Determination of the Catalytic Activities of mTOR and Other Members of the Phosphoinositide-3-Kinase-Related Kinase Family

Members of the phosphoinositide-3-kinase-related kinase (PIKK) family, which includes mTOR, ATM, ATR, and hSMG-1, play important roles in regulating the cellular response to environmental stimuli. Despite the similarity of their catalytic domain to that of phosphoinositide-3-kinase, these extremely large (>250 kDa) polypeptides function as serine/threonine protein kinases. The catalytic activities of these PIKK family members can now be measured in immune-complex kinase assays. This assay involves isolation of the kinase by immunoprecipitation and the in vitro phosphorylation of a specific substrate in the presence of radio-labeled ATP. Here we describe, in detail, the determination of PIKK catalytic activity with a standardized immune-complex kinase assay protocol.