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PubMed2005

Evidence of recombination in the norovirus capsid gene.

Noroviruses are single-stranded RNA viruses with high genomic variability. They have emerged in the last decade as a major cause of acute gastroenteritis. It remains so far unclear whether norovirus evolution is driven by sequence mutation and/or recombination. In this study, we have assessed the occurrence of recombination in the norovirus capsid gene. For this purpose, 69 complete capsid sequences of norovirus strains accessible in GenBank as well as 25 complete capsid sequences generated from norovirus-positive clinical samples were examined. Unreported recombination was detected in about 8% of norovirus strains belonging to genetic clusters I/1 (n = 1), II/1 (n = 1), II/3 (n = 1), II/4 (n = 3), and II/5 (n = 1). Recombination breakpoints were mainly located at the interface of the putative P1-1 and P2 domains of the capsid protein and/or within the P2 domain. The recombination region displayed features such as length, sequence composition (upstream and downstream GC- and AU-rich sequences, respectively), and predicted RNA secondary structure that are characteristic of homologous recombination activators. Our results suggest that recombination in the norovirus capsid gene may naturally occur, involving capsid domains presumably exposed to immunological pressure.

Amino Acid Sequence↗
PubMedDate not supplied

Immunocytochemical detection of HPV high-risk type L1 capsid proteins in LSIL and HSIL as compared with detection of HPV L1 DNA.

OBJECTIVE: To investigate the prevalence of HPV L1 capsid proteins in HPV-infected HSIL and LSIL. STUDY DESIGN: Cervical smears from 74 women with cytologically and histologically confirmed LSIL (n = 32) and HSIL (n = 42) were collected prospectively to detect HPV high-risk (hr) types 16, 18, 33, 35, 39, 45, 56 and 58 L1-DNA by standardized L1-consensus primer PCR (MY 09/11) and L1 capsid proteins by immunocytochemistry using monoclonal antibodies T31 (HPV16) and T16 (HPV hr) in a standardized protocol. RESULTS: In HSIL and LSIL, L1 DNA was found for HPV hr in 93% and 59% and for HPV16 in 69% and 37% of the specimens, respectively. HPV L1 capsid proteins were detected in HSIL and LSIL for HPV hr in 33% and 44% and for HPV16 in 29% and 31% of the specimens, respectively. Expression of L1 capsid proteins was significantly reduced, by 59.6% for HPV hr L1 DNA-positive HSIL (P < .01) and by 40.4% for HPV 16 L1 DNA-positive HSIL (P < .01). In HPV 16 DNA-positive and HPV hr DNA-positive LSIL, no significant reduction of corresponding L1 capsid protein expression could be demonstrated. CONCLUSION: These data suggest a disturbed viral cellular interaction in HPV 16 and HPV hr-infected HSIL, with loss of viral L1 capsid antigen. In this context there is a possible role of T31 and T16 as prognostic markers to predict the prognosis of CIN.

Adolescent↗
PubMed1992

Unique region of the minor capsid protein of human parvovirus B19 is exposed on the virion surface.

Capsids of the B19 parvovirus are composed of major (VP2; 58 kD) and minor (VP1; 83 kD) structural proteins. These proteins are identical except for a unique 226 amino acid region at the amino terminus of VP1. Previous immunization studies with recombinant empty capsids have demonstrated that the presence of VP1 was required to elicit virus-neutralizing antibody activity. However, to date, neutralizing epitopes have been identified only on VP2. Crystallographic studies of a related parvovirus (canine parvovirus) suggested the unique amino-terminal portion of VP1 assumed an internal position within the viral capsid. To determine the position of VP1 in both empty capsids and virions, we expressed a fusion protein containing the unique region of VP1. Antisera raised to this protein recognized recombinant empty capsids containing VP1 and VP2, but not those containing VP2 alone, in an enzyme-linked immunosorbent assay. The antisera immunoprecipitated both recombinant empty capsids and human plasma-derived virions, and agglutinated the latter as shown by immune electron microscopy. The sera contained potent neutralizing activity for virus infectivity in vitro. These data indicate that a portion of the amino terminus of VP1 is located on the virion surface, and that this region contains intrinsic neutralizing determinants. The external location of the VP1-specific tail may provide a site for engineered heterologous epitope presentation in novel recombinant vaccines.

Animals↗
PubMed1985

Mutual spatial orientation of hexons in the adenovirus capsid by electron microscopy and modelling.

On the basis of electron microscopic analysis of the virion and model experiments there are four hexon species in the adenovirus capsid which have different positional status and spatial orientation as compared to the six nearest neighbouring capsomers. The binding pattern of the polypeptide subunits according to their orientation is also different. The four hexon species are situated beside one another in the capsid and they form the capsid of 240 hexons besides the penton by repeating symmetrically 60 times, as a group of four hexons (GOF). Electron microscopic analysis and model experiments lead to the conclusion that the mutual spatial orientation of GOF hexons can be parallel (their longitudinal axes are parallel), or the direction of their longitudinal axes is different in one or two planes, thus they form different angles with the longitudinal axes of the neighbouring hexons. In the latter cases therefore, a bend and torsion of the bindings (connective elements) can be supposed as compared to the tendency-plane of parallel bindings. Based on the different combinations of the three kinds of rotational orientation of polypeptide subunits and of the different spatial orientation of hexons, six kinds of interhexonal bindings can be found in the adenovirus capsid. The distribution and characteristics of the 690 interhexonal bindings are the following; 240 bindings of "one-to-two" polypeptide orientation with parallel spatial orientation; 180 bindings of "one-to-two" polypeptide with spatial orientation divergent in two planes; 30 bindings of "one-to-one" polypeptide orientation with parallel spatial orientation; 60 bindings of "one-to-one" polypeptide orientation with spatial orientations divergent in one plane; 60 bindings of "one-to-one" polypeptide orientation with spatial orientation divergent in two planes; 120 bindings of "two-to-two" polypeptide orientation with spatial orientation divergent in two planes. The dissociational sequence of the capsid suggests that the firmest bindings are the ones having "one-to-two" polypeptide orientation and parallel spatial orientation. It may be supposed that any two hexons are able to connect to each other according to all three polypeptide orientations and the connections can tolerate the bends and torsions arising from the icosahedral structure. This suggests that only one kind of hexon is existing and the adenovirus capsid is formed by a homogeneous hexon population.

Adenoviridae↗
PubMed1978

Determination of capsid size by satellite bacteriophage P4.

Satellite bacteriophage P4 requires all morphogenic gene products provided by a helper phage, such as coliphage P2, to assemble its own capsid, which is one-third the volume of the larger helper capsid. We have isolated a satellite phage P4 sid (size determination) mutant that is unable to direct the assembly of the small wild-type-size P4 capsid. Instead, this mutant produces P4 plaque-forming units with large P2-size capsids which contain two or three copies of the P4 sid1 genome. P4 sid1 is evidently mutated in a protein that is specifically responsible for determining the precise size and symmetry of the structure into which the helper P2 gene products will assemble. In addition, we have found that the physical size of the genome does not appear to play an essential role in the proper assembly of the icosahedral capsid, since the majority of the P4 sid1 plaque-forming units do not contain a complete capsidful of DNA.

Coliphages↗
PubMed1994

Modulation of thermoprotection and translational thermotolerance induced by Semliki Forest virus capsid protein.

Low amounts of Semliki Forest virus capsid protein transferred into target cells by electroporation-mediated delivery (10(3)-10(4) molecules incorporated/cell) confer thermal resistance resulting in enhanced survival. Furthermore, when exposed to 43 degrees C, these cells display an enhanced expression of heat-shock protein-70 and a translational thermotolerance. Similarly, low amounts of capsid protein transferred into cells in which transcription is blocked by actinomycin D, also protect the translational machinery at 43 degrees C. In a cell-free translation system, added capsid protein appears to modulate translational efficiency of endogenous mRNAs. At approximately 1 molecule/ribosome, capsid protein is able to enhance translation at 30 degrees C and at 43 degrees C. In contrast, high concentrations of capsid protein are responsible for a marked inhibition of protein synthesis at 30 degrees C, but only hamper translational thermotolerance at 43 degrees C. Our results favor the hypothesis that small amounts of capsid protein trigger a chaperone-like activity that is able to protect the translational machinery from thermal damage.

Adaptation, Biological↗
PubMed1994

The lumazine synthase/riboflavin synthase complex of Bacillus subtilis. X-ray structure analysis of hollow reconstituted beta-subunit capsids.

The lumazine synthase/riboflavin synthase complex of Bacillus subtilis consists of an icosahedral capsid of 60 beta subunits enclosing a triplet of alpha subunits. An X-ray structure of 0.32 nm resolution has been obtained for the icosahedral capsid of the native alpha 3 beta 60 complex [Ladenstein, R., Schneider, M., Huber, R., Bartunik, H. D., Wilson, K., Schott, K. & Bacher, A. (1988) J. Mol. Biol. 203, 1045-1070]. beta subunits were isolated after denaturation of the alpha 3 beta 60 complex and were subsequently reconstituted in a ligand-driven reaction yielding artifactual, hollow beta 60 capsids with icosahedral symmetry. Hexagonal crystals (space group P6(3)22) of the reconstituted capsids diffracted X-rays to a resolution of 0.32 nm. Crystallographic intensity data were obtained using synchrotron radiation. Freeze-etched electron-microscopic images and rotation function calculations showed that the hexagonal crystal forms of the artifactual beta 60 capsids and the native alpha 3 beta 60 complex are isomorphous. Orientation and translation parameters of the beta-subunit model were refined by XPLOR rigid-body refinement. The electron-density map was improved by cyclic icosahedral averaging and phase extension from 0.5-0.32 nm resolution. The beta-subunit structure was partially refined by energy minimization and crystallographic refinement (XPLOR) assuming strict icosahedral symmetry (final R factor 30.9% for data at 0.8-0.32 nm resolution). The topology and chain folding of the beta subunits in the artifactual beta 60 capsid are similar to the native alpha 3 beta 60 enzyme. Structural features of the substrate-binding site and the binding of the substrate-analogous ligand 5-nitro-6-ribitylamino-2,4(1H,3H)-pyrimidinedione are discussed. Ligand binding occurs at the pentamer interfaces and includes van der Waals' interactions and hydrogen bonding. The binding pocket shows a hydrophobic region which accomodates the pyrimidinedione ring and a hydrophilic region to which the ribityl side chain binds. Most amino acid residues involved in the active site are conserved as shown by sequence comparisons with the putative lumazine-synthase genes of Escherichia coli and Photobacterium leiognathi. In the final electron-density map, a residual density feature was tentatively assigned to a bound phosphate ion which mimics the binding of the second substrate, 3,4-dihydroxy-2-butanone 4-phosphate. This putative phosphate-binding site involves a highly conserved amino acid sequence containing three basic residues.

Bacillus subtilis↗
PubMed1972

Complement-fixing antigens of herpes simplex virus types 1 and 2: reactivity of capsid, envelope, and soluble antigens.

Capsid, envelope, and nonvirion-associated soluble components of type 1 and type 2 herpes simplex virus (HSV) were obtained from infected monolayer cell cultures and used as complement fixation (CF) antigens. Capsids were prepared by treatment of cells with the nonionic detergent Nonidet-P40, envelope material by treatment of virions with ether and high pH, and soluble components were obtained from culture fluids of untreated cells. Serological studies with experimental anti-herpesvirus sera indicate that these serotypes share cross-reacting envelope, capsid, and soluble antigens with each other and with herpesvirus B but not with varicella virus. In addition, animals immunized with crude HSV preparations contain high levels of CF antibody (1:32 to 1:64) to soluble antigens, whereas sera from humans who have experienced natural infection contain low levels of antibody (</=1:8) to this antigen. Further testing with reference, capsid, and envelope antigens indicates that antibody levels to reference and capsid antigens are about the same in sera from healthy humans, whereas antibody to the envelope is decidedly lower in these sera. Herpes convalescent-phase sera contain higher levels of antibody to reference and envelope antigens than to capsid antigen.

Animals↗
PubMed1972

Structure of the bluetongue virus capsid.

Seven polypeptides were found to be present in the capsid of the bluetongue virus (BTV), four of which are major and three are minor components. This number and size distribution is the same as that found in reovirus, which has a similar segmented, double-stranded ribonucleic acid genome. In both viruses an excellent correlation is found between the molecular weights of certain genome segments and those of the polypeptides, suggesting a direct coding relationship between them. Careful comparison, however, indicates that the correlation patterns of the two viruses are different, suggesting that different genome segments may be utilized to code for certain capsid proteins. This phenomenon may be related to the fact that these viruses can be distinguished morphologically, reovirus having a larger diameter and possessing a double-layered capsid against the smaller single capsid of the BTV. Utilizing the degrading effect of CsCl gradients, it was demonstrated, however, that two of the BTV polypeptides are present as a diffuse protein layer surrounding the capsid. This outer layer probably has some of the functions of the reovirus outer capsid, one being the "masking" of a viral transcriptase which could be demonstrated after its removal.

Animals↗
PubMed1972

Gene order of the poliovirus capsid proteins.

Two methods were used to determine the genetic map of the poliovirus capsid proteins. The first method uses pactamycin, a drug which selectively inhibits the initiation of protein synthesis and causes a change in the relative amounts of capsid proteins synthesized. This differential effect on each of the capsid proteins is interpreted as indicating the relative distance of each protein from the initiation site of protein synthesis. The second method involves an analysis of coat precursor molecules released from polyribosomes after a series of short pulses of different length terminated by addition of emetine, a drug which stops all protein synthesis almost immediately after its addition. As the pulse length is increased, each of the capsid proteins within the precursor gains radioactivity with different kinetics. From these kinetics, it is possible to determine the gene order of the capsid proteins within the precursor as well as a rate of protein synthesis. Both methods indicate a gene order for the region of the ribonucleic acid coding for the capsid proteins as (5' --> 3') VP 4 - VP 2 - VP 3 - VP 1.

Anti-Bacterial Agents↗
PubMed2000

Expression of recombinant Norwalk-like virus capsid proteins using a bacterial system and the development of its immunologic detection.

The capsid protein of Norwalk-like virus (NLV) isolates NLV-36 (Mexico virus type, genogroup II [GII]), NLV-21 (Lordsdale virus type, GII), NLV-114 (untyped GII virus), and NLV-96-908 (KY89 virus type, GI) have been expressed in an Escherichia coli system. The expressed recombinant NLV capsid proteins, fused with maltose binding protein (MBP-rV) and thioredoxin (TRX-rV) in E. coli lysate, were analyzed using sodium dodecyl sulfate-polyacrylamide gel eletrophoresis. Rabbit IgG (R-IgG) in hyperimmune serum has been raised against MBP-rV-36 capsid protein and was purified before further study. Detection of TRX-rVs using an enzyme-linked immunosorbent assay (ELISA) showed that R-IgG had immunologic reactivity to GII as well as to the GI rV capsid proteins TRX-rV-36, TRX-rV-21, TRX-rV-114, and TRX-rV-96-908. Results of Western immunoblot (WB) analysis showed the same broad recognition of R-IgG when using the same samples. The results of the ELISA tests on serum samples obtained from patients involved in confirmed outbreaks of NLV proved that expressed NLV capsid proteins in E. coli can be detected by NLV-infected human serum. In addition, purified NLVs (LD virus types) derived from patients' stool could be detected using anti-NLV R-IgG, whereas normal R-IgG did not react when using WB. Our results strongly suggest that the immunologic detection of NLV antigens using anti-rV R-IgG is possible and seems a significant step toward simplification of an NLV detection test.

ATP-Binding Cassette Transporters↗
PubMed1996

Phage display of intact domains at high copy number: a system based on SOC, the small outer capsid protein of bacteriophage T4.

Peptides fused to the coat proteins of filamentous phages have found widespread applications in antigen display, the construction of antibody libraries, and biopanning. However, such systems are limited in terms of the size and number of the peptides that may be incorporated without compromising the fusion proteins' capacity to self-assemble. We describe here a system in which the molecules to be displayed are bound to pre-assembled polymers. The polymers are T4 capsids and polyheads (tubular capsid variants) and the display molecules are derivatives of the dispensable capsid protein SOC. In one implementation, SOC and its fusion derivatives are expressed at high levels in Escherichia coli, purified in high yield, and then bound in vitro to separately isolated polyheads. In the other, a positive selection vector forces integration of the modified soc gene into a soc-deleted T4 genome, leading to in vivo binding of the display protein to progeny virions. The system is demonstrated as applied to C-terminal fusions to SOC of (1) a tetrapeptide; (2) the 43-residue V3 loop domain of gp120, the human immunodeficiency virus type-1 (HIV-1) envelope glycoprotein; and (3) poliovirus VP1 capsid protein (312 residues). SOC-V3 displaying phage were highly antigenic in mice and produced antibodies reactive with native gp120. That the fusion protein binds correctly to the surface lattice was attested in averaged electron micrographs of polyheads. The SOC display system is capable of presenting up to approximately 10(3) copies per capsid and > 10(4) copies per polyhead of V3-sized domains. Phage displaying SOC-VP1 were isolated from a 1:10(6) mixture by two cycles of a simple biopanning procedure, indicating that proteins of at least 35 kDa may be accommodated.

Animals↗
PubMed1996

The capsid architecture of channel catfish virus, an evolutionarily distant herpesvirus, is largely conserved in the absence of discernible sequence homology with herpes simplex virus.

Although herpesviruses have a wide host range and their genomes vary substantially in size, the nucleocapsid appears to be a conservative element of viral design. The capsid shell is icosahedrally symmetric (T = 16), and 125 nm in diameter and 15nm thick in the case of herpes simplex virus 1 (HSV-1). Channel catfish virus (CCV) has the gross morphology of a herpesvirus, although no relationship to other herpesviruses is evident from the sequences of its proteins. To examine CCV capsid architecture more closely, we have determined its structure by cryoelectron microscopy and three-dimensional image reconstruction. The CCV capsid is smaller than that of HSV-1, but its 12% smaller genome is packed to essentially the same average density; its icosahedral facets are flatter, and its shell is about 20% thinner, consistent with the smaller size of its major capsid protein. Otherwise, their major features are remarkably similar: CCV has the same triangulation number; its hexons and pentons also have chimney-like protrusions with an axial channel through each capsomer; and there are "triplexes" on the outer surface at the sites of local threefold symmetry. The basic herpesvirus capsid architecture is, therefore, remarkably well conserved in CCV and implies a utilitarian basis to this design. The protein composition of CCV mirrors that of HSV-1, except for the absence of the 12-kDa protein, VP26, which is dispensable for assembly in the HSV-1 system and, apparently, wholly dispensable for CCV.

Animals↗
PubMed1996

Bacteriophage P4 capsid-size determination and its relationship to P2 helper interference.

The sid (size determination) gene product of phage P4 is known to be involved in capsid-size determination. Moreover, the capsid-size determination function interferes with the lytic development of its helper P2, presumably because the helper genome is too large to be packaged into P4-size capsids. In order to study P4-specified helper interference, we cloned the sid gene for expression during phage infection. Even though gpSid restores the capsid-size determination function of a sid defective P4 mutant, we find that gpSid alone is not sufficient to establish full interference of helper P2 phage production. Complete helper interference requires some P4 function in addition to gpSid. Complementation tests show that none of the known P4 genes display this property. We propose that P4 encodes a yet-unidentified function that in concert with gpSid establishes full P2 helper interference at the level of capsid-size determination.

Bacteriophage P2↗
PubMed1996

Carboxyl terminus of bovine papillomavirus type-1 L1 protein is not required for capsid formation.

The papillomavirus major capsid protein L1 can assemble into capsids in vitro. To identify areas within the bovine papillomavirus type-1 L1 (BPV L1) protein that are important for virus assembly, we constructed a set of 24 baculovirus recombinants expressing BPV L1 deletion mutants that span the entire L1 open reading frame. Virus-like particle (VLP) formation of the L1 mutants was examined by electron microscopy. Wild-type (wt) BPV L1 expressed in recombinant baculovirus formed VLPs, while capsomeres and aggregates were seen for most of the mutants screened. However, the C-terminal truncation mutant, lacking the last 24 amino acids (delta C2), was observed to form VLPs (threefold more efficiently than wt BPV L1). This suggests that this C-terminal region of L1 protein is not critical for capsid formation. As capsids assembled from BPV L1 are able to agglutinate mouse red blood cells (RBC) by binding to a membrane protein, we tested the ability of the mutants to hemagglutinate mouse RBCs. Most aberrant capsids or aggregates derived from deletion mutants were unable to agglutinate the RBCs with the exception of deletion mutants delta 11 (aa 231-271), delta 14 (aa 291-331), delta 21 (aa 431-471), and the carboxyl-terminus truncation mutant delta C2.

Amino Acid Sequence↗
PubMed1985

The structure of the adenovirus capsid. II. The packing symmetry of hexon and its implications for viral architecture.

The orientation and location of the 240 hexons comprising the outer protein shell of adenovirus have been determined. Electron micrographs of the capsid and its fragments were inspected for the features of hexon known from the X-ray crystallographic model as described in the accompanying paper. A capsid model is proposed with each facet comprising a small p3 net of 12 hexons, arranged as a triangular sextet with three outer hexon pairs. The sextet is centrally placed about the icosahedral threefold axis, with its edges parallel to those of the facet. The outer pairs project over the facet edges on one side of the icosahedral twofold axes at each edge. The model capsid is defined by the underlying icosahedron, of edge 445 A, upon which hexons are arranged. The hexons are thus bounded by icosahedra with insphere radii of 336 A and 452 A. A quartet of hexons forms the asymmetric unit of an icosahedral hexon shell, which can be closed by the addition of pentons at the 12 vertices. Considering the hexon trimer as a complex structure unit, its interactions in the four topologically distinct environments are very similar, with conservation of at least two-thirds of the inter-hexon bonding. The crystal-like construction explains the flat facets and sharp edges characteristic of adenovirus. Larger "adenovirus-like" capsids of any size could be formed using only one additional topologically different environment. The construction of adenovirus illustrates how an impenetrable protein shell can be formed, with highly conserved intermolecular bonding, by using the geometry of an oligomeric structure unit and symmetry additional to that of the icosahedral point group. This contrasts with the manner suggested by Caspar & Klug (1962), in which the polypeptide is the structure unit, and for which the number of possible bonding configurations required of a structure unit tends to infinity as the continuously curved capsid increases in size. The known structures of polyoma and the plant viruses with triangulation number equal to 3 are evaluated in terms of hexamer-pentamer packing, and evidence is presented for the existence of larger subunits than the polypeptide in both cases. It is suggested that spontaneous assembly can occur only when exact icosahedral symmetry relates structure units or sub-assemblies, which would themselves have been formed by self-limiting closed interactions. Without such symmetry, the presence of scaffolding proteins or nucleic acid is necessary to limit aggregation.

Adenoviridae↗
PubMed2005

Transcapsidation and the conserved interactions of two major structural proteins of a pair of phytoreoviruses confirm the mechanism of assembly of the outer capsid layer.

The strongly conserved amino acid sequences of the P8 outer capsid proteins of Rice dwarf virus (RDV) and Rice gall dwarf virus (RGDV) and the distribution of electrostatic potential on the proteins at the interfaces between structural proteins suggested the possibility that P8-trimers of RGDV might bind to the 3-fold symmetrical axes of RDV core particles, with vertical interaction between heterologous P3 and P8 proteins and lateral binding of homologous P8 proteins, thereby allowing formation of the double-layered capsids that are characteristic of viruses that belong to the family Reoviridae. We proved this hypothesis using chimeric virus-like particles composed of the P3 core capsid protein of RDV and the P8 outer capsid protein of RGDV, which were co-expressed in a baculovirus expression system. This is the first report on the molecular biological proof of the mechanism of the assembly of the double-layered capsids with disparate icosahedral lattices.

Amino Acid Sequence↗
PubMed2005

Control of virus assembly: HK97 "Whiffleball" mutant capsids without pentons.

The capsid of Escherichia coli bacteriophage HK97 assembles as a 420 subunit icosahedral shell called Prohead I which undergoes a series of maturation steps, including proteolytic cleavage, conformational rearrangements, and covalent cross-linking among all the subunits to yield the highly stable mature Head II shell. Prohead I have been shown to assemble from pre-formed hexamers and pentamers of the capsid protein subunit. We report here the properties of a mutant of the capsid protein, E219K, which illuminate the assembly of Prohead I. The mutant capsid protein is capable of going through all of the biochemically and morphologically defined steps of capsid maturation, and when it is expressed by itself from a plasmid it assembles efficiently into a Prohead I that is morphologically indistinguishable from the wild-type Prohead I, with a full complement of both hexamers and pentamers. Unlike the wild-type Prohead I, when the mutant structure is dissociated into capsomers in vitro, only hexamers are found. When such preparations are put under assembly conditions, these mutant hexamers assemble into "Whiffleballs", particles that are identical with Prohead I except that they are missing the 12 pentamers. These Whiffleballs can even be converted to Prohead I by specifically binding wild-type pentamers. We argue that the ability of the mutant hexamers to assemble in the absence of pentamers implies that they retain a memory of their earlier assembled state, most likely as a conformational difference relative to assembly-naive hexamers. The data therefore favor a model in which Prohead I assembly is regulated by conformational switching of the hexamer.

Bacteriophages↗