Cyclic AMP-dependent protein kinases from normal and SV40-transformed 3T3 cells

Abstract

THE apparent molecular mechanism by which cyclic AMP regulates eukaryotic physiology is through the activation of cyclic AMP-dependent protein kinase enzymes¹. Two cytosolic protein kinase isozymes, called types I and II², have been described in mammalian tissues. They differ in their charge², dissociability by salt², capacity to undergo an autophosphorylation reaction³ and in their antigenic properties⁴. Evidence suggests that cyclic AMP regulates cell division. Intracellular levels of cyclic AMP are higher in normal than in transformed cells^5,6, and exogenous addition of cyclic AMP analogues to the culture medium inhibits the growth of transformed cells⁷. Cyclic AMP levels⁸, protein kinase specific activity⁹, and the relative amounts of types I and II isozymes¹⁰ vary during the cell cycle. A genetic lesion in lymphoma cells making them insensitive to cyclic AMP may be due to a defective protein kinase enzyme¹¹. We report here that cytosol from SV40-transformed 3T3 cells contains a type I protein kinase not found in BALB 3T3 cytosol. Chromatographic separation of these enzymes indicates that this isozymic difference is the result of an additional protein kinase regulatory subunit in SV3T3 cytosol.