Atomic force microscopy investigation of a chlorella virus, PBCV-1

A virus PBCV-1, which infects certain fresh water algae and has been shown by transmission and cryo-electron microscopy to exist as a triskaidecahedron, was imaged using atomic force microscopy (AFM). From AFM the particles have diameters of about 190 nm and the overall structure is in all important respects consistent with existing models. The surface lattice of the virion is composed of trimeric capsid proteins distributed according to p3 symmetry to create a honeycomb arrangement of raised edges forming quasi-hexagonal cells. At the pentagonal vertices are five copies of a different protein forming an exact pentagon, and this has yet another unique protein in its center. The apical protein exhibits some unusual mechanical properties in that it can be made to retract into the virion interior when subjected to AFM tip pressure. When PBCV-1 virions degrade, they give rise to small, uniform, spherical, and virus like particles (VLP) consistent with T = 1 or 3 icosahedral products. Also observed upon disintegration are strands of linear dsDNA. Fibers of unknown function are also occasionally seen associated with some virions.