The active conformation of the PAK1 kinase domain

Area of Science:

• Molecular Biology
• Cell Signaling
• Structural Biology

Background:

• p21-activated kinases (PAKs) are serine/threonine kinases involved in cytoskeletal regulation.
• PAKs function downstream of Rho-family GTPases, playing roles in cell structure and motility.
• The autoinhibited state of PAK1 involves its N-terminal regulatory region inactivating the kinase domain and promoting dimerization.

Purpose of the Study:

• To elucidate the structural basis of PAK1 activation.
• To investigate the conformation of the PAK1 kinase domain in the absence of activation loop phosphorylation.
• To understand the transition of PAK1 to an active state upon Cdc42 binding.

Main Methods:

• X-ray crystallography was used to determine the structures of the PAK1 kinase domain.
• Mutagenesis was employed to create an inactive mutant and a phosphomimetic mutant in the activation loop.
• Structural analysis focused on the kinase domain conformation and dimerization interfaces.

Main Results:

• Structures of the free PAK1 kinase domain revealed an essentially active conformation, even without Thr423 phosphorylation.
• A phosphomimetic mutation in the activation loop did not significantly alter the kinase domain structure.
• The results suggest an intermediate-active state for PAK1 upon Cdc42-induced dimer dissociation, preceding activation loop modification.

Conclusions:

• The PAK1 kinase domain possesses an active conformation independent of activation loop phosphorylation.
• Cdc42 binding and dimer dissociation are key events initiating PAK1 activation.
• PAK1 can engage in autophosphorylation or substrate phosphorylation in an intermediate-active state before full activation loop modification.