Morphogenesis of the T4 tail and tail fibers

The structures of bacteriophages are unique among viruses in that most of them have tails, the specialized host cell attachment organelles. Phages that possess a tail are collectively called "Caudovirales" [1]. The family Caudovirales is divided into three sub-families according to the tail morphology: Myoviridae (long contractile tail), Siphoviridae (long non-contractile tail), and Podoviridae (short non-contractile tail). Of these, Myoviridae phages have the most complex tail structures with the greatest number of proteins involved in the tail assembly and function. Bacteriophage T4 belongs to this sub-family and has a very high efficiency of infection, likely due to its complex tails and two sets of host-cell binding fibers (Figure 1). In laboratory conditions, virtually every phage particle can adsorb onto a bacterium and is successful in injecting the DNA into the cytosol [2].Since the emergence of conditional lethal mutants in the 1960's [3], assembly of the phage as well as its molecular genetics have been extensively studied as reviewed in "Molecular biology of bacteriophage T4" [4]. During the past ten years, remarkable progress has been made in understanding the conformational transformation of the tail baseplate from a "hexagon" to a "star" shape, which occurs upon attachment of the phage to the host cell surface. Three-dimensional image reconstructions have been determined of the baseplate, both before [5] and after [6] tail contraction using cryo-electron microscopy and complete or partial atomic structures of eight out of 15 baseplate proteins have been solved [7-14]. The atomic structures of these proteins were fitted into the reconstructions [15]. The fact that the crystal structures of the constituent proteins could be unambiguously placed in both conformations of the baseplate indicated that the gross conformational change of the baseplate is caused by a rearrangement or relative movement of the subunit proteins, rather than associated with large s