Pseudokinase domain of STE20-related kinase adapter protein alpha
The pseudokinase domain shows similarity to protein kinases but lacks crucial residues for catalytic activity. The structure of STRAD-alpha is available and shows that this protein binds ATP, has an ordered activation loop, and adopts a closed conformation typical of fully active protein kinases. It does not possess activity due to nonconservative substitutions of essential catalytic residues. ATP binding enhances the affinity of STRAD for MO25. The conformation of STRAD-alpha, stabilized through ATP and MO25, may be needed to activate LKB1. A mutation which results in a truncation of a C-terminal part of the human STRAD-alpha pseudokinase domain and disrupts its association with LKB1, leads to PMSE (polyhydramnios, megalencephaly, symptomatic epilepsy) syndrome. Several splice variants of STRAD-alpha exist which exhibit different effects on the localization and activation of LKB1. STRAD forms a complex with the scaffolding protein MO25, and the serine/threonine kinase (STK), LKB1, resulting in the activation of the kinase. In the complex, LKB1 phosphorylates and activates adenosine monophosphate-activated protein kinases (AMPKs), which regulate cell energy metabolism and cell polarity. The STRAD alpha subfamily is part of a larger superfamily that includes the catalytic domains of STKs, protein tyrosine kinases, RIO kinases, aminoglycoside phosphotransferase, choline kinase, and phosphoinositide 3-kinase.