Abstract
Contractile injection systems (CISs) mediate cell-cell interactions by a phage tail-like apparatus. Their conserved mechanism relies on the anchoring of the proximal end of a sheath-tube module to a membrane, followed by contraction of the sheath towards the attachment site and ejection of the inner tube. Here we reveal a major variation of the CIS mechanism in the type six secretion system (T6SS) of enteroaggregative Escherichia coli (EAEC). We show that both ends of the sheath-tube module are attached to opposite sides of the cell, enabling the structure to contract in two opposite directions. The protein TssA1 mediates the interaction of the distal end with the cell envelope, the termination of tail elongation, and non-canonical contraction towards the distal end. We provide a framework for the molecular processes at the T6SS distal end. Further research will address whether bidirectional contraction allows for bidirectional effector secretion. The unrecognized concept of non-canonical contractions could be relevant to biofilms of the human intestine.
Highlights
Contractile injection systems (CISs) mediate cell-cell interactions by a phage tail-like apparatus
The analysis of fluorescence light microscopy data revealed that T6SS structures that were parallel to the imaging plane usually polymerized until they reached the opposite side of the cell and stopped growing
17% of the monitored events showed T6SS sheaths that entirely contracted toward the distal end (Fig. 1b). Another 16.6% of all events accounted for a third pattern of T6SS structures that split into two fragments, which simultaneously contracted toward the proximal end and distal end, respectively (Fig. 1c, d)
Summary
Contractile injection systems (CISs) mediate cell-cell interactions by a phage tail-like apparatus Their conserved mechanism relies on the anchoring of the proximal end of a sheath-tube module to a membrane, followed by contraction of the sheath towards the attachment site and ejection of the inner tube. We reveal a major variation of the CIS mechanism in the type six secretion system (T6SS) of enteroaggregative Escherichia coli (EAEC) We show that both ends of the sheath-tube module are attached to opposite sides of the cell, enabling the structure to contract in two opposite directions. All CISs are structurally, functionally and evolutionarily related to contractile phages2,3 Their conserved mechanistic theme relies on the attachment of one end of a sheath-tube module to a membrane, followed by contraction of the tail sheath toward this proximal attachment site, which in turn ejects the inner tube. A recent study suggested a role of the T6SS protein TagA in terminating T6SS tail elongation
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