Transition Fibers Play a Central Role in Import of a Glucose Transporter into the Flagellar Membrane

Leishmania parasite promastigotes have a motile flagellum that encompasses a variety of integral membrane proteins, but how such proteins traffic selectively to the flagellum is largely unknown. The flagellar-specific glucose transporter, GT1, binds to the KHARON protein via the transporter hydrophilic amino terminal domain, and KHARON is essential for trafficking of GT1 from the flagellar pocket membrane at the base of the flagellum into the flagellar membrane. A major objective of this study was to determine the mechanism whereby KHARON transports GT1 into the flagellar membrane. We have determined that KHARON is a novel component of the transition fibers that project from the basal body at the base of the flagellum toward the flagellar membrane. We propose that KHARON binds GT1 at the transition fibers and then transfers it to intraflagellar transport (IFT) trains as they enter the flagellum at this site. The IFT trains, associated with the membrane tethering protein TULP, then move GT1 through the transition zone filter into the body of the flagellum. These results provide evidence for a role for transition fibers in transport of membrane proteins into flagella.