Firing ofStreptomyces coelicolorcontractile injection system requires the membrane protein CisA

Abstract

AbstractBacterial contractile injection systems (CIS) are phage tail-like macromolecular complexes that mediate cell-cell interactions by injecting effector proteins into target cells. CISScfromStreptomyces coelicolorare distinct because they induce regulated cell death under stress, impacting the organism’s life cycle. It remains unknown whether CISScrequire accessory proteins to function.Here we characterize the putative membrane adaptor CisA, a conserved factor in CISScgene clusters acrossStreptomycesspecies. We show by cryo-electron tomography imaging andin vivoassays that CISSccontraction and function depend on CisA. Using single-particle cryo-electron microscopy, we provide an atomic model of the extended CISScapparatus; however, CisA is not part of the complex. Instead, our findings show that CisA is a membrane protein with a cytoplasmic N-terminus predicted to interact with CISSccomponents, thereby providing a possible mechanism for mediating CISScrecruitment to the membrane and subsequent firing.Our work shows that CIS function in multicellular bacteria is distinct from Type 6 Secretion Systems and extracellular CIS, and possibly evolved due to the role of CISScin regulated cell death.