In the inactive state, the juxtamembrane region of EphB2 is ordered, and folds against the kinase domain, primarily making contact with the small lobe. Two juxtamembrane tyrosine residues which are subject to autophosphorylation upon EphB2 activation are inserted into hydrophobic pockets, formed by residues from the juxtamembrane region itself as well as the catalytic domain. In the autoinhibited state, the juxtamembrane region appears to repress kinase activity by creating a kink in the aC-helix of the small lobe of the kinase domain; this movement is transmitted to the ATP-binding site, so that although ATP can bind, the phosphates are not productively positioned for phosphotransfer. In addition, the juxtamembrane region may inhibit the movement of the activation segment of the kinase domain into an active conformation. EphB2 activation leads to autophosphorylation of the juxtamembrane tyrosines, which we predict are therefore expelled from their pockets as a result of electrostatic repulsion and steric clash. We speculate that this leads to the disordering of the juxtamembrane region, freeing the kinase domain to snap into an active state; coincidentally the juxtamembrane phosphotyrosines are exposed for potential interactions with SH2 domain proteins.

This model is remarkably similar to that proposed for the activation of the type I TGF b-receptor, and the subsequent interaction of its phosphorylated juxtamembrane region with R-Smad targets (Huse M., Muir T.W., Xu L., Chen Y.G., Kuriyan J., and Massague J. (2001) Mol Cell 8, 671-682). The principal difference involves the observation that the autoinhibted state of the TGF b-receptor involves an accessory protein, FKBP12, which is required for the folding of the juxtamembrane region and occlusion of the ATP-binding site.