Cryo-Electron Microscopy and Image Reconstruction of PBCV-1, an ALGAL Virus with T=169, Icosahedral Lattice Symmetry

Abstract

Paramecium bursaria Chlorella virus l(PBCV-l), the type species of the Pycodnaviridae family, is a large, plaque-forming virus that replicates in certain unicellular, exsymbiotic, Chlorella-like green algae. The virion (∼lxl09 daltons) has a 330,740 base pair dsDNA genome encapsulated within an icosahedral shell of ∼190nm diameter. Purified virions contain more than 50 different proteins, which account for 65% of the total virion mass and range in size from 10 to more than 200kDa. Vp54, the major capsid protein (40% of total virion protein) is a myristilated glycoprotein but it is not phosphorylated. Three other proteins are located on the virus surface. Virions contain 5-10% lipid, located inside the glycoprotein shell, which is required for virus infectivity. Little is known about the detailed ultrastructure of PBCV-1 virions, though some information has been obtained with sectioned, negatively-stained, and metal-shadowed specimens. We have used cryo-electron microscopy and three-dimensional (3D) image reconstruction methods to examine the native morphology of PBCV-1.