How Vav proteins discriminate the GTPases Rac1 and RhoA from Cdc42

Area of Science:

• Molecular Biology
• Cell Signaling
• Protein Structure and Function

Background:

• Vav proteins function as guanine nucleotide exchange factors (GEFs) for Rho/Rac GTPases.
• Activation of Vav proteins occurs via tyrosine phosphorylation.
• Vav proteins specifically activate Rac1, RhoG, and RhoA, but not Cdc42, with the underlying structural basis unknown.

Purpose of the Study:

• To elucidate the structural mechanisms underlying Vav protein substrate selectivity.
• To identify specific amino acid residues in Rho/Rac GTPases critical for Vav binding and activation.
• To understand the sequential events governing Vav-mediated GDP/GTP exchange.

Main Methods:

• Structural analysis of Vav protein interactions with Rho/Rac GTPases.
• Site-directed mutagenesis to probe the role of specific amino acid residues.
• Biochemical assays to measure GDP/GTP exchange activity.

Main Results:

• Vav proteins require specific residues (D49 and E54) in the beta2/beta3 region of Rho/Rac GTPases for substrate binding.
• A second signal, residue K118 in the beta5 region, is necessary for Vav-mediated GDP/GTP exchange.
• These findings explain the discrimination between Cdc42 and other Rho/Rac substrates by Vav proteins.

Conclusions:

• Substrate selectivity of Vav proteins is determined by distinct structural features in Rho/Rac GTPases.
• Vav-mediated activation involves a two-step process: specific binding followed by GDP/GTP exchange.
• The identified residues and mechanism provide a structural explanation for Vav substrate specificity.