Metastable aggregates in the polymerisation of tobacco-mosaic-virus protein

Dynamic mechanism of the self-assembly process of tobacco mosaic virus protein studied by rapid temperature-jump small-angle X-ray scattering using synchrotron radiation

The self-assembly process of tobacco mosaic virus protein (TMVP) was observed by rapid temperature-jump time-resolved solution X-ray small-angle scattering using synchrotron radiation. The temperature-jump device used for the X-ray measurements is rapid enough to cope with even the fastest-assembling process of TMVP, and accumulates data of reasonable signal-to-noise ratios with a minimum total counting time of 7.5 seconds. The measurements suggested that the 20 S disk of TMVP polymerized to stacked disks (short rods). The time to complete stacking varied from approximately 25 seconds to approximately 1200 seconds, depending on the solution condition and magnitude of the temperature gap. Higher protein concentration, ionic strength and temperature favoured faster association. The results were analysed in terms of a set of kinetic equations that describe the two-stage aggregation of TMVP with an equilibrium constant K1, and two rate constants k+2 and k-2 for association and dissociation of disks, respectively. The consistency of the analysis suggests that the TMVP assembly proceeds in two steps of: (1) the aggregation of A-proteins into double-layered disks; and (2) the stacking of double-layered disks. The kinetic analysis indicated that the stacking belongs to the lowest range of protein-protein interaction system.

Temperature dependence of the structure of aggregates of tobacco mosaic virus protein at pH 7.2. Static synchrotron small-angle X-ray scattering

The small-angle X-ray scattering (SAXS) method using a synchrotron radiation source was applied to the study of the self-aggregation process of tobacco mosaic virus protein (TMVP) at a concentration of 5.0 or 12.0 mg ml-1 in 50 mM or 100 mM-phosphate buffer (ionic strengths approx. 0.1 and 0.2, respectively) at pH 7.2 in the temperature region of 4.8 to 25.0 degrees C. This paper presents the results of static measurements of SAXS. Sedimentation velocity experiments were performed simultaneously under the same conditions. These results are qualitatively parallel to those of the SAXS measurements, although the size of stacked disks derived from the SAXS measurements is larger than that derived from the sedimentation experiments, suggesting a change in the equilibrium conditions in the centrifugal field. Qualitative analysis of the SAXS data with model simulation calculations implies that the aggregation of TMVP consists of two steps: (1) the aggregation of A-protein comprising a few subunits to form double-layered disks; and (2) the random polymerization of double-layered disks by disk-stacking. Increase in temperature, ionic strength or protein concentration induced TMVP to polymerize to form a double-layered disk or a quadruple-layered short rod with consumption of A-proteins, accompanied by a small number of multi-layered short rods. The SAXS results indicate that the A-protein and the multilayered short rods are polydisperse with respect to size and shape, i.e. the mixture of A-protein, double-layered disks and multi-layered short rods coexists in the equilibrium state without pressure-induced partial dissociation of TMPV as observed during normal ultracentrifugation, and even under solution conditions in which the formation of double-layered disks or higher-order aggregates is favored.

Scanning calorimetric investigation of the polymerization of the coat protein of tobacco mosaic virus

The endothermic polymerization of the coat protein of tobacco mosaic virus has been studied by high-sensitivity differential scanning calorimetry, with control experiments involving turbidimetry and sedimentation velocity measurements. The variation of the apparent extent of polymerization under conditions close to equilibrium as the temperature is raised follows a course which is difficult to duplicate on the basis of simple models for the process. The enthalpy of polymerization at low protein concentration varies from 12.5 kcal (mol of monomer)-1 (17500 daltons) under conditions where the product is largely a mixture of short helical rods to 6.0 kcal ol-1 for the formation of double disks containing 34 monomer units. In the former case, the polymerization is accompanied by a decrease in apparent heat capacity of 350 cal K-1 mol-1 while in the latter there is an increase of 150 cal K-1 mol-1. These results constitute evidence that these two types of polymerization involve intersubunit bonds of quite different chemical character.

Titration behaviour of three strains of tobacco mosaic virus

Hydrogen ion titration curves of the virions and proteins of three strains of tobacco mosaic virus (Y-TAMV, U2, and cowpea) were measured in the absence and the presence of Ca2+, Mg2+, or Mn2+ ions, and compared with the analogous curves for the type strain (vulgare). Extinction coefficients were also measured for all four strains' virions and proteins. Y-TAMV is very like vulgare in its cation affinities: the virion has probably three groups per protein subunit that titrate near neutral pH and significantly bind metal ions; the RNA-free protein has very little affinity for Ca2+, although moderate Ca2+ concentrations favour the existence of larger polymers. U2 and cowpea strain virions bind cations significantly more strongly than do Y-TAMV or vulgare virions: their polymerized proteins, too, have significant affinities for Ca2+ ions, which make their titration and sedimentation behaviours relatively sensitive to added calcium. These cation-binding differences correspond well with the differences between the strains' protein sequences. The features common to all four strains are that the virions are apparently structurally invariant and have at least one site per subunit with Ca2+ affinity in the region of 10(-5)M, while the RNA-free proteins lack the high-affinity sites but have weaker Ca2+ affinities in the region of 10(-3)M. Some of the cation-binding sites probably lie near the central holes of the virions.

Hydrogen-ion binding by tobacco-mosaic-virus protein polymers

Hydrogen ion titration curves of tobacco mosaic virus protein have been measured in various conditions of protein concentration, temperature, ionic strength, and rate of pH change. The polymers present at each stage are deduced from turbidity and sedimentation data, plus published information. A simple semi-quantitative analysis of the curves is given, and the pK values of the two abnormal carboxylates in single helix are estimated as 6.4 and about 7.0. Disks, and some faster-forming unknown polymers in the same size range, have been abnormal carboxylate with pK 6.9. These results are most easily interpreted in terms of electrostatic interactions between carboxylates, probably at the axial ends of the protein subunits.