BSF FAZ proteins operate in PCF coat environment

Throughout the life cycle of the unicellular parasite Trypanosoma brucei, its single flagellum remains laterally attached to the cell body by the proteins FLA and FLABP, even as the parasite differentiates from the bloodstream form (BSF), found in the mammalian host, to the procyclic form (PCF), in the insect midgut. This differentiation is accompanied by changes in the dominant surface coat protein from the variable surface glycoprotein to procyclins. There are stage-specific variants of the FLA and FLABP proteins, with FLA2 and FLA2BP found in BSFs and FLA1 and FLA1BP in PCFs. Yet, how these proteins maintain flagellum attachment during the differentiation from BSFs to PCFs and accompanying change in surface coat environment is unknown. Here, we used a double-induction system to test whether FLA2 and FLA2BP can maintain flagellum attachment in cells expressing procyclins. While FLA2 was able to compensate for the loss of FLA1, FLA2BP was mis-localised in PCFs and could not compensate for the loss of FLA1BP. Interestingly, when FLA2 was expressed alongside FLA2BP, FLA2BP localised to the FAZ and flagellum attachment was maintained. Thus, we conclude that FLA2 and FLA2BP, together, are able to maintain flagellum attachment as the surface coat environment changes during BSF to PCF differentiation.