Figure 1.

Routes responsible for trafficking of variant surface glycoproteins (VSG). (A) The major intracellular compartments that are known to be associated with VSG trafficking, with the flagellar pocket shown at top left. Rab GTPase proteins, which are convenient markers for many subcellular compartments, are shown as green ovals, whereas arrows indicate the major known transport routes and red indicates several molecular complexes within the endocytic system that are of significance. (B) The overall structure of a VSG dimer, colour coded with α-helical elements in red, β-sheets in blue, comprising the X-ray and NMR structures for the N- and C-terminal domains [83,84] (note that the arrangement of these domains with each other is arbitrary because this is not experimentally known). (C) A hypothetical array of VSGs with the tips of the C-terminal domains just touching. (D) A simplified version of (A) but rekeyed to illustrate the significant concentration gradient of VSG as it exits the ER and is then concentrated at the plasma membrane. Arrows indicate major routes, with blue for biosynthesis, red for degradation and grey for recycling. The approximate number of VSG molecules that are in flux through each route is also indicated. Abbreviation: CAPS, clathrin-associated proteins; ERES, endoplasmic reticulum exit sites; ESCRT, endosomal-sorting complex required for transport.