Characterization of two mutants of the LLC-PK1 porcine kidney cell line affected in the catalytic subunit of the cAMP-dependent protein kinase

Abstract

The catalytic (C) subunit activity of the cAMP-dependent protein kinase (cAMP-PK) from the mutant cell lines, FIB4 and FIB6, is only 10% compared with the parent cell line, LLC-PK1 [Jans and Hemmings (1986) FEBS Lett. 205, 127-131]. In order to understand the nature of the mutant phenotypes the cAMP-PK from parent and mutant cell lines was studied in more detail. Analysis of mutant cAMP-PK activity by ion-exchange chromatography revealed that kinase activity associated with type I holoenzyme of both FIB4 and FIB6 was only 5% parental, and the activity of the type II holoenzyme was about 20% parental. The type I regulatory (RI) subunits associated with the type I were also found to be reduced by 70-80% in both mutants, whereas the type II R subunit levels were similar to that of the parent. The residual kinase activity associated with the type I holoenzyme from FIB4 and FIB6 could not be activated by cAMP whereas the type II holoenzyme was activated by cAMP (Ka of 5.5 .times. 10-8M), and showed normal affinities for Kemptamide and ATP. A polyclonal antibody to the catalytic subunit was used to quantify the level of this protein in wild-type and mutant cells. This analysis showed that FIB4 and FIB6 had nearly normal levels of C subunit, suggesting that the C subunit synthesized by the mutants was mostly inactive. As both type I and type II cAMP-PK holoenzymes were abnormal, the most likely explanation of the mutant phenotype is a defect either in the structural gene for the C subunit or in an enzyme involved in its post-translational processing. However, a second lesion affecting the RI subunit cannot be ruled out this moment.