Open AccessS6 kinase in quiescent Swiss mouse 3T3 cells is activated by phosphorylation in response to serum treatment.
Author(s) -
Lisa M. Ballou,
Michel Siegmann,
George Thomas
Publication year - 1988
Publication title -
proceedings of the national academy of sciences of the united states of america
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 5.011
H-Index - 771
eISSN - 1091-6490
pISSN - 0027-8424
DOI - 10.1073/pnas.85.19.7154
Subject(s) - phosphoserine , phosphorylation , kinase , phosphatase , protein kinase a , microbiology and biotechnology , biochemistry , biology , serine , map2k7 , cyclin dependent kinase 2 , mitogen activated protein kinase kinase , c raf , threonine , in vivo , map kinase kinase kinase , chemistry
To investigate the role of phosphorylation in the activation of S6 kinase, the enzyme was isolated from 32P-labeled Swiss mouse 3T3 cells before and after stimulation with serum. The kinase activity was followed through several purification steps, and a radioactive protein of Mr 70,000 was obtained from the stimulated cells. This band was not detected in resting cells. The Mr 70,000 protein exhibited the same size upon NaDodSO4/PAGE as the homogeneous kinase, and it comigrated with the in vitro autophosphorylated form of the enzyme. Treatment of the in vivo-labeled material with phosphatase 2A led to a loss of kinase activity concomitant with a release of 32Pi from the Mr 70,000 protein. The partially dephosphorylated protein migrated faster during PAGE, displaying distinct species of Mr 69,000 and 68,000. Most importantly, phospho amino acid analysis of the labeled S6 kinase showed only phosphoserine and phosphothreonine. These results argue that the S6 kinase is phosphorylated at multiple sites in vivo and that it is activated by serine/threonine phosphorylation.