Electron microscopic and crystallographic studies of bacteriophage Sf6 procapsid-like particles assembled from heterologously expressed capsid protein gp5

Abstract

AbstractMany double-stranded DNA (dsDNA) viruses undergo a capsid maturation process during assembly of infectious virus particles, which involves transformation of a metastable capsid precursor called procapsid into a stable, DNA-filled capsid usually with a larger size and a more angular shape. Sf6 is a tailed dsDNA bacteriophage that infectsShigella flexneri. The phage Sf6 capsid protein gp5 was heterologously expressed and purified. Electron microscopy showed that the gp5 spontaneously assembled into spherical, procapsid-like particles. We also observed tube-like and cone-shaped particles reminiscent of human immunodeficiency virus. The gp5 procapsid-like particles were crystallized and crystals diffracted beyond 4.3 Å resolution. X-ray data at 5.9 Å resolution were collected with a completeness of 31.1% and an overallRmerge of 15.0%. The crystals belong to the space groupC2 with unit cell dimensions of a=973.326 Å, b=568.234 Å, c=565.567 Å, and β=120.540°. Self-rotation function showed the 532 symmetry, confirming formation of icosahedral particles. The particle was situated at the origin of the crystal unit cell with the icosahedral 2-fold axis coinciding with the crystallographicbaxis, and there is a half of the icosahedral particle in the crystallographic asymmetric unit.