Flotillins and ERM proteins function to promote uropod formation in T cells

Migrating and adherent T cells form a protruding leading edge and a constricted tail-like structure termed the uropod. The formation of these two structures involves the segregation of specific cytoskeletal elements and cell surface molecules, including proteins that organize cell polarity in other systems. Among the proteins that are segregated to the uropod are ezrin and moesin, ERM family proteins that organize cell membrane domains by linking cytoplasmic and cytosolic proteins to the actin cytoskeleton. Using conditional ezrin-deficient mice in conjunction with siRNA for moesin, we show that ezrin and moesin are required for uropod formation. In addition to ERM proteins, we find that the lipid-raft associated actin-binding proteins flotillins/reggies are also associated with the T cell uropod. Flotillins form a polarized cap even under conditions where a constricted uropod has not formed. Suppression of flotillin expression inhibits uropod formation and the polarization of ERM proteins and their binding partners. These data support a model in which flotillins mark a cell surface domain to which ERM proteins are recruited, and ERM proteins then promote formation of a constricted uropod structure. Since flotillins are not known to interact directly with ERM proteins, intermediary molecules, such as their common binding partner, PSGL-1, may be involved.