Role of bacteriophage T4 baseplate in regulating assembly and infection

Abstract

Bacteriophage T4 consists of a head for protecting its genome and a sheathed tail for inserting its genome into a host. The tail terminates with a multiprotein baseplate that changes its conformation from a "high-energy" dome-shaped to a "low-energy" star-shaped structure during infection. Although these two structures represent different minima in the total energy landscape of the baseplate assembly, as the dome-shaped structure readily changes to the star-shaped structure when the virus infects a host bacterium, the dome-shaped structure must have more energy than the star-shaped structure. Here we describe the electron microscopy structure of a 3.3-MDa in vitro-assembled star-shaped baseplate with a resolution of 3.8 Å. This structure, together with other genetic and structural data, shows why the high-energy baseplate is formed in the presence of the central hub and how the baseplate changes to the lowenergy structure, via two steps during infection. Thus, the presence of the central hub is required to initiate the assembly of metastable, high-energy structures. If the high-energy structure is formed and stabilized faster than the low-energy structure, there will be insufficient components to assemble the low-energy structure.bacteriophage T4 | cryo-EM reconstruction | baseplate assembly | conformational changes | near-atomic resolution M ost bacteriophages have a tail. At the distal end of the tail there is usually a baseplate that is decorated by some fibers (1). The baseplate initiates infection when the tail fibers bind to a host cell. Signals are transmitted from the tail fibers via the baseplate to the tail that then trigger the ejection of the phage genome from the head into the host cell through the tail tube. Two evolutionary related structures, of pyocin (2, 3) and of the type VI secretion system (4, 5), are found in bacteria as defense systems to kill competing bacteria. These structures are remarkably similar to the tail baseplate structure of bacteriophages, suggesting that tail baseplate-like structures are effective organelles for infecting bacteria (6, 7).T4 is a member of the Myoviridae family of bacteriophages. These phages have a sheath around the tail tube that contracts during infection (Fig. 1) (8). T4 has a complex baseplate that is essential for assuring a highly efficient infection mechanism (8). After recognition of an Escherichia coli host cell by some of the six long-tail fibers (LTF), the short-tail fibers (STF) that are a part of the baseplate, bind irreversibly to the cell. This process is accompanied by a large conformational change in the baseplate from a "high-energy" dome-to a "low-energy" star-shaped structure (9, 10), although each of these structures represent an energy minimum in the energy landscape of the baseplate assembly. This change triggers contraction of the tail sheath, driving the tail tube into the outer host cell membrane and further across the periplasmic space to the inner membrane. The genomic DNA is then ejected into the host's cytoplasm. Hence, the base...

TssA forms a gp6‐like ring attached to the type VI secretion sheath

“…Our BTH approach did not detect interactions between TssA1 and TssE1 or TssG1 which are other potential baseplate candidates (Fig ), but instead an interaction with VgrG1a (Fig ). VgrG proteins are gp5‐gp27 homologues with the latter described as the central hub in the phage baseplate (Yap et al , ). Our observation thus strengthens the concept of TssA1 being part of a larger T6SS complex which may display functional similarities with the phage baseplate.…”

Cryo-EM Elucidation of the Structure of Bacteriophage P22 Virions after Genome Release

“…The double-stranded DNA bacteriophages carry one of three types of tail: long contractile (members of the family Myoviridae), long noncontractile (Siphoviridae), and short noncontractile (Podoviridae). Although the very substantial tail contraction during Myoviridae DNA delivery into the host cell is best understood (2)(3)(4)(5), the apparently subtler changes in long and short noncontractile tails during this process are less clear (1,6). In addition to structural protein rearrangements, DNA ejection is accompanied by the release of a few proteins.…”

Common Evolutionary Origin of Procapsid Proteases, Phage Tail Tubes, and Tubes of Bacterial Type VI Secretion Systems

“…The T4 virion consists of a 1200 Å-long and 860 Å-wide prolate head (Fokine et al, 2004) encapsidating a 172-kbp double-stranded DNA molecule, and a 1000 Å-long tail (Kostyuchenko et al, 2005; Leiman et al, 2004) containing a rigid inner tube surrounded by a contractile sheath. The tail terminates in a baseplate (Kostyuchenko et al, 2003; Taylor et al, 2016; Yap et al, 2016) to which are attached six long tail fibers. When the phage baseplate attaches to an E. coli cell the tail sheath contracts and drives the tail tube across the cell’s periplasmic space.…”