TI - Identification and characterization of a mitogen -activated S6 kinase . AB - Treatment of Swiss mouse 3T3 cells with epidermal growth factor , orthovanadate , or serum results in the activation of a kinase that phosphorylates protein S6 of the 40S ribosomal subunit in vitro . This kinase is eluted as a single peak of activity from either a Mono Q anion-exchange column at 0.34 M NaCl or a Mono S cation-exchange column at 0.20 M NaCl . Treatment of the peak fraction from the Mono S column with phosphatase 2A completely abolishes the activity of the enzyme . The kinase appears to be distinct from protein kinase C , cAMP -dependent protein kinase , and two protease -activated kinases , PAK II and H4P . The kinase has been purified to apparent homogeneity and migrates as a single band at Mr 70,000 in NaDodSO4/polyacrylamide gels . The kinase exhibits the ability to autophosphorylate , and this activity directly parallels S6 phosphorylation activity on the final step of purification . In vitro , the kinase incorporates up to 5 mol of phosphate into S6 , and the tryptic phosphopeptide maps obtained are equivalent to those from S6 phosphorylated in vivo . Most important , treatment of the purified kinase with phosphatase 2A results in complete inactivation of the enzyme , arguing that the activity of the kinase is directly controlled by phosphorylation .