A new type of retrovirus (Sm-MTV) released by cultured cells of a spontaneous mammary tumor from a house musk shrew, Suncus murinus, is described. The Sm-MTV is distinct morphologically from type C particles. In spite of certain morphological similarities to type B and type D retroviruses, the Sm-MTV is readily distinguishable. The extracellular virions had a spikeless envelope containing a centrally located nucleoid with a small electron-dense core surrounded by an inner membrane. The budding particles contained a doughnut-shaped nucleoid. Intracytoplasmic type A particles similar in profile to those associated with mouse mammary tumor cells were also found, and tended to form a small cluster of several particles in the cytoplasm. The virus banded at 1.169 g/cm3 in isopycnic centrifugation and possessed constitutive Mg2+-dependent reverse transcriptase. The viral RNA had a molecular size ranging from 50 to 70 S in its native form and 30 to 40 S in its denatured form by a glycerol gradient ultracentrifugation. Major viral polypeptides were 72K, 69K, 47K, 44K/43K, 27K, 20.5K, and 15K.
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