Tracking the baseplate of contractile machines
ALAIN FILLOUX1 and PAUL FREEMONT2
1MRC Centre for Molecular Bacteriology and Infection (CMBI), Department of Life Sciences, Imperial College London, London SW7 2AZ, United Kingdom.
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2Section Structural Biology, Department of Medicine, Imperial College London, London SW7 2AZ, United Kingdom.
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Supra-molecular multi-protein complexes are fascinating objects that can be studied in their most intimate details thanks to recent transformative developments in single particle cryo-electron microscopy. The atomic structure of the T4 phage baseplate, and its6-megadalton size, is an extraordinary achievement in structural biology paving the way to understanding themolecular dynamics of contractile machines at near atomic resolution, such as the phage tail or bacterial type VI secretion system.
Simple organisms like bacterial viruses, developed sophisticated molecular machines thatenable their life cycle. These viruses store genetic material ina capsid with the aim of transferring it into bacterial preys after landing on their surface. Attachment relies on fibresemerging from a baseplate connected to the bacteriophage tail tube and spike. Upon landingconformational changes occurring within the baseplatetrigger the contraction of the tail sheath, puncturing the prey’s cell envelope and transferring the phage DNA from the capsid, through the tail tube and into the bacterial cytosol.It has been a long journey since the description of the genetic structure of the T4 phage back in the 50’s1. This led to meticulous structural reconstructions of the whole phage2,3culminating in the paper by Taylor et al. published in the Nature issue of May 19th 20164. Thestudy reports an unprecedented level of detail for the bacteriophage T4 baseplate-tailimaged at near atomic resolution using single particle cryo-electron microscopy,a real tour de force in structural biology.
The phage baseplate-tail is made of a complex assembly of gpproteins, which identify all the different phagegene products.In their study, Taylor et al4 have completed 3D structures for all the gp’s, notably gp25, and using modelling programs, have fitted all of these structures into an improved EM image of the T4 tail-baseplate generated by cryo-electron microscopy at 3.8Å resolution. The result is stunning and shows how each of the 15 different proteins contributes to the sequential construction of this impressive edifice 49 nm wide by 32 nm high edifice (excluding the tube part). The shape of the baseplate is roughly conical, with the tail tube emerging at the narrowest end. One remarkable feature revealed in this study4 is the distinction of 3 layers, which are called inner, intermediate and peripheral,the latter corresponding to the widest part of the baseplate where the fibres network is connected. Each layer is made by the complex entanglement of specific set of gp’s, which are present in multi-copies and sometime in different conformation, such as gp65 in the inner baseplate.
One hurdle in the mechanistic analysis of highly complex but dynamic molecular machines is to understand how they move. How dosmall conformational changes withinproteins network dramatically impact the equilibrium of the whole structureand generate sufficient forces for rapid contraction?Taylor et al4. capitalized on their success in imaging a mixture of pre- and post-attachment baseplate particles and resolved both conformations allowing a direct comparison. As could be expected the most significant changes reside in the peripheral baseplate connecting with the fibres network in direct contact with the cell surface. The long-tail-fibres (LTFs) are rooted into the baseplate via attachment to gp proteins, such as gp96 which connect LTFs to the (gp10-gp11-gp12)-containing peripheral baseplate. One remarkable observation by Taylor et al4.in the post-attachment stage is the rotation of the gp10-gp11-gp12 complexwhich results in conformational changes within the gp7 protein in the context of(gp6)2-gp7 heterotrimer ring in the inner baseplate. This results in a widening of the ring and dissociation of gp25 and gp53 from the tail tube. This makes perfect sense in the post-attachment state,as the tube should be able to glide through the baseplate upon sheath contraction.
The bacteriophage tail is acontractile machine that has been adapted through evolution to achieve a larger set of biological functions7.R-type pyocins resemble the tail/baseplate part of the T4 phage though in this case no injection of material into the prey cells but piercing allows the formation of a membrane channel which compromises target cell survival. The type VI secretion system (T6SS) is another analog of a phage tail/baseplate-like complex8. The device is assembled within the cytosol of the bacterium and ejects toxins through the bacterial cell envelope and into either prokaryotic or eukaryotic target cells. For T6SS and pyocins, the tail/baseplate-likeassemblageis a minimal versionof the bacteriophage. The tail and spike are identified but the composition of the baseplate structure remains under debate, andis likely restricted to what is the inner/intermediate T4 baseplate. Taylor et al4., using previous information on the T6SS9,10,purified a complex of 4 proteins, TssEFGK,and tentatively confirmed these as baseplate components. It is unclear as to whether this complex could form a circular structure similar to the T4 phage baseplate and it ispossible that additional T6SS proteins like TssA are required for the complete assemblyof the baseplate since TssA forms a gp6-like ring structure11.
It is of course tempting to try to establish a perfect parallel between eachgp and theircognate T6SS ortholog, such as further speculated by Taylor et al4. However, this is not without risk giventhe ancient divergence of both systems wheresequence conservation is very poor if non-existent. Furthermore, a number of domain swaps and protein rearrangements have occurred such that the single gp18 corresponds to TssBCin the T6SS12, or the T6SS VgrG protein corresponds to both gp5 and gp2713.This is likely to be a recurrent theme such thatproposing one-to-one assignments is hazardous. TssE is a genuine gp25 homolog12 with both proteinsacting as natural extensions of the sheath to firmly anchor it in the baseplate, howeverTssA has been proposed as a full component of the baseplate11while others propose it resides at the other end of the T6SS tail14. Tracking similarities is helpful but because of drastically distinct biological functions, it is unlikely that T4 phage and T6SS work in a strictly similar manner.Instead distinct and specific features may clearly be reflectedonce the atomic structure of the T6SS baseplate isresolved. The outstanding cryo-EM structures of pre- and post-contracted sheath of R-type pyocins15, already point at differences as compared to the T6SS or the T4 phage. Yet, the extraordinary results of Taylor et al4should be seen as a firm basis for exploring similarities and differences between the phage and bacterial contractile machines.
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Figure 1 Structure of contractile naomachines
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