A myoepithelial-like cell line (Rama 401), isolated from rat mammary gland, has been transformed with a temperature-sensitive mutant of Rous sarcoma virus (tsRSV). Rama 401-tsRSV cells adopt a spindle morphology and fail to deposit basement membrane proteins when grown at the permissive temperature (35 degrees C). When switched to the non-permissive temperature (41 degrees C), the cells flatten (with a 5-fold increase in area), and deposit an extracellular matrix containing basement membrane proteins. When the cells are switched from 35 degrees C to 41 degrees C in the presence of monensin (an ionophore that inhibits protein secretion), basement membrane proteins are no longer deposited extracellularly although the cells flatten, their area increasing by ninefold. Cells switched from 35 degrees C to 41 degrees C in the presence of cycloheximide still flatten and deposit basement membrane proteins, whereas the morphological change on switching from 41 degrees C to 35 degrees C is inhibited by cycloheximide. These experiments indicate that the ability of Rama 401-tsRSV cells to spread on a plastic substratum is not dependent on the de novo synthesis and deposition of basement membrane proteins.

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