The Structure of the T4 Bacteriophage

Bacteriophage T4 is a type of virus that infects Escherichia coli and related bacteria. It is a member of the Myoviridae family, a group known for its complex and contractile tails. As one of the most studied viruses, T4 serves as a model organism for understanding viral assembly and infection. Its structure is composed of proteins that self-assemble into a functional particle, allowing the virus to protect its genome and efficiently deliver it into a host cell.

The Main Components of T4

The T4 virion has a structure often compared to a lunar lander, which can be divided into three main sections: a head, a tail, and tail fibers. The head, or capsid, is an elongated structure housing the virus’s genetic material. It connects to a cylindrical tail that acts as a conduit for DNA delivery, which ends in a hexagonal baseplate where six long tail fibers extend. These components are assembled independently within the host bacterium before they are joined together to form the complete virus particle.

The Head and Its Genetic Cargo

The head of the T4 bacteriophage is a protein shell, or capsid, that protects the viral genome. Its specific shape is a prolate icosahedron, which resembles a 20-sided die that has been stretched along one of its axes. This structure is composed of multiple copies of several proteins, primarily the major capsid protein gp23, which forms the main hexagonal lattice of the capsid surface. Eleven of the twelve vertices are made of a pentameric arrangement of the protein gp24.

Inside the capsid is the virus’s genome, a single, linear molecule of double-stranded DNA (dsDNA) that is approximately 171,000 base pairs long. The DNA is packed into the head under extreme pressure, reaching about 25 atmospheres, which is many times greater than the pressure inside a champagne bottle. This dense packing is accomplished by a molecular motor that forces the DNA into the pre-formed capsid shell during assembly.

The capsid is stable, capable of withstanding this internal force due to the extensive interactions between its protein subunits. The twelfth vertex of the head is unique, containing a portal protein structure made of gp20. This portal serves as the entry and exit point for the DNA and as the attachment site for the tail.

The Tail Assembly as a Molecular Machine

The T4 tail is a molecular machine designed to penetrate a bacterial cell wall. It is composed of several sub-assemblies that work in concert. At the top, a collar structure connects the tail to the portal vertex of the head. Below this is the main part of the tail, which consists of a rigid inner tube surrounded by a contractile outer sheath.

The outer sheath, assembled from 138 copies of the protein gp18, is about 925 angstroms long in its extended state. It stores mechanical energy similar to a compressed spring. The inner tube, made of the protein gp19, is a hollow, rigid structure that provides a channel for the viral DNA to pass through during infection.

At the bottom of the tail is a hexagonal baseplate, a complex hub composed of subunits from 15 different proteins. This baseplate serves as the control center for the infection process. Attached to the corners of the baseplate are six long tail fibers, which are the primary sensors for host recognition. Six shorter tail fibers are folded underneath the baseplate, ready to be deployed for irreversible attachment to the host cell surface.

The Infection Mechanism

The infection process begins when the long tail fibers of the T4 phage recognize specific receptor sites on the surface of an E. coli bacterium. This initial binding is reversible but triggers a signal that is transmitted to the baseplate. In response, the short tail fibers unfold from beneath the baseplate and attach firmly to the bacterial cell surface, locking the phage in place.

This irreversible binding causes a conformational change in the baseplate, shifting it from a dome to a star-like configuration. This change acts as a trigger, causing the outer sheath to contract rapidly. As the sheath contracts, it shortens to less than half its original length, driving the rigid inner tube forward like a syringe. The tip of the tube punctures the outer membrane of the bacterium.

A lysozyme enzyme located at the tip of the tail then degrades the peptidoglycan layer of the bacterial cell wall, clearing a path. The high pressure of the DNA inside the capsid helps propel the viral genome through the hollow tail tube and into the host cell’s cytoplasm. Once inside, the viral DNA takes over the cell’s machinery to replicate itself, leading to the production of new phage particles.