| Proline-rich tyrosine kinase-2 (Pyk2), along with focal adhesion kinase (FAK), comprise an important subfamily of protein tyrosine kinases (PTKs) which are key mediators of cellular signaling in response to a variety of stimuli. The fact that Pyk2 and FAK are so similar in sequence, yet behave so differently highlights the importance of determining what factors are involved in distinguishing the biological roles of the two molecules. Structures of the FERM, kinase, and FAT domain of FAK have all been solved by X-ray crystallography and have aided in elucidating some of the mechanisms by which FAK activity is regulated [1-5]. Thus far, very little structural data exists for Pyk2 and such information would serve as a starting point in dissecting the intra- and intermolecular interactions which influence the behavior and activity of Pyk2. Comparison of the structures of the individual domains of Pyk2 and FAK, either alone or in complex with interacting molecules, has provided much insight about the factors which help to determine the cellular localization and selectivity of these two closely related protein tyrosine kinases. Here, we describe two crystal structures of the C-terminal FAT domain of Pyk2 in addition to the crystal structure of the Pyk2 FAT domain in complex with two paxillin LD4 motif-derived peptides. We have highlighted the structural features within Pyk2-FAT which are conserved throughout the FAK subfamily of tyrosine kinases, as well as a region which distinguishes Pyk2 from FAK and may account for differences in selectivity between the two molecules. We also show that Tyr-881, the Grb2 SH2 domain binding site within the Pyk2 FAT domain, is not required for the constitutive association between Pyk2 and paxillin. We further show that the Pyk2/paxillin interaction is not dependent upon Pyk2 kinase activity, that paxillin is a substrate of Pyk2, and that phosphorylation of paxillin by Pyk2 requires Pyk2 kinase activity. Finally, in order to shed light on the manner by which the FERM domain of Pyk2 is involved in Pyk2 regulation, we have solved the X-ray crystal structure of the Pyk2 FERM domain in complex with the C-terminus of the Pyk2 N-terminal domain-interacting receptor-2 (Nir2-CT), which binds uniquely to the FERM domain of Pyk2. |
