Authors

J Bangs¹;  ¹ Department of Microbiology & Immunology, University of Buffalo, United States

Discussion

African trypanosomes survive in the bloodstream and tissues in the face of the humoral immune system of the mammalian host by a remarkable process called antigenic variation. The lynchpin of this process is the variant surface glycoprotein (VSG), which is 10% of total cell protein. There is a repertoire of ~1500 VSG genes, only one of which can be expressed at time. Switching occurs primarily by gene conversion whereby the active gene is over-written by another gene from the silent repertoire. Often this process is segmental leading to fusion of sequences from multiple VSG genes. This will ultimately lead to assembly of incompatible segments that are incapable of folding and subsequent forward trafficking from the ER - a catastrophic event unless the cell can cope with the accumulated cargo. Misfolded secretory proteins are generally retained by ER quality control (ERQC) and degraded in the proteasome by ER associated degradation (ERAD). However, in yeast and mammals misfolded glycosylphosphatidylinositol-anchored proteins (GPI-APs) are preferentially degraded in the vacuole/lysosome. To determine if ERAD can handle misfolded GPI-APs in trypanosomes we exploit a misfolded GPI-anchored subunit (HA:E6) of the trypanosome transferrin receptor. HA:E6 is N-glycosylated and GPI-anchored, and accumulates in the ER as aggregates. Treatment with MG132, a proteasome inhibitor, generates a smaller protected polypeptide (HA:E6*), consistent with turnover in the proteasome. HA:E6* partitions between membrane and cytosol fractions, and both pools are proteinase K-sensitive, indicating cytosolic disposition of membrane associated HA:E6*. HA:E6* is de-N-glycosylated and has a full GPI-glycan structure from which dimyristoylglycerol has been removed, indicating that complete GPI removal is not a prerequisite for proteasomal degradation. However, HA:E6* is apparently not ubiquitin modified. Like yeast and mammals, the trypanosome GPI anchor is a forward trafficking signal, thus the dynamic tension between ERQC and ER exit favors degradation by ERAD. These results differ markedly from the standard eukaryotic model systems, and may indicate an evolutionary advantage related to pathogenesis.