Polypeptides of the tail fibres of bacteriophage T4

A model is presented for the mechanism of packaging double-stranded DNA into phage heads. The model is based on, and rationalizes, the mismatch in symmetry between the heads and tails of large bacteriophages. DNA movement is postulated to be mediated by a rotating protein structure at the tail-proximal vertex of the head.

https://www.pnas.org/content/pnas/75/10/4779.full.pdf

Symmetry mismatch and DNA packaging in large bacteriophages.

The introduction of the use of T-even bacteriophages as genetic and biochemical experimental systems by Max Delbruck in the late 1930s has led to the intense study of many aspects of bacteriophage biology. Of these, two related endeavors, the study of the structure and the assembly of the virions, have been very important models in the development of our current understanding of macromolecular assembly processes. Twenty years ago, Edgar, Kellenberger, Epstein, and collaborators nucleated these studies by showing that phage assembly follows defined pathways that can accumulate assembly intermediates when blocked and that the assembly-naive components of phage T4 thus accumulated could join properly in vitro (Epstein et al., 1963; Edgar and Wood, 1966; Wood et al., 1968). Since that time, the structure and assembly of many bacteriophages and other viruses have been studied. The possibility of completely defining the genetic systems, and therefore the proteins involved, has made phage assembly a particularly popular and tractable area in which to study macromolecular assembly. We will not consider it the mission of this chapter to collect the details of this myriad of studies. The reader should consult other chapters in this volume or other reviews for such details (e.g., Casjens and King, 1975; Murialdo and Becker, 1978a; Eiserling, 1979; Wood and King, 1979; King, 1980; DuBow, 1981; Mathews et al., 1983; Hendrix et al., 1983; Casjens, 1985c; Carrascosa, 1986). Instead we will focus on general questions currently under study and attempts to answer them in the various dsDNA phage systems. We will not discuss the problem of DNA packaging in detail and will not cover the lipid-containing dsDNA phages.

Control Mechanisms in dsDNA Bacteriophage Assembly

We have decorated neurofilaments with antibodies against three polypeptides (designated here as H [mol wt = 195,000], 45[mol wt = 145,000], and 46[mol wt = 73,000]) in an effort to understand the arrangement of these polypeptides within neurofilaments. The three polypeptides were purified and antibodies were raised against each. The cross-reactivity of the antibodies suggested that each polypeptide contains both shared and unique antigenic determinants. The differential reactivities of each antibody preparation were enhanced by adsorption with the two heterologous polypeptides, and the resulting preparations were used to decorate purified neurofilaments, which were then negatively stained and examined in an electron microscope. The appearance of the antibody-decorated structures led to the following conclusions: All three polypeptides are physically associated with the same neurofilament. However, the disposition of H and 46 within a filament is different; 46 antigens appear to be associated with a "central core" of the filament, whereas H antigens compose a structure more loosely and peripherally attached to the central core and periodically arranged along its axis. The antibody-decorated H-containing structure assumes variable configurations; in some cases it appears asa bridge connecting two filaments; in other cases it appears as a helix wrapping the central core with a period of approximately 1,000 A and an apparent unit length of approximately 1.5 periods. These configurations suggest several functional implications, including the possibility that H is a component of the cross-bridges observed between filaments in situ. We also note that the central core-helix relationship could be used in the design of an intracellular transport motor.

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Antibody decoration of neurofilaments.

The first stages of productive bacteriophage infections of bacterial host cells require efficient adsorption to the cell surface followed by ejection of phage DNA into the host cytoplasm. To achieve this goal, a phage virion must undergo significant structural remodeling. For phage T4, the most obvious change is the contraction of its tail. Here, we use skinny E. coli minicells as a host, along with cryo-electron tomography and mutant phage virions, to visualize key structural intermediates during initiation of T4 infection. We show for the first time that most long tail fibers are folded back against the tail sheath until irreversible adsorption, a feature compatible with the virion randomly walking across the cell surface to find an optimal site for infection. Our data confirm that tail contraction is triggered by structural changes in the baseplate, as intermediates were found with remodeled baseplates and extended tails. After contraction, the tail tube penetrates the host cell periplasm, pausing while it degrades the peptidoglycan layer. Penetration into the host cytoplasm is accompanied by a dramatic local outward curvature of the cytoplasmic membrane as it fuses with the phage tail tip. The baseplate hub protein gp27 and/or the ejected tape measure protein gp29 likely form the transmembrane channel for viral DNA passage into the cell cytoplasm. Building on the wealth of prior biochemical and structural information, this work provides new molecular insights into the mechanistic pathway of T4 phage infection.

https://www.pnas.org/content/pnas/112/35/E4919.full.pdf

C. Ahmad-Zadeh

Papers

Determination of gene product positions in bacteriophage T4 by specific antibody association.