High-resolution structure of the Tiam1 PH_n-CC-Ex domain

The T-lymphoma and metastasis gene 1 (TIAM1) encodes a guanine nucleotide-exchange factor protein (Tiam1) that is specific for the Rho-family GTPase Rac1 and is important for cell polarity, migration and adhesion. Tiam1 is a large multi-domain protein that contains several protein–protein binding domains that are important for regulating cellular function. The PH_n-CC-Ex domain is critical for plasma-membrane association and interactions with protein-scaffold proteins (e.g. Par3b, spinophilin, IRSp53 and JIP2) that direct Tiam1–Rac1 signaling specificity. It was determined that the coiled-coil domain of Par3b binds the PH_n-CC-Ex domain with a dissociation constant of ∼30 µM. Moreover, the structures of two variants of the Tiam1 PH_n-CC-Ex domain were solved at resolutions of 1.98 and 2.15 Å, respectively. The structures indicate that the PH_n, CC and Ex regions form independent subdomains that together provide an integrated platform for binding partner proteins. Small-angle X-ray scattering (SAXS) data indicate that the Tiam1 PH_n-CC-Ex domain is monomeric in solution and that the solution and crystal structures are very similar. Together, these data provide the foundation necessary to elucidate the structural mechanism of the PH_n-CC-Ex/scaffold interactions that are critical for Tiam1–Rac1 signaling specificity.