Ultrastructural characterization of the retrovirus particles (Sm-MTV) liberated from the mammary tumor cell line (Sm-MT) of a house musk shrew, Suncus murinus (Insectivora)

Detailed ultrastructure of a new type of retrovirus (Sm-MTV) released by cultured cells (Sm-MT) of a spontaneous mammary tumor from a house musk shrew Suncus murinus, Insectivora, is described. The virus particles were revealed as three forms: intracellular; budding; and extracellular. The intracellular type A particles were similar in profile to those associated with mouse mammary tumor cells and tended to form a small cluster of several particles in the cytoplasm. In addition, horseshoe-shaped particles as well as smaller particles in clusters, with doughnut-shaped morphology similar in structure to type A particles, were identified near the clusters of type A particles, although in smaller numbers. The budding particles contained a doughnut-shaped nucleoid, although the nucleoids decreased in size as compared with intracytoplasmic type A particles. The extracellular virions consisted of an envelope and a centrally located nucleoid. In routinely fixed specimens, the former was covered with irregularly distributed fuzzy materials in its surface, and the latter was further composed of a small electron dense core surrounded by an intermediate layer. Tannic acid treatment of cells resulted in the visualization of surface projections on the envelope of virions. Similar projections were also detected exclusively on the plasma membrane where virus budding took place, and not on the normal plasma membrane. The presence of surface projections on the viral envelope was further confirmed by the whole-cell-mounting technique. Together with our previous results of biochemical and immunological investigations, we concluded that Sm-MTV seemed to have closer phylogenetic relatedness with type D viruses of primates than with murine mammary tumor virus.