Envelope of mouse mammary tumor virus studied by freeze-etching and freeze-fracture techniques

As part of a study of the cell surface changes associated with the production of murine mammary tumor virus, the structure of the envelope of this virus has been examined by using freeze-fracture techniques. Both fracture and deep-etch surfaces were examined. The fracture faces contain 10-nm spheres comparable to those observed on fractured plasma membranes, although fewer in number. Surfaces exposed by etching possess a highly regular hexagonal array of pits 25 nm apart. By examining freeze-fracture and freeze-etch preparations of virus with ferritin covalently bound to its surface, it has been determined that the surface exposed by etching is the outer surface of the virus. The pitted exterior surface of the mammary tumor virus appears to be a unique surface structure.