Stretching the arms of the type VI secretion sheath protein

The bacterial type VI secretion system (T6SS) is a multicomponent complex responsible for the translocation of effector proteins into the external milieu. The T6SS consists of an external sheath, an internal rigid tube, a baseplate, and a T6SS-specific membrane complex. Secretion is accomplished by the contraction of the sheath, which expels the effector-loaded tube. In this issue of EMBO reports, Brackmann et al [1] show how modifications of the sheath subunits can lock the T6SS assembly in the extended state. These findings allowed Wang et al [2] and Nazarov et al [3] to purify the T6SS sheath–tube–baseplate complex in the extended pre-secretion state and to analyze its structure using cryo-electron microscopy (cryoEM).