Structures and roles of the polymorphic forms of tobacco mosaic virus protein. VII. Lengths of the growing rods during assembly into nucleoprotein with the viral RNA

Production and applications of engineered viral capsids

As biological agents, viruses come in an astounding range of sizes, with varied shapes and surface morphologies. The structures of viral capsids are generally assemblies of hundreds of copies of one or a few proteins which can be harnessed for use in a wide variety of applications in biotechnology, nanotechnology, and medicine. Despite their complexity, many capsid types form as homogenous populations of precise geometrical assemblies. This is important in both medicine, where well-defined therapeutics are critical for drug performance and federal approval, and nanotechnology, where precise placement affects the properties of the desired material. Here we review the production of viruses and virus-like particles with methods for selecting and manipulating the size, surface chemistry, assembly state, and interior cargo of capsid. We then discuss many of the applications used in research today and the potential commercial and therapeutic products from engineered viral capsids.

A kinetic Zipper model and the assembly of tobacco mosaic virus

We put forward a modified Zipper model inspired by the statics and dynamics of the spontaneous reconstitution of rodlike tobacco mosaic virus particles in solutions containing the coat protein and the single-stranded RNA of the virus. An important ingredient of our model is an allosteric switch associated with the binding of the first protein unit to the origin-of-assembly domain of the viral RNA. The subsequent addition and conformational switching of coat proteins to the growing capsid we believe is catalyzed by the presence of the helical arrangement of bound proteins to the RNA. The model explains why the formation of complete viruses is favored over incomplete ones, even though the process is quasi-one-dimensional in character. We numerically solve the relevant kinetic equations and show that time evolution is different for the assembly and disassembly of the virus, the former exhibiting a time lag even if all forward rate constants are equal. We find the late-stage assembly kinetics in the presence of excess protein to be governed by a single-exponential relaxation, which agrees with available experimental data on TMV reconstruction.

Differences of reconstitution process between tobacco mosaic virus and cucumber green mottle mosaic virus by synchrotron small angle X-ray scattering using low-temperature quenching

The differences of the reconstitution process of tobacco mosaic virus (TMV) and its mutant, cucumber green mottle mosaic virus (CGMMV) were investigated by the solution X-ray scattering measurements with the synchrotron radiation source using low-temperature quenching. The reconstitution in an aqueous solution is completely stopped below 5 degrees C. The TMV and CGMMV assembly was traced by the small-angle X-ray scattering (SAXS) measurements at 5 degrees C on a series of solutions prepared by low-temperature quenching after incubation at 20 degrees C for an appropriate interval between 0 and 60 min. The SAXS results were analyzed by the Guinier plot, the Kratky plot and the distance distribution function. The incubation of RNA and protein of CGMMV did not reconstitute at the initial reaction stages below 5 min and then began to reconstitute gradually. After 60 min, the radius of gyration for CGMMV reconstitution process reached almost the value for the initial stage of TMV reconstitution process. This is due to the fact the formation of double-layered disk in CGMMV protein is much slower than in TMV protein.

Solution X-ray scattering study of reconstitution process of tobacco mosaic virus particle using low-temperature quenching

The reconstitution process of tobacco mosaic virus (TMV) was investigated by the solution X-ray scattering measurements with the synchrotron radiation source using low-temperature quenching. TMV assembly in an aqueous solution is completely stopped below 5 degrees C. The TMV assembly was traced by the small-angle X-ray scattering (SAXS) measurements at 5 degrees C on a series of solutions prepared by low-temperature quenching after incubation either at 15, 20 or 25 degrees C for an appropriate interval between 0 and 60 min. The SAXS results were analyzed by the Guinier plot, the Kratky plot and the distance distribution function. In order to account the time course of SAXS profiles in terms of the elongation of TMV assembly, a model calculation was performed to simulate the Guinier plot, the Kratky plot and the distance distribution function by applying Glatter's multibody method using models that were constituted of the spheres representing a column of piled two-layer disks of TMV-protein. The three simulated functions thus obtained support the conclusion derived from the three functions calculated from the experimental results that the incubation of the RNA and protein of TMV began to reconstitute TMV instantly after mixing, proceeded steeply to a long rod, and then extended asymptotic to the full length of the TMV particle. This process is in good agreement with that obtained from electron microscopic studies.

RNA-protein interactions in the assembly of tobacco mosaic virus

Assembly of tobacco mosaic virus is initiated by the binding of a specific loop of the RNA into the central hole of the disk aggregate of protein subunits. Since the nucleation loop is located about five-sixths along the RNA molecule, subsequent elongation must be bidirectional. We have now measured the rates of elongation in the two directions by determining the lengths of RNA protected from nuclease digestion at different times and using either intact TMV rNA, or RNA with most of the longer tail removed. Comparison of the rates with the protein supplied as either a mixture of disks with A-protein (a mixture of less aggregated states) or just A-protein, shows that different mechanisms and protein aggregates are used for the most rapid growth. When disks are present, they add more rapidly along the longer RNA tail but do not appear to add directly on the shorter tail. In contrast, smaller aggregates (A-protein) can add at both ends of the rod, but do so more slowly. Mechanisms for these processes are discussed. Preliminary results on the binding of the specific hexanucleotide AAGAAG to the disk are given and compared with the known changes on binding nonspecific hexanucleotides or the trinucleotide AAG.

Location and encapsidation of the coat protein cistron of tobacco mosaic virus. A bidirectional elongation of the nucleoprotein rod

The coat protein cistron of tobacco mosaic virus has been located on the viral RNA starting between 975 and 1050 nucleotides from the 3'-hydroxyl end. This locates its 5' end close to the origin for virus assembly, where the first protein disk interacts with RNA. It also means that the coat protein mRNA must have a short 5'-untranslated tail and a long (over 500 nucleotides) 3' one. The recovery of characteristic oligonucleotides in nuclease-protected rods during the growth from RNA and a protein disk preparation shows that elongation of the nucleated rods proceeds independently in both directions though, on average, much more rapidly along the longer 5' tail than the shorter 3' tail. Protected RNA of length equal to that in the complete virion is first seen within 6 min, showing that the most rapidly elongated particles are substantially complete by this time.

Configuration of tobacco mosaic virus, RNA during virus assembly

When TMV reassembles, the uncoated RNA is folded back along the growing rod, probably down the central hole. This surprising configuration is essential for rapid elongation--presumably supplying RNA to its site of incorporation while keeping the bulk of the free RNA out of the way.

Structure and immunogenic behaviour of methylated tobacco mosaic virus

Tobacco mosaic virus was methylated, using various concentrations of dimethylsulfate. The methylated virus sample with still intact particles was subjected to sequential analysis. The sites and the degree of methylation were determined in the tryptic peptides. Tyrosine 139 and cysteine 27 are more accessible to methylation than tyrosine 72, lysine 68 and tyrosine 2. A limited number of carboxyl groups was also methylated. The ability of methylated and original tobacco mosaic virus to initiate the formation of humoral antibodies and the capacity to induce a delayed-type hypersensitivity reaction were investigated in STU mice. Original tobacco mosaic virus could not induce a delayed-type hypersensitivity reaction but methylated tobacco mosaic virus induced a delayed-type reaction, not depending on whether the virus particles were intact or disintegrated. This phenomenon was strictly linked with the presence of methylester groups.