The mechanism by which the epidermal growth factor receptor (EGFR) is activated upon dimerization has eluded definition. We find that the EGFR kinase domain can be activated by increasing its local concentration or by mutating a leucine (L834R) in the activation loop, the phosphorylation of which is not required for activation. This suggests that the kinase domain is intrinsically autoinhibited, and an intermolecular interaction promotes its activation. Using further mutational analysis and crystallography we demonstrate that the autoinhibited conformation of the EGFR kinase domain resembles that of Src and cyclin-dependent kinases (CDKs). EGFR activation results from the formation of an asymmetric dimer in which the C-terminal lobe of one kinase domain plays a role analogous to that of cyclin in activated CDK/cyclin complexes. The CDK/cyclin-like complex formed by two kinase domains thus explains the activation of EGFR-family receptors by homo- or heterodimerization.

Protein kinases operate in a large number of distinct signaling pathways, where the tight regulation of their catalytic activity is crucial to the development and maintenance of eukaryotic organisms. The catalytic domains of different kinases adopt strikingly similar structures when they are active. By contrast, crystal structures of inactive kinases have revealed a remarkable plasticity in the kinase domain that allows the adoption of distinct conformations in response to interactions with specific regulatory domains or proteins.

The inadvertent activation of the Abelson tyrosine kinase (Abl) causes chronic myelogenous leukemia (CML). A small-molecule inhibitor of Abl (STI-571) is effective in the treatment of CML. We report the crystal structure of the catalytic domain of Abl, complexed to a variant of STI-571. Critical to the binding of STI-571 is the adoption by the kinase of an inactive conformation, in which a centrally located "activation loop" is not phosphorylated. The conformation of this loop is distinct from that in active protein kinases, as well as in the inactive form of the closely related Src kinases. These results suggest that compounds that exploit the distinctive inactivation mechanisms of individual protein kinases can achieve both high affinity and high specificity.

Molecular dynamics was used to refine macromolecular structures by incorporating the difference between the observed crystallographic structure factor amplitude and that calculated from an assumed atomic model into the total energy of the system. The method has a radius of convergence that is larger than that of conventional restrained least-squares refinement. Test cases showed that the need for manual corrections during refinement of macromolecular crystal structures is reduced. In crambin, the dynamics calculation moved residues that were misplaced by more than 3 angstroms into the correct positions without human intervention.

Through sequencing analysis of blood or bone marrow samples from patients with chronic myeloid leukemia, we identified BCR-ABL kinase domain mutations in 29 of 32 patients whose disease relapsed after an initial response to the tyrosine kinase inhibitor imatinib. Fifteen different amino acid substitutions affecting 13 residues in the kinase domain were found. Mutations fell into two groups-those that alter amino acids that directly contact imatinib and those postulated to prevent BCR-ABL from achieving the inactive conformational state required for imatinib binding. Distinct mutations conferred varying degrees of imatinib resistance. Mutations detected in a subset of patients with stable chronic phase disease correlated with subsequent disease progression. Multiple independent mutant clones were detected in a subset of relapsed cases. Our data support a clonal selection model of preexisting BCR-ABL mutations that confer imatinib resistance.

The crystal structure of the haematopoietic cell kinase Hck has been determined at 2.6/2.9 A resolution. Inhibition of enzymatic activity is a consequence of intramolecular interactions of the enzyme's Src-homology domains SH2 and SH3, with concomitant displacement of elements of the catalytic domain. The conformation of the active site has similarities with that of inactive cyclin-dependent protein kinases.

c-Abl is normally regulated by an autoinhibitory mechanism, the disruption of which leads to chronic myelogenous leukemia. The details of this mechanism have been elusive because c-Abl lacks a phosphotyrosine residue that triggers the assembly of the autoinhibited form of the closely related Src kinases by internally engaging the SH2 domain. Crystal structures of c-Abl show that the N-terminal myristoyl modification of c-Abl 1b binds to the kinase domain and induces conformational changes that allow the SH2 and SH3 domains to dock onto it. Autoinhibited c-Abl forms an assembly that is strikingly similar to that of inactive Src kinases but with specific differences that explain the differential ability of the drug STI-571/Gleevec/imatinib (STI-571) to inhibit the catalytic activity of Abl, but not that of c-Src.

The crystal structure of the human DNA polymerase delta processivity factor PCNA (proliferating cell nuclear antigen) complexed with a 22 residue peptide derived from the C-terminus of the cell-cycle checkpoint protein p21(WAF1/CIP1) has been determined at 2.6 angstrom resolution. p21 binds to PCNA in a 1:1 stoichiometry with an extensive array of interactions that include the formation of a beta sheet with the interdomain connector loop of PCNA. An intact trimeric ring is maintained in the structure of the p21-PCNA complex, with a central hole available for DNA interaction. The ability of p21 to inhibit the action of PCNA is therefore likely to be due to its masking of elements on PCNA that are required for the binding of other components of the polymerase assembly.