Evidence for a rapid structural change in TMV-A-protein near neutrality

Tobacco mosaic virus protein aggregates in solution: structural comparison of 20S aggregates with those near conditions for disk crystallization

Previous X-ray studies (2.8-A resolution) on the crystals of tobacco mosaic virus protein (TMVP) grown from solutions containing high salt have characterized the structure of the protein aggregate as a bilayered cylindrical disk formed by 34 identical subunits [Bloomer, A.C., Champness, J.N., Bricogne, G., Staden, R., & Klug, A. (1978) Nature (London) 276, 362-368]. Under low-salt conditions, 20S aggregates are in equilibrium with 4S species and involved in the efficient nucleation of TMV assembly in vitro [Butler, P.J.G. (1984) J. Gen. Virol. 65, 253-279]. We have investigated by sedimentation velocity and near-UV circular dichroism (CD) measurements the structure of 20S aggregates in low salt (I = 0.1 potassium phosphate at pH 7.0 and 20 degrees C) and the aggregates in high salt [0.2 M (NH4)2SO4 in I = 0.1 tris(hydroxymethyl)aminomethane hydrochloride at pH 8.0 and 20 degrees C, close to the conditions under which TMVP crystallizes as disk aggregates]. At high salt, we observe structures (presumably stacks of disks) having s20,w values around 40, 45, and 50 S, but not the 20S species present in low-salt buffers. The near-UV CD spectrum of 20S aggregates has been obtained for the first time, using computer techniques, from the spectra of the 4S-20S equilibrium mixture and the 4S species. This spectrum of 20S aggregates differs dramatically from that of the stacks of disks examined at both high and low salt (into which the stacks can be returned by dialysis), indicating that the difference is not a solvent effect.(ABSTRACT TRUNCATED AT 250 WORDS)

Studies on the mechanism of assembly of tobacco mosaic virus

Sedimentation and proton binding studies on the endothermic self-association of tobacco mosaic virus (TMV) protein indicate that the so-called "20S" sedimenting protein is an interaction system involving at least the 34-subunit two-turn yield cylindrical disk aggregate and the 49-subunit three-turn helical rod. The pH dependence of this overall equilibrium suggests that disk formation is proton-linked through the binding of protons to the two-turn helix which is not present as significant concentrations near pH 7. There is a temperature-induced intramolecular conformation change in the protein leading to a difference spectrum which is complete in 5 x 10(-6) s at pH 7 and 20 degrees C and is dominated at 300 nm by tryptophan residues. Kinetics measurements of protein polymerization, from 10(-6) to 10(3) s, reveal three relaxation processes at pH 7.0, 20 degrees C, 0.10 M ionic strength K (H) PO4. The fastest relaxation time is a few milliseconds and represents reactions within the 4S protein distribution. The second fastest relaxation is 50-100 x 10(-3) s and represents elementary polymerization steps involved in the formation of the approximately 20 S protein. Analysis of the slowest relaxation, approximately 5 x 10(4) s, suggests that this very slow formation of approximately 20 S protein may be dominated by some first order process in the overall dissociation of approximately 20S protein. Sedimentation measurements of the rate of TMV reconstitution, under the same conditions, show by direct measurements of 4S and approximately 20S incorporation at various 4S to approximately 20S weight ratios that the relative rate of approximately 20S incorporation decreases almost linearly, from 0 to 50% 4S. There appears to be one or more regions of TMV-RNA, approximately 1-1.5 kilobases long, which incorporates approximately 20S protein exclusively. Solutions of approximately 95-100% approximately 20S protein have been prepared for the first time and used for reconstitution with RNA. Such protein solutions yield full size TMV, but at a slower rate than if 4S protein is added. Thus the elongation reaction in TMV assembly, following nucleation with approximately 20S protein, is not exclusively dependent upon the presence of either 4S or approximately 20S protein aggregates. The initial, maximum, rate of reconstitution increases about threefold when the protein composition is changed from 5% to 30% 4S protein, at constant total protein concentration at pH 7.0, 20 degrees C in 0.10 M ionic strength K (H)PO4. The probable binding frame at the internal assembly nucleation site of TMV-RNA has been determined by measuring the association constants for the binding of various trinucleoside diphosphates to helical TMV protein rods. The -CAG-AAG-AAG-sequence at the nucleation site is capable of providing at least 10-14 kcal/mol of sites of binding free energy for the nucleation event in TMV self-assembly.

Circular-dichroism and absorption spectroscopic studies on specific aromatic residues involved in the different modes of aggregation of tobacco-mosaic-virus protein

Conformational changes accompanying the different modes of aggregation of tobacco mosaic virus protein (TMV-protein) were investigated using circular dichroism (CD) and absorption difference spectra in the range of aromatic absorption. Comparing wild-type protein and mutant Ni 2068 (Tyr-139 leads to Cys-139) a tentative localization of aromatic amino acids in the three-dimensional structure is rendered possible. In all modes of aggregation the CD spectra are determined by intrasubunit interactions between aromatic residues, in particular Trp-17 and Trp-52 as well as Tyr-70, Tyr-72 and Tyr-139. The Trp-17-Trp-52 interaction was found to be highly sensitive towards changes of the quaternary structure especially with respect to helical aggregates. This suggests that the environment of the two tryptophan residues is of crucial importance in the three-dimensional structure of the subunit; in the course of aggregation intersubunit interactions compete with the specific intrasubunit Trp-17--Trp52 interactions. It is suggested that Try-70 and Tyr-72 form hydrogen bonds in a strongly hydrophobic environment. Formation of the double disc decreases the rotatory strength, pointing to an increase in conformational flexibility. Spectroscopic and chemical evidence prove that Tyr-70, Tyr-72 and Tyr-139 are in close neighbourhood. Double disc formation by lowering the pH (pH 8 LEADS TO 6.9, I = 0.1 M) or increasing the ionic strength (pH 8, I = 0.1 LEADS TO 0.6 M) is reflected by identical spectral effects in the environment of Tyr-70 - Tyr-72. However the interaction between Trp-17 and Trp-52 indicates significant differences in the conformation which may be important for the formation of higher aggregates, i.e. 'lockwashers', helices, and 'stacked discs'.