Baseplate assembly of phage Mu: Defining the conserved core components of contractile-tailed phages and related bacterial systems

Abstract

Contractile phage tails are powerful cell puncturing nanomachines that have been co-opted by bacteria for self-defense against both bacteria and eukaryotic cells. The tail of phage T4 has long served as the paradigm for understanding contractile tail-like systems despite its greater complexity compared with other contractile-tailed phages. Here, we present a detailed investigation of the assembly of a "simple" contractile-tailed phage baseplate, that of Escherichia coli phage Mu. By coexpressing various combinations of putative Mu baseplate proteins, we defined the required components of this baseplate and delineated its assembly pathway. We show that the Mu baseplate is constructed through the independent assembly of wedges that are organized around a central hub complex. The Mu wedges are comprised of only three protein subunits rather than the seven found in the equivalent structure in T4. Through extensive bioinformatic analyses, we found that homologs of the essential components of the Mu baseplate can be identified in the majority of contractile-tailed phages and prophages. No T4-like prophages were identified. The conserved simple baseplate components were also found in contractile tailderived bacterial apparatuses, such as type VI secretion systems, Photorhabdus virulence cassettes, and R-type tailocins. Our work highlights the evolutionary connections and similarities in the biochemical behavior of phage Mu wedge components and the TssF and TssG proteins of the type VI secretion system. In addition, we demonstrate the importance of the Mu baseplate as a model system for understanding bacterial phage tail-derived systems.baseplate | assembly | contractile tail | phage Mu

Cryo‐ EM reconstruction of Type VI secretion system baseplate and sheath distal end

“…The central spike is surrounded by six copies of a structure resembling a phage baseplate wedge (Leiman & Shneider, 2012; Büttner et al , 2016; Taylor et al , 2016; Fig 2) and is decorated by a protein density, which is likely a connector between the baseplate and the membrane complex (Fig 2A–E). Overall, the T6SS baseplate resembles so‐called minimal baseplate of contractile tail‐like structures and is similar to the T4 phage inner baseplate in its pre‐injection state (Appendix Fig S2; Leiman & Shneider, 2012; Taylor et al , 2016).…”

“…Although the 8 Å resolution of the baseplate reconstruction prevents precise segmentation of individual components, the overall baseplate structure clearly resembles T4 inner baseplate and represents the simplest type of baseplates conserved among other contractile tail‐like systems (Leiman & Shneider, 2012; Büttner et al , 2016; Taylor et al , 2016). …”

“…Our reconstruction also suggests that TssK protein may be involved in recruitment of certain effectors to the spike. Additionally, it is possible that some organisms evolved proteins that would serve as extensions for the wedge and would thus further increase the size of the baseplate cavity similarly to the variations in baseplate sizes seen in phages (Fu et al , 2011; Schwarzer et al , 2012; Hu et al , 2015; Büttner et al , 2016; Nováček et al , 2016; Taylor et al , 2016). …”

Molecular Analysis of Arthrobacter Myovirus vB_ArtM-ArV1: We Blame It on the Tail

“…For many years, bacteriophage T4, the most thoroughly investigated myovirus, served as a model system for all contractile phage tail-like structures. However, quite a number of myoviruses, especially those with smaller genomes, have tails significantly less complex than that of T4 (27,68). Therefore, by analyzing the available genomic and protein structural data for three well-studied myoviruses (T4, P2, and Mu), Leiman and Shneider identified a minimal set of 12 proteins that comprise a functional contractile phage tail (66) Based on the data provided in Results, we tentatively predict that at least 9 ArV1 structural proteins represent the aforementioned conserved set (Table 3), and this suggests, in accordance with the TEM results, that bacteriophage ArV1 has a "simple" baseplate likely somewhat similar to that of Mu.…”

“…The baseplate protein ArV1 gp19 contains a C-terminal LysM superfamily domain (cl23764) and is similar to LysM and cell wall-binding domain-containing proteins from a wide range of diverse bacteria (128 hits) as well as phages active against Arthrobacter (9 hits) or Clostridium (2 hits). Notably, it has been reported recently that the LysM domain is present in many phage baseplates, and it is often fused to the tail tube initiator (27). Based on its genomic position and the presence of LysM, ArV1 gp19 is a probable candidate for a tail tube initiator.…”