Structural Insight into the 14-3-3 Protein-dependent Inhibition of Protein Kinase ASK1 (Apoptosis Signal-regulating kinase 1).

Apoptosis signal-regulating kinase 1 (ASK1, also known as MAP3K5), a member of the mitogen-activated protein kinase kinase kinase (MAP3K) family, regulates diverse physiological processes. The activity of ASK1 is triggered by various stress stimuli and is involved in the pathogenesis of cancer, neurodegeneration, inflammation, and diabetes. ASK1 forms a high molecular mass complex whose activity is, under non-stress conditions, suppressed through interaction with thioredoxin and the scaffolding protein 14-3-3. The 14-3-3 protein binds to the phosphorylated Ser-966 motif downstream of the ASK1 kinase domain. The role of 14-3-3 in the inhibition of ASK1 has yet to be elucidated. In this study we performed structural analysis of the complex between the ASK1 kinase domain phosphorylated at Ser-966 (pASK1-CD) and the 14-3-3ζ protein. Small angle x-ray scattering (SAXS) measurements and chemical cross-linking revealed that the pASK1-CD·14-3-3ζ complex is dynamic and conformationally heterogeneous. In addition, structural analysis coupled with the results of phosphorus NMR and time-resolved tryptophan fluorescence measurements suggest that 14-3-3ζ interacts with the kinase domain of ASK1 in close proximity to its active site, thus indicating this interaction might block its accessibility and/or affect its conformation.