Firing of Streptomyces coelicolor contractile injection system requires the membrane protein CisA

Abstract

Bacterial contractile injection systems (CIS) are phage tail-like macromolecular complexes that mediate cell-cell interactions by injecting effector proteins into target cells. CIS^Sc from Streptomyces coelicolor are distinct because they induce regulated cell death under stress, impacting the organism’s life cycle. It remains unknown whether CIS^Sc require accessory proteins to function.

Here we characterize the putative membrane adaptor CisA, a conserved factor in CIS^Sc gene clusters across Streptomyces species. We show by cryo-electron tomography imaging and in vivo assays that CIS^Sc contraction and function depend on CisA. Using single-particle cryo-electron microscopy, we provide an atomic model of the extended CIS^Sc apparatus; 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 CIS^Sc components, thereby providing a possible mechanism for mediating CIS^Sc recruitment 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 CIS^Sc in regulated cell death.