Trypanosoma brucei is an extracellular parasite which lives in the bloodstream of infected mammalian hosts and is in continuous exposure to the immune system. Its surface is covered in a dense glycoprotein coat which is continually endo- and exocytosed in order to remove any bound antibodies. Nutrients such as LDL (low density lipoprotein) and transferrin are scavenged by endocytosis. Remarkably, all endo- and exocytic activity is restricted to a single small subdomain of the plasma membrane - an invagination called the flagellar pocket. Even though trypanosomes are capable of internalising very large macromolecular complexes such as LDL, the exact size limit for macromolecule entry to the flagellar pocket is unknown. Cytoskeleton-associated protein complexes that are coiled around the neck of the flagellar pocket, such as the hook complex, are suspected to influence the size limit and endocytic activity. In this study, the size limits for fluid phase and surface-bound cargo entry were systematically measured. Depletion of specific protein components of the hook complex did not influence the defined size limit. Surprisingly, surface-bound cargo accumulated at the flagellar pocket entry after depletion, prompting a re-evaluation of the proposed function of these complexes.
