The Tiam1 Guanine Nucleotide Exchange Factor is Auto-inhibited by its Pleckstrin Homology Coiled-Coil Extension Domain [Protein Structure and Folding]

T-cell lymphoma invasion and metastasis 1 (Tiam1) is a Dbl-family guanine nucleotide exchange factor (GEF) that specifically activates the Rho-family GTPase Rac1 in response to upstream signals, thereby regulating cellular processes including cell adhesion and migration. Tiam1 contains multiple domains, including an N-terminal Pleckstrin homology coiled-coiled extension (PHn-CC-Ex) and catalytic Dbl-homology and C-terminal Pleckstrin homology (DH-PHc) domain. Previous studies indicate that larger fragments of Tiam1, such the region encompassing the N-terminal to C-terminal Pleckstrin homology domains (PHn-PHc) are auto-inhibited. However, the domains in this region responsible for inhibition remain unknown. Here, we show that the PHn-CC-Ex domain inhibits Tiam1 GEF activity by directly interacting with the catalytic DH-PHc domain, preventing Rac1 binding and activation. Enzyme kinetics experiments suggested that Tiam1 is auto-inhibited through occlusion of the catalytic site, rather than by allostery. Small angle X-ray scattering and ensemble modeling yielded models of the PHn-PHc fragment that indicate it is in equilibrium between open and closed conformational states. Finally, single-molecule experiments support a model in which conformational sampling between the open and closed states of Tiam1 contributes to Rac1 dissociation. Our results highlight the role of the PHn-CC-Ex domain in Tiam1 GEF regulation, and suggest a combinatorial model for GEF inhibition and activation of the Rac1 signaling pathway.

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