Regulation of BORG2 by Cdc42 through disruption of internal structure

The Binder of Rho GTPases (BORG)/CDC42 effector proteins (CDC42EP) family has been associated with multiple cellular processes and diseases. The family comprises five Rho GTPase binding proteins, but we focus here on BORG2. Despite BORG2's emerging role in actin dynamics and various cancers, it is poorly understood at the structure-function level. BORG2 contains two well-known domains separated by what is expected to be a disordered region. The CRIB (CDC42/Rac Interactive Binding) domain binds to CDC42 (Cell Division Control protein 42 homolog), while the BORG homology 3 (BH3) domain binds to the septin family of GTPases. Though these domains can bind their partners independently, our biochemical work found that binding is negatively cooperative: that the binding of a ligand to one site decreases the affinity for subsequent ligand binding at other sites. We focus on finding the structural differences and proof of internal interactions within BORG2 that indicate broader functional and regulatory mechanisms for the family. Multiple fluorescent spectroscopic probes were used to show a novel N- and C-terminal interaction disrupted upon CDC42-GTP (active CDC42) binding.