Touching the Surface: Diverse Roles for the Flagellar Membrane in Kinetoplastid Parasites.

While flagella have been studied extensively as motility organelles, with a focus on internal structures such as the axoneme, more recent research has illuminated the roles of the flagellar surface in a variety of biological processes. Parasitic protists of the order Kinetoplastida, which include trypanosomes and Leishmania species, provide a paradigm for probing the role of flagella in host-microbe interactions and illustrate that this interface between the flagellar surface and the host is of paramount importance. An increasing body of knowledge indicates that the flagellar membrane serves a multitude of functions at this interface: attachment of parasites to tissues within insect vectors, close interactions with intracellular organelles of vertebrate cells, transactions between flagella from different parasites, junctions between the flagella and the parasite cell body, emergence of nanotubes and exosomes from the parasite directed to either host or microbial targets, immune evasion, and sensing of the extracellular milieu. Recent whole-organelle or genome-wide studies have begun to identify protein components of the flagellar surface that must mediate these diverse host-parasite interactions. The increasing corpus of knowledge on kinetoplastid flagella will likely prove illuminating for other flagellated or ciliated pathogens as well.