Discovery of a novel flagellar filament system underpinning Leishmania adhesion to surfaces.

Adhesion to surfaces is a common strategy employed across biology, especially by pathogens. Within their sand fly vector, Leishmania parasites undergo multiple developmental stages, including the understudied haptomonad form, which adheres to the sand fly stomodeal valve via a highly modified flagellum. This adhesion, likely critical for efficient transmission, is mediated by a complex adhesion plaque from which filaments in the modified flagellum extend toward the cell body and likely connect to the flagellum attachment zone (FAZ), a cytoskeletal structure important for cell morphogenesis. However, the role of the FAZ in adhesion and its relationship with the kinetoplastid-insect adhesion proteins (KIAPs) and the filamentous structures of the plaque itself remain unclear. Here, to examine the role of the FAZ in adhesion, we generated FAZ2, FAZ5, and FAZ34 deletion mutants. Deletion of any of these FAZ proteins impaired parasite adhesion in vitro. Furthermore, we identified a novel and distinct set of extra-axonemal flagellar filaments important for adhesion and demonstrated that KIAP2 is an essential component of these filaments. Our findings underscore the importance of a robust connection from the cell body to the adhesion plaque for stable Leishmania adhesion via the highly modified flagellum.