The Structural Basis for Cdc42-Induced Dimerization of IQGAPs

Area of Science:

• Molecular Biology
• Cell Signaling
• Structural Biology

Background:

• Rho-family GTPases are key regulators of cell signaling.
• GTPase-activating protein (GAP)-related domains (GRDs) interact with GTPases to modulate their activity.
• IQGAP proteins are scaffold proteins involved in various cellular processes.

Purpose of the Study:

• To determine the structural basis of Cdc42·GTP interaction with the IQGAP2 GRD.
• To elucidate the role of Cdc42 in IQGAP protein complex formation and dimerization.

Main Methods:

• X-ray crystallography to determine the structure of Cdc42·GTP bound to IQGAP2 GRD.
• Isothermal titration calorimetry to quantify binding affinities.
• Site-directed mutagenesis to investigate the role of specific residues.

Main Results:

• The crystal structure revealed four Cdc42 molecules bound to two IQGAP2 GRD molecules, forming a parallel dimer.
• Cdc42 binding to IQGAP GRDs involves both Ras/RasGAP interaction sites and "extra domain" sequences, facilitating GRD dimerization.
• Calorimetry confirmed two-site binding for both IQGAP1 and IQGAP2.
• Rac1·GTP showed only single-site binding, indicating Cdc42 specifically promotes IQGAP dimerization.

Conclusions:

• Cdc42 binding induces IQGAP dimerization through a novel mechanism involving extra domains.
• This study reveals an unexpected role for Cdc42 in mediating protein-protein interactions and expanding the functional repertoire of Ras family proteins.