Membrane-bound and free polysomes in transformed and untransformed fibroblast cells

Abstract

A rapid and quantitative fractionation procedure has been used to measure the amounts of free and membrane-bound polysomes in growing and stationary Py3T3 and 3T3 (mouse) cells. A comparison of growing 3T3 and Py3T3 cells does not reveal any significant differences with regard to the ratio of the two polysome fractions. The amount of free and membrane-bound polysomes decreases in both 3T3 and Py3T3 cells as they approach the stationary state, an effect which is much more pronounced for free polysomes. At greatly reduced growth rates or in stationary cells, however, the amount of membrane-bound polysomes doubles in Py3T3 cells while it decreases even further in 3T3 cells. By contrast, the amount of free polysomes remains at a reduced level in both 3T3 and Py3T3 cells when cell multiplication is inhibited. Based on the hypothesis that membrane proteins are selectively synthesized on membrane-bound polysomes, an attempt is made to relate the results to accumulated data in the literature and discuss its possible significance with respect to the loss of growth control in Py3T3 cells.