protein trafficking within trypanosomes has lagged behind the more standard eukaryotic model systems such as yeast and polarized epithelial cells. Trypanosomes are also highly polarized cells in which all endocytosis and exocytosis intersect at a discrete domain of the plasma membrane, the flagellar pocket. Within these convergent pathways trafficking of GPI anchored proteins correlates strongly with valence: homodimeric VSG with two GPIs is stably incorporated into the cell surface coat, heterodimeric transferrin receptor with a single GPI is found in the flagellar pocket and is slowly delivered to the lysosome for degradation, and recombinant GPI minus VSG reporters are rapidly degraded in the lysosome. Here we summarize recent data confirming this correlation using a tool kit of recombinant GPI anchored reporters, including a reporter designed to be conditionally modulated between a GPI valence of one and two.