Potassium inhibition of transforming protein P85 gag‐mos and reversal of the transformed phenotype in 6m2 cells

K + at high concentrations (52–72 mM hypertonic KCI) has been reported to induce reverse transformation in the 6m2 cell, which is a clone of normal rat kidney cells (NRK) infected with a temperature‐sensitive transformation virus. When exposed to high K + , 6m2 cells grown at the permissive temperature (33°C) exhibit normal morphology and reduced soft agar growth, characteristics of cells grown at nonpermissive temperature (39°C). In the current study, flattening of cells and rearrangement of surface microvilli were demonstrated by scanning electron microscopy to occur within 6 hr of exposure to high K + , similar to the effect of temperature shift to 39°C. Exposure to K + resulted in a 90% inhibition of P85 gag‐mos ‐associated serine kinase activity within 5 min, with a subsequent reduction of up to 75% of the synthesis of this protein. These alterations in the putative transforming protein were similar to those induced by temperature shift and were considered to be the basis for retrotransformation. The cell microtubular system and F‐actin cables were affected more slowly by K + than by a temperature shift to 39°C. The former did not achieve the fine reticulum network seen in NRK cells until 72 hr later, but the latter remained aberrant. The effect on the enzyme might be mediated by alteration in phosphorylation, but the mechanism by which kinase inactivation induces retrotransformation is not yet known.