A VETTIGER¹, M BASLER<sup>1

1</sup>Focal Area Infection Biology, Biozentrum, University of Basel, Basel, Switzerland

Bacterial Type VI secretion system (T6SS) is a nanomachine that works similarly to a speargun and shares structural and functional homology to a membrane attached bacteriophage. T6SS physically punctures a target cell membrane by a tip composed of valine-glycine repeated protein G (VgrGs) and effector molecules located at the end of a long shaft composed of haemolysin co-regulated protein (Hcp). This is driven by a rapid contraction of a sling called sheath. The deletion of essential structural components (e.g. Hcp, VgrG2 or certain effectors) leads to a complete absence of T6SS activity. Using live-cell imaging of T6SS assembly in Vibrio cholerae, we show that the T6SS substrates are exchanged between neighboring sister cells within tens of seconds after initial contact. Upon T6SS-dependent translocation, both tip and Hcp shaft dissociate and are subsequently reused to form a fully functional T6SS assembly. Detailed analysis of T6SS dynamics in trans-complemented cells revealed that the availability of tip associated proteins (e.g. VgrGs) regulates the number of sheath assemblies per cell, whereas sheath length correlates with Hcp concentration.
We describe here a novel form of direct protein exchange and recycling among by-standing sister cells that could lower the overall cost of constitutively active secretion machinery for the bacterial community. Furthermore, we gained intriguing insights into the regulation and assembly of the T6SS.