The overall structure adopts a typical bi-lobal kinase architecture. Both the N-lobe and the C-lobe have the a/b fold. Each lobe consists of a central b-sheet flanked by a-helices on both sides of the sheet. The last helix C-terminal to the C-lobe is part of the Nlobe and perpendicular to the interfacial helix of the C-lobe. Residues 475-498 of the N-lobe are invisible in the electron density map (Figure 2 ).