Novel structural components generate distinct type VI secretion system anchoring modes

Abstract

20 21 2 These authors have contributed equally to this work. 22 23 24 KEYWORDS 25 26 Type VI secretion system, interbacterial competition, contact-dependent killing, toxin 27 delivery, TssA, TagB, TagA, sheath stabilization, sheath contraction, Pseudomonas. 28 29 30 ABSTRACT 31 32The type VI secretion system (T6SS) is a phage-derived contractile nanomachine primarily 33 involved in interbacterial competition. Its pivotal component, TssA, is indispensable for the 34 assembly of the T6SS sheath structure, the contraction of which propels a payload of effector 35proteins into neighboring cells. Despite their key function, TssA proteins exhibit unexpected 36 diversity and exist in two major forms, a short (TssAS) and a long (TssAL) TssA. Whilst 37TssAL proteins interact with a partner, called TagA, to anchor the distal end of the extended 38sheath, the mechanism for the stabilization of TssAS-containing T6SSs remains unknown. 39Here we discover a novel class of structural components that interact with short TssA 40proteins and contribute to T6SS assembly by stabilizing the polymerizing sheath from the 41 baseplate. We demonstrate that the presence of these components is important for full sheath 42 extension and optimal firing. Moreover, we show that the pairing of each form of TssA with 43 a different class of sheath stabilization proteins results in T6SS apparatuses that either reside 44in the cell for a while or fire immediately after sheath extension, thus giving rise to different 45 aggression behaviors. We propose that this functional diversity could contribute to the 46 specialization of the T6SS to suit bacterial lifestyles in diverse environmental niches. 47 48 49Bacteria live in complex polymicrobial communities that are shaped by interspecies 50 cooperation and competition. As resources are limited, antagonistic strategies are a major 51 driver of survival and success for bacterial populations. One of the most elaborate bacterial 52weapons is the type VI secretion system (T6SS), which not only promotes inter-bacterial and 53inter-kingdom competition (1-3), but is also involved in the interaction of bacteria with their 54