Kinetics of endocytosis and recycling of the GPI-anchored variant surface glycoprotein in Trypanosoma brucei.

The dense coat of glycosylphosphatidylinositol (GPI)-anchored variant surface glycoprotein (VSG) covering parasitic African trypanosomes is essential for survival in mammalian hosts. VSG is internalised and recycled exclusively via a specialised part of the plasma membrane, the flagellar pocket. Direct measurement of the kinetics of VSG endocytosis and recycling shows that the VSG cell-surface pool is turned over within 12 minutes. Correspondingly, the turnover of the intracellular pool (9+/-4% of total VSG) requires only 1 minute, and this is an exceptionally high rate considering that endocytosis and exocytosis are limited to only 5% of the cell surface area. Kinetic 3D co-localisation analysis using biotinylated VSG and a panel of compartmental markers provides consistent evidence for the itinerary of VSG through the cell: VSG is endocytosed in large clathrin-coated vesicles, which bud from the flagellar pocket membrane at a rate of 6-7 vesicles per second, and is then delivered to RAB5-positive early endosomes. From there, VSG is recycled to RAB11-positive recycling endosomes at two stages, either directly or via RAB7-positive, late endosomes. Small clathrin-coated vesicles carrying fluid-phase cargo and being depleted of VSG bud from early and recycling endosomes. These vesicles are postulated to deliver their content to late endosomes and/or the lysosome. The recycling endosomes give rise to RAB11-positive exocytic carriers that fuse with the flagellar pocket and thereby return VSG to the cell surface. VSG recycling provides an interesting model for studies on the cellular trafficking and sorting of GPI-anchored proteins.