Mutational activation of ErbB2 reveals a new protein kinase autoinhibition mechanism

Abstract

Autoinhibition plays a key role in the control of protein kinase activity. ErbB2 is a unique receptor-tyrosine kinase that does not bind ligand but possesses an extracellular domain poised to engage other ErbBs. Little is known about the molecular mechanism for ErbB2 catalytic regulation. Here we show that ErbB2 kinase is strongly autoinhibited, and a loop connecting the αC helix and β4 sheet within the kinase domain plays a major role in the control of kinase activity. Mutations of two Gly residues at positions 776 and 778 in this loop dramatically increase ErbB2 catalytic activity. Kinetic analysis demonstrates that mutational activation is due to ∼10- and ∼7-fold increases in ATP binding affinity and turnover number, respectively. Expression of the activated ErbB2 mutants in cells resulted in elevated ligand-independent ErbB2 autophosphorylation, ErbB3 phosphorylation, and stimulation of mitogen-activated protein kinase. Molecular modeling suggests that the ErbB2 kinase domain is stabilized in an inactive state via a hydrophobic interaction between the αC-β4 and activation loops. Importantly, many ErbB2 human cancer mutations have been identified in the αC-β4 loop, including the activating G776S mutation studied here. Our findings reveal a new kinase regulatory mechanism in which the αC-β4 loop functions as an intramolecular switch that controls ErbB2 activity and suggests that loss of αC-β4 loop-mediated autoinhibition is involved in oncogenic activation of ErbB2.