Macromolecular structural transitions in Pf1 filamentous bacterial virus

Abstract

The filamentous bacterial virus Pf1 is a simple model for biological filaments. We have studied the structure of the virion and report here that the helix parameters of Pf1 change sharply with temperature at about 8 degrees C. Local interactions between protein subunits change by only a few tenths of an angstrom, but the changes are amplified between one end and the other of the virion to a rotation of 15 turns and a translation of 1,000 A. The limited nature of the phase transition is probably due to the constraints of 'knobs-into-holes' interaction between side chains of adjacent alpha-helical protein subunits. Treatment of the virion with ether causes a rearrangement of protein subunits into sheets, with the alpha-helices normal to the plane of the sheet. This phase transition suggests a model for virion assembly in the bacterial membrane.

Molecular models and structural comparisons of native and mutant class I filamentous bacteriophages

“…Several attempts, in part successful, were made to infer the symmetry (Marvin & Wachtel, 1976;Day & Wiseman, 1978;Makowski & Caspar, 1978, but none gave the correct enantiomorph. The development of methods using the diamagnetic anisotropy of lnovirus to improve alignment in fibres (Nave et al, 1979;Torbet & Maret, 1979;Maret & Dransfeld, 1985) enabled closely spaced layer lines to be resolved and small intensity differences between native and heavy-atom derivative vir.iorus to be measured. These experimental advances enabled resolution of the symmetry ambiguities (Banner et al, 1981;Bryan et al, 1983;Bryan, 1987;Marvin et al, 1987;Glucksman et ai., 1992).…”

Intersubunit Hydrophobic Interactions in Pf1 Filamentous Phage

“…An approach to the full determination and understanding of the Pf1 structure is through the use of a well known reversible structural phase transition near 10°C between two ordered states that were first observed by x-ray fiber diffraction (8,9). This transition temperature is well below the physiological temperature, and the structural response of Pf1 is associated with a change in the helical pitch and rise.…”

“…The transition is clearly a property of a single virion particle, as shown by studies done at varying concentrations of 100 mg/ml by Raman optical activity (16), 50 mg/ml by NMR (17), 20 mg/ml by diffraction from oriented gels (10), and 0.5 mg/ml and lower by calorimetry and CD (18). However, the transition is apparently blocked at the very high concentrations of 400 mg/ml or more in fibers; dry fibers made at the high temperature exhibit the same diffraction pattern when the temperature is lowered to 4°C and vice versa (9,19). The electronic and vibrational spectroscopies revealed little if any change in protein conformation, although changes involving DNA have been observed by CD and Raman optical activity (16,20).…”

Conformational transitions in Pf3 and their implications for the structure and assembly of filamentous bacterial viruses.

“…The 'low-temperature Pf1 transition is a shift in the arrangement of protein subunits, causing marked changes in x-ray diffraction patterns but no major change in a-helicity of the subunits (24). (Unpublished laser Raman and CD spectra obtained by us confirm that there is only a marginal change in protein a-helicity associated with this transition.)…”

“…A quite different type of thermally induced transition occurs in Pfl virus structure at about 80C (24). The 'low-temperature Pf1 transition is a shift in the arrangement of protein subunits, causing marked changes in x-ray diffraction patterns but no major change in a-helicity of the subunits (24).…”