Actin and myosin IIA have been implicated in the inward movement of receptor clusters at the immunological synapse of T lymphocytes. This study defines their spatial organization and quantifies their relative contributions to the dynamics of receptor clusters at the immunological synapse.

Murugesan et al. report that actomyosin arcs at the T cell synapse are formin-generated structures that directly propel T cell receptor cluster movement. The authors reveal the origin, organization, and functions of a major cytoskeletal network during synapse maturation.