The intracellular pathway and assembly of newly formed variable surface glycoprotein of Trypanosoma brucei

Pulse-chase experiments using L-[³⁵S]methionine suggests that Trypanosoma brucei MIT at 1.2 variable surface glycoprotein (VSG) synthesized in the rough endoplasmic reticulum, a process that takes 6-8 min, is shuttle4d to the Golgi complex 8 min later. Labeling of ultrathin frozen sections with affinity-purified anti-cross-reacting determinant (CRD) IgG followed by protein A-colloidal gold shows that the CRD is localized in the trans-Golgi region, cis-Golgi is not labeled. VSG, when solubilized by treatment with the detergent Nonidet P-40, behaves on sucrose density gradients as a non-membrane protein with a sedimentation value of 5 S. In contrast, VSG solubilized in the presence of Zwittergent TM 3-14 yielded several VSG-containing fractions >5 S, and only the 5S fraction contained the CRD. Lack of the CRD in VSG complexes with sedimentation values >5 S suggests that this determinant is either masked from antibody, perhaps by involvement in polymer formation, or represents the membrane form of VSG recently described by Cardoso de Almeida and Turner [Cardoso de Almeida, M.L. & Turner, M.J. (1983) Nature (London) 302, 349-352].