Structural localizaton of the proteins of the head to tail connecting region of bacteriophage φ29

Self-assembly of heptameric nanoparticles derived from tag-functionalized phi29 connectors

The structure of an induced macromolecular assembly was characterized and found to consist of an ordered heptameric arrangement of recombinant phi29 gp10 connector molecules. Insertion of an N-terminal Strep-II/His(6) tag to the connectors led to the spontaneous formation of large nanoparticles that were distinct from free, wild-type phi29 connectors in both size and symmetry elements. The determination of single-molecule tomograms and image-averaged reconstructions allowed for the stoichiometric and topological characterization of the ordered assemblage, revealing that the nanoparticle is composed of five equatorial connectors arranged with pseudo-5-fold rotational symmetry, capped on its ends by two polar connectors. Additionally, all seven connectors are oriented with their narrower N-terminal necks into the nanoparticle core and wider C-terminal ends out toward the nanoparticle surface, a geometric arrangement accommodated by the shape complementarity of the conical connector profiles. A significant amount of conformational heterogeneity was detected, ranging from changes in overall nanoparticle diameter, to tilting of individual connectors, to variations in connector stoichiometry. Nevertheless, a stable, heptameric nanoparticle was resolved, revealing the significant potential of guided, peptide-mediated supramolecular self-assembly. With this construct, we anticipate the further design of variable N-terminal tags to allow for the generation of nanoparticles with tailored connector stoichiometry and topological arrangements. By modifying the surface-exposed C-terminal ends with application-appropriate moieties, the consistent structure and compact nature of these nanoparticles may prove beneficial in nanotechnological and nanomedical approaches.

DNA poised for release in bacteriophage phi29

We present here the first asymmetric, three-dimensional reconstruction of a tailed dsDNA virus, the mature bacteriophage phi29, at subnanometer resolution. This structure reveals the rich detail of the asymmetric interactions and conformational dynamics of the phi29 protein and DNA components, and provides novel insight into the mechanics of virus assembly. For example, the dodecameric head-tail connector protein undergoes significant rearrangement upon assembly into the virion. Specific interactions occur between the tightly packed dsDNA and the proteins of the head and tail. Of particular interest and novelty, an approximately 60A diameter toroid of dsDNA was observed in the connector-lower collar cavity. The extreme deformation that occurs over a small stretch of DNA is likely a consequence of the high pressure of the packaged genome. This toroid structure may help retain the DNA inside the capsid prior to its injection into the bacterial host.

Detection and identification of viruses by electron microscopy

Electron microscopy can aid in the rapid diagnosis of viral diseases, as it can be performed in a matter of hours, but on a routine basis it should be used in conjunction with other techniques. Initially, the specimen source and patient symptoms should be ascertained, as these will lend suggestions of possible agents while eliminating others; however, this information should not be allowed to prejudice observation in such a way as to cause oversight of an unlikely pathogen. Second, selection of the method of preparation should be based on sample consistency; extraction, debris clarification, concentration, tissue culture amplification, or embedment may be necessary. Finally, false-positive results must be avoided by differentiating viruses from cell organelles or debris, mycoplasmal or bacterial contamination, and bacteriophages.

Structural organisation of the head-to-tail interface of a bacterial virus

In tailed icosahedral bacteriophages the connection between the 5-fold symmetric environment of the portal vertex in the capsid and the 6-fold symmetric phage tail is formed by a complex interface structure. The current study provides the detailed analysis of the assembly and structural organisation of such an interface within a phage having a long tail. The region of the interface assembled as part of the viral capsid (connector) was purified from DNA-filled capsids of the Bacillus subtilis bacteriophage SPP1. It is composed of oligomers of gp6, the SPP1 portal protein, of gp15, and of gp16. The SPP1 connector structure is formed by a mushroom-like portal protein whose cap faces the interior of the viral capsid in intact virions, an annular structure below the stem of the mushroom, and a second narrower annulus that is in direct contact with the helical tail extremity. The layered arrangement correlates to the stacking of gp6, gp15, and gp16 on top of the tail. The gp16 ring is exposed to the virion outside. During SPP1 morphogenesis, gp6 participates in the procapsid assembly reaction, an early step in the assembly pathway, while gp15 and gp16 bind to the capsid portal vertex after viral chromosome encapsidation. gp16 is processed during or after tail attachment to the connector region. The portal protein gp6 has 12-fold cyclical symmetry in the connector structure, whereas assembly-naïve gp6 exhibits 13-fold symmetry. We propose that it is the interaction of gp6 with other viral morphogenetic proteins that drives its assembly into the 12-mer state.

Composition and mass of the bacteriophage phi29 prohead and virion

The protein composition of the Bacillus subtilis bacteriophage phi29 prohead and virion was determined by combustion of gel bands of (3)H-labeled proteins. Copy numbers of individual proteins were calculated relative to the 12 copies of the head-tail connector protein. The mean numbers of copies of the major capsid protein in the prohead and virion were 241 and 218, respectively, approaching the 235 copies determined previously by cryoelectron microscopy. The mean numbers of copies of the dimeric head fiber on the prohead and virion were 24 and 31, respectively, demonstrating partial occupancy of the 55 fiber binding sites. Measured copies of neck and tail proteins in the virion included 11 of the lower collar, 58 of the appendage, and 9 of the tail; if the true copies of these proteins are 12, 60, and 9, respectively, the entire neck and tail of phi29 has quasi-sixfold symmetry. The mass of the fiberless prohead with pRNA was about 14.2 MDa, and the mass of the prohead determined by scanning transmission electron microscopy was consistent with the biochemical data. The mass of the fiberless virion containing the 12.8-MDa DNA genome was about 30.4 MDa. A full complement of dimeric fibers on the prohead or virion would increase the mass of the particle by about 3.2 MDa. The data complement studies relating the structure of phi29 components to dynamic functions in morphogenesis and infection.

Phi29 family of phages

Continuous research spanning more than three decades has made the Bacillus bacteriophage phi29 a paradigm for several molecular mechanisms of general biological processes, such as DNA replication, regulation of transcription, phage morphogenesis, and phage DNA packaging. The genome of bacteriophage phi29 consists of a linear double-stranded DNA (dsDNA), which has a terminal protein (TP) covalently linked to its 5' ends. Initiation of DNA replication, carried out by a protein-primed mechanism, has been studied in detail and is considered to be a model system for the protein-primed DNA replication that is also used by most other linear genomes with a TP linked to their DNA ends, such as other phages, linear plasmids, and adenoviruses. In addition to a continuing progress in unraveling the initiation of DNA replication mechanism and the role of various proteins involved in this process, major advances have been made during the last few years, especially in our understanding of transcription regulation, the head-tail connector protein, and DNA packaging. Recent progress in all these topics is reviewed. In addition to phi29, the genomes of several other Bacillus phages consist of a linear dsDNA with a TP molecule attached to their 5' ends. These phi29-like phages can be divided into three groups. The first group includes, in addition to phi29, phages PZA, phi15, and BS32. The second group comprises B103, Nf, and M2Y, and the third group contains GA-1 as its sole member. Whereas the DNA sequences of the complete genomes of phi29 (group I) and B103 (group II) are known, only parts of the genome of GA-1 (group III) were sequenced. We have determined the complete DNA sequence of the GA-1 genome, which allowed analysis of differences and homologies between the three groups of phi29-like phages, which is included in this review.

Structural analysis of the bacteriophage T3 head-to-tail connector

The connector protein of bacteriophage T3, p8, has been overexpressed in Escherichia coli. Purification of the oligomers built by several copies of p8 reveals a mixed population of dodecamers and tridecamers. The percentages of these two types of oligomers differ in every culture growth, indicating that assembly of this protein depends upon the conditions of the expression system. Those cultures that generated a majority of dodecamers allowed, after purification of the connectors, the two-dimensional crystallization of the dodecamers in a tetragonal arrangement, while the tridecamers did not form crystals. The processing and averaging of several images of frozen-hydrated crystals and their internal phase comparison shows that the crystals are arranged in a P42(1)2 space group, with cell unit dimensions of 165 x 165 A. The three-dimensional reconstruction generated with images of crystals ranging from 0 degrees to 60 degrees tilt reveals a wide domain surrounded by 12 protrusions and a narrow domain that serves to interact with the tail of the bacteriophage. A channel runs along the connector wide enough to allow the translocation of a double-stranded DNA molecule into the prohead. The general structure of the T3 connector is very similar to those obtained for other nonrelated bacteriophages and strongly suggests that the shape of this important viral structure is intimately related to its function.

Domain architecture of the bacteriophage phi29 connector protein

Viral connectors are essential components of the DNA packaging machinery. They interact with nucleic acids and other viral components to translocate DNA inside the viral head. We have attempted to locate the different structural and functional domains of the phage Phi29 connector using a combination of approaches to generate different antigenic probes. Complexes of native connectors with either monoclonal or monospecific antibodies were studied by immunoelectron microscopy and image averaging methods. The data were merged in a model of the connector domain structure at 2-3 nm resolution. This epitope mapping provides a general outline of the folding architecture of the connector polypeptide, following a complicated threading that places the amino and carboxyl-terminals in close alignment in the narrower domain at 2-3 nm from the top of the connector. The appendages are built up by a long and highly immunogenic sequence (amino acid residues 153 to 206). The RNA binding domain forms part of the top of the narrow conical area of the connector, a flexible region that undergoes structural changes during viral morphogenesis. The DNA binding domain is located not far away, 2-3 nm below, in the outer side of the narrow conical part. The precise location of the functional domains of the connector, as well as their relative positions provide the first experimental framework for understanding the connector function.

The three-dimensional structure of a DNA translocating machine at 10 A resolution

BACKGROUND

Head-tail connectors are viral substructures that are very important in the viral morphogenetic cycle, having roles in the formation of the precursor capsid (prohead), DNA packaging, tail binding to the mature head and in the infection process. Structural information on the connector would, therefore, help us to understand how this structure is related to a multiplicity of functions.

RESULTS

Recombinant bacteriophage phi29 connectors have been crystallized in two-dimensional aggregates. An average projection image and a three-dimensional map have been obtained at 8 A and 10 A resolution, respectively, from untilted and tilted images of vitrified specimens of the two-dimensional crystals. The average projection image reveals a central mass surrounding a channel with 12 appendages protruding from the central mass. The three-dimensional map reveals a wide domain surrounded by 12 appendages that interact with the prohead vertex, and a narrow domain that interacts with the bacteriophage tail. At the junction of the two domains, 12 smaller appendages are visualized. A channel runs along the axis of the connector structure and is sufficiently wide to allow a double-stranded DNA molecule to pass through.

CONCLUSIONS

The propeller-like structure of the phi29 connector strengthens the notion of the connector rotating during DNA packaging. The groove formed by the two lanes of large and small appendages may act as a rail to prevent the liberation of the connector from the prohead vertex during rotation.

Structure of viral connectors and their function in bacteriophage assembly and DNA packaging

The viruses have been an attractive model for the study of basic mechanisms of protein/protein and protein/nucleic acid interactions involved in the assembly of macromolecular aggregates. This has been due primarily to their relative genetic simplicity as compared to their structural and functional complexity. Although most of the initial studies were carried out on bacterial and plant viruses, increasing data has also been accumulated from animal viruses, which has led to an understanding of some basic principles, as well as to many specific strategies in every system. The study of virus assembly has been a source of ideas that underlie our present knowledge of the organization of biological systems. It has also provided, since the production of bacteriophage mutants which have allowed the study of assembly intermediates, the first system in which the genetic studies played a dominant role. The increasing volume of data over the last years has revealed how the structural components can interact sequentially through an ordered pathway to yield macromolecular assemblies that satisfy the demands of stability required for a successful transfer of genetic information from host to host.

Three-dimensional structure of T3 connector purified from overexpressing bacteria

The bacteriophage T3 connector has been purified from overexpressed protein in Escherichia coli, harboring a plasmid containing the gene encoding p8 protein. The connector, which is composed of 12 copies of p8, has been crystallized in two-dimensional sheets and studied by electron microscopy from negatively stained specimens. A two-dimensional Fourier filtering and averaging procedure was performed with crystalline specimens. In addition, single particle averaging techniques were used with other preparations. The average images obtained from these two approaches gave similar results. A three-dimensional reconstruction from two-dimensional crystals of T3 connectors was obtained by collecting several sets of tilted views and using standard Fourier procedures. The resolution of the three-dimensional map was 1.65 nm. The reconstructed connector shows two main domains: a wider one with 12 small units in the periphery and with an external diameter of 14.9 nm, and a smaller one with 8.5 nm diameter. The height of the reconstructed connector has been determined to be around 8.5 nm. The reconstruction clearly shows an internal open channel running along the longitudinal axis of the particle and having an average diameter of 3.7 nm.

Novel second-site suppression of a cold-sensitive defect in phage P22 procapsid assembly

The DNA packaging portal of the phage P22 procapsid is formed of 12 molecules of the 90,000 dalton gene 1 protein. The assembly of this dodecameric complex at a unique capsid vertex requires scaffolding subunits. The mechanism that ensures the location of the 12-fold symmetrical portal at only one of the 12 5-fold vertices of an icosahedral virus capsid presents a unique assembly problem, which, in some viruses, is solved by the portal also acting as initiator of procapsid assembly. Phage P22 procapsids, however, are formed in the absence of the portal protein. The 1-csH137 mutation prevents the incorporation of the portal protein into procapsids. In a mixed infection with cs+ phage, the mutant subunits are able to form functional portals, suggesting that the cold-sensitivity does not affect portal-portal interactions, but affects the interaction of portal subunits with some other molecular species involved in the initiation of portal assembly. Interestingly, the cs defect is suppressed by temperature-sensitive folding mutations at four sites in the P22 tailspike gene 9. The suppression is allele-specific; other tailspike tsf mutations fail to suppress the cs defect. Translation through a suppressor site is required for suppression. This observation is unexpected, since analysis of nonsense mutations in this gene indicates that it is not required for procapsid assembly. Examination of the nucleic acid sequences in the neighborhood of each of the suppressor sites shows significant sequence similarity with the scaffolding gene translational initiation region on the late message. This supports a previously proposed model, in which procapsid assembly is normally initiated in a region on the late messenger RNA that includes the gene 8 start site. By this model, the suppressor mutations may be acting through protein-RNA interactions, changing sequences that identify alternative or competing sites at which the mutant portal subunits may be organized for assembly into the differentiated vertex of the phage capsid.

Conformational changes in bacteriophage phi 29 connector prevents DNA-binding activity

In vitro DNA packaging activity in a defined system derived from bacteriophage phi 29 depends upon the chemical integrity of the connector protein p10. Proteolytic cleavage of p10 rendered the proheads inactive for DNA packaging. A similar treatment on isolated connectors abolished the DNA-binding activity of the native p10, but the general shape and size of the connector was not changed as revealed by electron microscopy. Analytical ultracentrifugation showed that the proteolyzed connectors had a smaller sedimentation coefficient, while amino acid analysis after dialysis of the proteolyzed p10 confirmed the loss of 16 and 19 amino acids from the amino and carboxy termini, respectively. Low angle X-ray scattering revealed that proteolysis was followed by a small decrease in the radius of gyration and a reorganization of the distal domain of the cylindrical inner part of the connector. Characterization of the cleavage sites in the primary sequence allowed us to propose the location of the DNA-binding domain in the connector model.

Bacteriophage T3 connector: three-dimensional structure and comparison with other viral head-tail connecting regions

The bacteriophage T3 connector, which consists of 12 copies of protein gp8, has been studied by image processing of electron micrographs from negatively stained ordered aggregates. A three-dimensional reconstruction of T3 connectors was obtained by collection of tilted views and using the direct Fourier method, up to 2.3 nm resolution. The reconstructed unit cell contains two connectors whose main structural features are essentially identical, but facing in opposite directions. The T3 connector has a height of about 10.9 nm, with two clearly defined domains: a wider one 14.4 nm in diameter, with 12 morphological units in the periphery, and a narrower one, 9.7 nm in diameter. There is a channel clearly defined in the narrower domain that almost closes along the wider domain. Comparison of the three-dimensional structure obtained for the connector of phages T3 and phi 29, and that of the neck extracted from phage phi 29 particles, reveals striking similarities and significant differences. A model for a general connector to account for the common functions carried out by these viral assemblies is discussed together with the possible role of the channel for DNA translocation.

Purification and organization of the gene 1 portal protein required for phage P22 DNA packaging

The gene 1 protein of Salmonella bacteriophage P22 is located at the DNA packaging vertex of the mature particle. The protein is incorporated into the procapsid shell during shell assembly and is required for DNA packaging. The unassembled precursor form of the gene 1 protein has been purified from cells infected with mutants blocked in procapsid assembly. The purified 90,000-dalton protein was dimeric or monomeric; upon storage in the cold it formed 20S cyclic dodecamers. Computer filtering of negatively stained electron micrographs revealed 12 arms and knobs projecting from a central ring, with a 30-A channel at the center. Similar dodecameric rings were released from disrupted procapsid shells. These results indicate that the gene 1 protein is organized as a cyclic dodecamer within the procapsid shell and serves as the portal through which P22 DNA is threaded during DNA packaging. The presence of a 12-fold ring located at a 5-fold portal vertex appears to be a conserved structural theme of the DNA packaging apparatus of double-stranded DNA phages.

Three-dimensional reconstruction of the connector of bacteriophage phi 29 at 1.8 nm resolution

The three-dimensional reconstruction of the connector of bacteriophage phi 29 has been obtained from tilt series of negatively stained tetragonal ordered aggregates under low-dose conditions and up to a resolution of (1/1.8) nm-1. These connectors are built up as dodecamers of only one structural polypeptide (p10). Two connectors form the crystal unit cell, each one facing in the opposite direction with respect to the plane of the crystal and partially overlapping. The main features of the two connectors that build the unit cell were essentially the same, although they were negatively stained in slightly different ways, probably due to their situations with respect to the carbon-coated support grid. The main features of the phi 29 connector structure revealed by this three-dimensional reconstruction are: the existence of two clearly defined domains, one with a diameter of around 14 nm and the other narrower (diameter approximately equal to 7.5 nm); an inner hole running all along the structure (around 7 to 8 nm in height) with a cylindrical profile and an average diameter of 4 nm; a general 6-fold symmetry along the whole structure and a 12-fold one in the wider domain; a clockwise twist of the more contrasted regions of both domains from the narrower towards the wider domain (the direction of DNA encapsidation). These features are compatible with an active role for the connector in the process of DNA packaging.

Computer graphic display method for visualizing three-dimensional biological structures

A computer graphic display method that produces two-dimensional perspective views of three-dimensional objects is presented. The method is applied to the reconstruction at a resolution of 2.2 nanometers of the neck of bacteriophage phi 29, obtained from transmission electron micrographs processed by the direct Fourier method. The combined use of directed illumination, reflectance models, color, and different levels of transparency provides a powerful tool for a better interpretation of the three-dimensional structure, allowing improved correlation with genetic, structural, and biochemical data.

Bacteriophage T3 gene 8 product oligomer structure

The structure of the connector of bacteriophage T3 (built up by the product of gene 8) has been studied in two dimensions by combined use of translational and rotational image filtering procedures applied to tetragonal ordered aggregates of the former oligomers. This analysis, performed up to 1/1.6 nm-1 resolution, has revealed the existence of a 12-fold symmetry in the outermost region of the specimen (mainly between radii 5.2 and 6.7 nm), a 6-fold one in the inner region (between radii 1.7 and 3.2 nm), and a hole in its center. These features are very similar to the ones described for the connectors of other phages, such as T4, lambda, and phi 29, thus suggesting a common mechanism for the functions carried out by this viral region.

Three-dimensional reconstruction of bacteriophage phi 29 neck particles at 2 X 2 nm resolution

The three-dimensional structure of the head-to-tail connecting region of bacteriophage phi 29 has been studied by analysing two-dimensional, hexagonal ordered aggregates of negatively stained viral necks to a resolution of 2 X 2 nm. These necks are composed of two proteins, p10 and p11; p10 being the connector protein. A 12-folded and a 6-folded axially symmetric domain are present in the specimen. The 12-folded domain is the larger part of the structure; it consists of 12 subunits associated in pairs. These subunits appear to be more closely paired towards the centre, where only six subunits are resolved forming the 6-folded domain. The pairs of subunits present an important twist between the 12-folded and the 6-folded areas. A conical hole is formed at the centre of the structure. This hole is more open at the 12-folded domain than at the level of the possible zone of interaction between p10 and p11, where it is almost closed. Protein p11 is very poorly represented in the reconstruction, probably due to lack of staining. The structure described for the phi 29 neck has many of the attributes expected for an active device involved in bacteriophage DNA encapsidation.

Structure of phage phi 29 connector protein assembled in vivo

The protein p10 that forms the connector of phage phi 29, has been produced in Escherichia coli harboring a plasmid that carried the gene coding for this protein. The connector protein is assembled in a 13.4-S oligomer that has an apparent molecular weight of 460,000, suggesting that it is a dodecamer. The purified oligomers have been studied by electron microscopy of the isolated particles as well as by image-processing techniques (Fourier and rotational filtering) of artificially induced two-dimensional aggregates. The results show that the purified p10 is assembled in a circular structure with a hole in its center and 12 morphological units in the periphery. Both the morphology and the dimensions of this p10 oligomer are very similar to those of the upper neck collar extracted from phi 29 viral particles. The results strongly suggest the close relationship between the p10 oligomers assembled in E. coli and the ones produced in phi 29 infected Bacillis subtilis.

Cloning, nucleotide sequence and high level expression of the gene coding for the connector protein of Bacillus subtilis phage phi 29

The phi 29 DNA restriction fragment HindIII-D, shown to contain gene 10 coding for the connector protein, has been cloned in plasmid pPLc28 under the control of the pL promoter of phage lambda. After heat induction to inactivate the lambda repressor, a protein with the electrophoretic mobility of the connector protein p10 was synthesized, accounting for about 30% of the total Escherichia coli protein after 3 h of induction. The 2205 nucleotide-long sequence of the cloned HindIII-D fragment has been determined. The sequenced region has an ORF coding for a protein of Mr 35881 that was shown to correspond to the connector protein by determination of the amino-terminal sequence of purified protein p10. Features of the nucleotide sequence and the amino acid sequence of protein p10 are discussed.

Structural study of tetragonal-ordered aggregates of phage φ29 necks

A new class of two-dimensional tetragonal aggregates of phage phi 29 necks has been studied by electron microscopy and a combination of Fourier filtering procedures and detailed rotational analysis. The results confirm the main features of the head-to-tail connecting region previously observed in hexagonal aggregates. There are several differences in the resulting pictures that can be attributed to the different way in which the aggregates are organized and stained.

Bacteriophage lambda preconnectors. Purification and structure

The morphogenesis of bacteriophage lambda proheads is under the control of the four phage genes B, C, Nu3 and E, and the two Escherichia coli genes groEL and groES . It has been shown previously that extracts prepared from cells infected with a lambda C-E- mutant accumulate a gpB polymer, which behaves as a biologically active intermediate in prohead assembly. This gpB activity has been called a preconnector , as it is probably a precursor to the head-tail connector. We now report the partial purification of biologically active preconnectors and the characterization of its structure. In the electron microscope, preconnectors appear as donut -like structures composed of several subunits displaying radial symmetry. Optical filtration of periodic arrays of preconnectors showed that the structure has 12-fold rotational symmetry. Side views of the preconnector reveal that it resembles an asymmetrical dumbell . This information has been used to construct a three-dimensional model of the preconnector . The implications of this structure for prohead shape and function, and for DNA packaging are discussed.