AtT20 pituitary tumour cells contain mouse mammary tumour virus and intracisternal A-type particles in addition to murine leukemia virus.

By electron microscopy and immunocytochemistry we have examined the retroviruses endogenous to AtT20 D16V cells, a cloned line of murine pituitary tumour cells. In addition to the C-type retrovirus particles related to Rauscher murine leukemia virus (MuLV) previously reported to bud from these cells we observed cytoplasmic A-type particles and intracisternal A-type particles (IAP). In the cytoplasm the A-type particles occur in large clusters often associated with sheets of material with a fine structure resembling the shells of the particles. At the plasma membrane individual A-type particles bud to give rise to extracellular virions. The IAP are restricted to the rough endoplasmic reticulum (RER) into which they bud: they are not transported out of the RER to the Golgi apparatus and beyond. We describe a new monoclonal antibody (designated 83E7) which is specific for an epitope of the major core protein (MTVp27) of mouse mammary tumour virus (MMTV). Using immunogold labelling procedures we have specifically labelled both the A-type particles and the associated sheets of material with this antibody. We conclude that the A-type particles and the virions they give rise to are MMTV. The sheets of material must also at least in part be made up of the major core protein of MMTV or its precursor polypeptide. AtT20 cells, therefore, contain endogenous MuLV and MMTV as well as IAP.