Bacteriophages are the most numerous organisms in the biosphere. In spite of their biological significance and the spectrum of potential applications, little high-resolution structural detail is available on their receptor-binding fibers. Here we present the crystal structure of the receptor-binding tip of the bacteriophage T4 long tail fiber, which is highly homologous to the tip of the bacteriophage lambda side tail fibers. This structure reveals an unusual elongated six-stranded antiparallel beta-strand needle domain containing seven iron ions coordinated by histidine residues arranged colinearly along the core of the biological unit. At the end of the tip, the three chains intertwine forming a broader head domain, which contains the putative receptor interaction site. The structure reveals a previously unknown beta-structured fibrous fold, provides insights into the remarkable stability of the fiber, and suggests a framework for mutations to expand or modulate receptor-binding specificity.

Minute virus of mice (MVM) is a single-stranded (ss) DNA-containing, murine parvovirus with a capsid built up of 60 icosahedrally related polypeptide chains, each of which consists of the C-terminal region common to two structural proteins, VP1 and VP2. In infectious virions, most VP2 molecules are cleaved to VP3 by the removal of about 20 amino acids from the N terminus. Of the 587 amino acids in VP2, approximately half are identical to those in the analogous capsid protein of the antigenically distinct canine parvovirus (CPV), the crystal structure of which has previously been determined. The three-dimensional structure determination of MVMi (the immunosuppressive strain of MVM) was previously reported to 3.5 A resolution. We report here an analysis of the MVMi virus structure and provide insights into tissue tropism, antigenicity and DNA packaging. Amino acids determining MVM tissue tropism were found to cluster on, or near, the viral surface. A conserved, glycine-rich, N-terminal peptide was seen to run through a cylindrical channel along each fivefold axis and may have implications for antigenicity. Density within the virion was interpreted as 29 ssDNA nucleotides per icosahedral asymmetric unit, and accounts for over one-third of the viral genome. The presence of the glycine-rich sequence in the fivefold channels of MVMi provides a possible mechanism to explain how the unique N-terminal region of VP1 becomes externalized in infectious parvovirions. Residues that determine tropism may form an attachment recognition site for a secondary host-cell factor that modulates tissue specificity. The ordering of nucleotides in a similar region of the interior surface in the CPV and MVMi capsids suggests the existence of a genomic DNA-recognition site within the parvoviral capsid.

A new Salmonella enterica phage, Det7, was isolated from sewage and shown by electron microscopy to belong to the Myoviridae morphogroup of bacteriophages. Det7 contains a 75-kDa protein with 50% overall sequence identity to the tail spike endorhamnosidase of podovirus P22. Adsorption of myoviruses to their bacterial hosts is normally mediated by long and short tail fibers attached to a contractile tail, whereas podoviruses do not contain fibers but attach to host cells through stubby tail spikes attached to a very short, noncontractile tail. The amino-terminal 150 residues of the Det7 protein lack homology to the P22 tail spike and are probably responsible for binding to the base plate of the myoviral tail. Det7 tail spike lacking this putative particle-binding domain was purified from Escherichia coli , and well-diffracting crystals of the protein were obtained. The structure, determined by molecular replacement and refined at a 1.6-Å resolution, is very similar to that of bacteriophage P22 tail spike. Fluorescence titrations with an octasaccharide suggest Det7 tail spike to bind its receptor lipopolysaccharide somewhat less tightly than the P22 tail spike. The Det7 tail spike is even more resistant to thermal unfolding than the already exceptionally stable homologue from P22. Folding and assembly of both trimeric proteins are equally temperature sensitive and equally slow. Despite the close structural, biochemical, and sequence similarities between both proteins, the Det7 tail spike lacks both carboxy-terminal cysteines previously proposed to form a transient disulfide during P22 tail spike assembly. Our data suggest receptor-binding module exchange between podoviruses and myoviruses in the course of bacteriophage evolution.

A single mutation in canine parvovirus (CPV) of VP2 residue 300 from alanine to aspartic acid causes a loss of canine host range and alters the antigenic properties of the virus. The three-dimensional structure of this mutant has been solved to 3.25 A resolution. Crystals of full particles were triclinic, with cell dimensions of a = 267.6, b = 268.5, c = 274.3 A. alpha = 61.9, beta = 62.6, and gamma = 60.2 degrees. The native structure of CPV was used as an initial model. Phases were improved by real-space electron density averaging. In spite of the relative low percentage of observed reflections (32.5% of the data between 15.0 and 3.25 A resolution), the presence of 60-fold noncrystallographic redundancy allowed the averaging procedure to converge smoothly. The mutant aspartic acid at residue 300 forms a salt bridge with Arg81 in an icosahedrally threefold-related subunit, inducing local changes within the antigenic site B on the CPV surface. In addition, the loop between residues 359 and 374 adopts a conformation similar to that displayed by feline panleukopenia virus. The ability of the Ala300-->Asp mutant to evade antibody binding can be associated with the change of charge distribution and structure in the antigenic binding site. The variation in host range behavior may be due to the increased stability as a result of formation of the salt bridge between adjacent subunits.