Tailed Bacteriophage Structure and Function

Capsids

In the tailed dsDNA bacteriophages, the capsid is the container for the phage genome that protects it from the environment until it is delivered to a new host by the phage tail, the organelle of attachment and genome injection. In many other types of viruses, the capsid is taken up by cells with the genome still inside and the virus particle uncoats or disassembles within the new host to initiate viral replication. The dsDNA bacteriophages' capsids do not have to disassemble in this manner to initiate infection, so they can be constructed to be highly stable -resistant to a wide variety of chemical and physical assaults from the environment - as they travel from one host to another. The dsDNA genome in bacteriophage capsids is packed to a very high density (^0.5gml-1) which results in high pressure, so the capsid shell has to be strong enough to withstand the high internal pressure from DNA. Most members of the Caudovirales have symmetric icosahedral capsids, but some have capsids with an elongated icosahe-dral shape, like T4 (Figures 1(a) and 2) that have icosa-hedral ends and an elongated tubular middle section.

Tails and Tail Fibers

Phage tails function to identify and bind to the correct host and to deliver the phage genome into the new host to initiate replication. Phage tails have a large variety of morphological forms with many parts and appendages, such as those labeled on the diagram of T4 in Figure 2 and visible in the electron micrograph in Figure 1. We know little about the functions of some tail parts, but others are understood in considerable detail. For example, in bacteriophage T4, both the long tail fibers and the short tail fibers (which are folded under the baseplate until after the long tail fibers are bound to the host; see Figure 2) recognize specific host receptors. The long tail fibers of phage T4 are the primary determinants of host range, and mutations that change the T4 host range cause alterations in the fibers near their distal tips. The whiskers that are attached to the top of the T4 tail have multiple functions. The whiskers act as assembly jigs for adding the long tail fibers to the virion, and they also act as environmental sensors that sequester the long tail fibers under unfavorable conditions and thus prevent attachment to a host.

The long tail fibers of T4 have an analog in bacterio-phage l, but in the ordinary laboratory strains of l these fibers are not made because of a mutation in the side tail fiber gene. The electron micrograph in Figure 1(b) shows the long side tail fibers present on Ur-l, a primordial l that lacks this mutation. These long side tail fibers of Ur-l speed up the adsorption of this phage to its host, but are not required for infection, because the primary interaction of l with its host is mediated by the central tail fiber (a trimer of the tail protein gpJ) protruding from the end of the lambda tail. The central tail fiber of l binds to a host maltose transport protein (called lamB or malB) in the outer membrane of the host. Binding of phage l to its receptor under suitable conditions acts as a signal that

Capsid

Figure 2 Schematic drawing of bacteriophage T4, with some of the structural components identified.

Capsid

Figure 2 Schematic drawing of bacteriophage T4, with some of the structural components identified.

triggers the injection of the phage DNA genome into the host to initiate replication.

The structure and functions of many parts of the contractile tails of Myoviridae have been revealed in depth, especially for the phage T4 tail. After the T4 long tail fibers attach to the host, the short tail fibers unfold from the baseplate and bind to a second receptor on the host surface. The baseplate remains attached to the cell and undergoes a dramatic reorganization in which it changes from a hexagon to a star shape, causing the sheath to contract and drive the internal tail tube into the cell. As the nail-shaped tip of the inner tail tube passes through the outer membrane, tail lysozyme molecules are released to create a small hole in the cell wall peptidoglycan layer. These combined actions allow the tip ofthe tail tube to reach the inner membrane, where a channel is created that allows the dsDNA genome to enter the cell and initiate replication.

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Responses

  • corey
    What is the function of the tail fibers of a bacteriophage?
    6 years ago

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