The bacteriophage T4 is a virus that specifically infects bacteria, making it one of the most studied biological nanomachines. This obligate parasite is a highly sophisticated, self-assembling structure, often likened to a molecular syringe. Its architecture is designed for the highly efficient purpose of delivering its genetic material into a host cell. The precise arrangement of its many protein components allows it to recognize a host, mechanically penetrate the cell envelope, and inject its genome. The structural complexity of this Myoviridae-family phage makes it a foundational model for virology and molecular biology research.
The Icosahedral Head and DNA Storage
The head, or capsid, of the T4 bacteriophage is the protective container for its genetic cargo. Its unique geometry is an elongated icosahedron, a shape known as prolate, measuring about 120 nanometers long and 86 nanometers wide. This shell is constructed primarily from three specialized proteins. The main hexagonal lattice of the capsid is formed by multiple copies of the major capsid protein, known as gene product 23 (gp23).
Eleven of the twelve vertices of this structure are capped by pentamers of the minor capsid protein, gp24. The remaining twelfth vertex forms the crucial link to the tail assembly. This specialized junction is the dodecameric portal vertex, constructed from protein gp20, which acts as the channel for DNA movement. The head encapsulates approximately 171 kilobase pairs of linear double-stranded DNA, which is packed with incredible density. This process uses a powerful terminase motor (gp17) to force the genetic material into the confined space until the head is full, a mechanism referred to as “headful” packaging.
The Contractile Tail Sheath and Tube
The tail functions as the dedicated injection apparatus. This assembly consists of a rigid, central inner tube surrounded by a flexible, contractile outer sheath. The core inner tube is assembled from subunits of gene product 19 (gp19) and acts as the conduit through which the viral DNA will pass into the host cell.
The surrounding outer sheath is a helical structure made up of 138 copies of the protein gp18. This sheath is held in a metastable, extended state before infection, acting like a molecular spring under tension. Upon receiving the signal from the baseplate, the sheath undergoes a conformational change, shortening from approximately 925 Å to a contracted state of about 420 Å. This irreversible action drives the inner tube through the bacterial outer membrane. During this process, the inner tube is translated downward and rotates by 345 degrees.
Baseplate Structure and Spikes
The baseplate is the multi-protein platform situated at the distal end of the tail assembly. It serves as the central regulatory hub, connecting the tail sheath, the inner tube, and the external appendages. In its resting state, the baseplate is a hexagonal, dome-shaped structure. This structure is composed of six identical wedge-like sectors surrounding a central plug.
The baseplate’s role as a regulatory switch is paramount to the infection process. Once the long tail fibers signal host recognition, the baseplate undergoes a conformational shift from its hexagonal dome to a planar, star-shaped configuration. This structural change triggers the irreversible contraction of the tail sheath. Furthermore, the baseplate houses the short tail fibers (STFs), made of gene product 12 (gp12), which are initially folded up underneath the plate. Once the baseplate transforms, these six short spikes rotate downward to bind irreversibly to the bacterial cell wall, securing the phage.
The central part of the baseplate also includes the cell-puncturing device, a complex of proteins like gp5 and gp27. This device acts as the needle to breach the outer membrane after the sheath contraction is complete.
Tail Fibers and Host Recognition
The long tail fibers (LTFs) are the sensory organs of the T4 bacteriophage, determining its host specificity. There are six of these long, flexible appendages, each measuring about 145 nanometers, extending outward from the baseplate periphery. Their primary function is to recognize and reversibly bind to specific receptor molecules, such as lipopolysaccharide (LPS) or outer membrane porin C (OmpC), found on the surface of the bacterial host cell.
The LTFs are segmented structures composed of proteins, including gp34, gp35, gp36, and gp37, with the tip of gp37 forming the actual receptor-binding domain. Successful binding of at least three of these fibers to the host surface generates a mechanical signal. This signal is transmitted up the tail structure to the baseplate, initiating the injection mechanism.