Clonal variation of adenylyl cyclase activity in a rat tumor cell line caused by change in G protein‐catalytic unit interaction

Abstract

Two subclones of the rat XC cell line characterized by different morphology exhibited quite different adenylyl cyclase responses upon various stimulations. Upon treatment with cholera toxin, clone RK1 accumulated a high level of intracellular cAMP thereby changing its polygonal morphology to an elongated morphology, while the other clone, LK1, with a fibroblastic morphology, failed to increase the intracellular cAMP and remained morphologically unchanged. When membrane fractions derived from these two clones were stimulated with 10 microM forskolin, 10 microM GTP gamma S, or 10 mM NaF, five- to 20-fold more cAMP was accumulated in RK1-derived membranes than in LK1-derived membranes. With the same membrane fractions, upon treatment with Mn(+)+, which directly stimulates the catalytic unit, a high level of cAMP was accumulated both in RK1 and LK1, indicating that the catalytic function inducible by Mn(+)+ was similar in both clones. There was no significant difference in the level of expression of G protein alpha 2, alpha i (at least alpha i1 and alpha i2), and beta subunits between LK1 and RK1. Cholate extracts of the membrane proteins of LK1 and RK1 reconstituted the adenylyl cyclase activity of the cyc- variant of S49 lymphoma cells to the same level. Therefore, it is inferred that the defect in LK1 resides in the interaction of stimulatory G proteins and the actual catalyst.