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Summary

The biogenesis of the ER Exit Site/Golgi Junction (EGJ) in bloodstream-form African trypanosomes is investigated using tagged markers for ER Exit Sites, the Golgi and the bilobe structure. The typical pattern is two EGJ in G1 phase (1 kinetoplast/1 nucleus, 1K1N) through S-phase (2K1N), duplication to four EGJ in post-mitotic cells (2K2N) and segregation of two EGJ to each daughter. Lesser cell percentages have elevated EGJ copy numbers in all stages, and blocking cell cycle progression results in even higher copy numbers. EGJs are closely aligned with the flagellar attachment zone (FAZ) indicating nucleation on the FAZ-associated ER (FAZ:ER). Only the most posterior EGJ in each cell is in proximity to the bilobe, which is located at the base of the FAZ filament near the mouth of the flagellar pocket. These results indicate that EGJ replication in bloodstream trypanosomes is not tightly coupled to the cell cycle. Furthermore, segregation of EGJ is not obligately mediated by the bilobe, rather assembly of the EGJ on the FAZ:ER, which is coupled to the flagellar cytoskeleton, apparently ensures segregation with fidelity during cytokinesis. These findings differ markedly from procyclic-form trypanosomes, and models highlighting these stage-specific differences in EGJ biogenesis are proposed.