The general interest of our lab is to understand mechanisms for regulation of signaling proteins through intra- and inter-molecular interactions. A major focus is on receptor-mediated signaling pathways involved in neuron development and axonal guidance, and more recently those involved in the innate immunity. We use biochemistry, X-ray crystallography, cryo-EM and cell biological approaches to seek understanding of the regulatory mechanisms at the atomic level. We have shown that the plexin intracellular region has GTPase activating protein (GAP) activity specific to the Ras homolog Rap, and this activity is critical for its signaling. Our crystal structures elucidate how dimerization allosterically drives activation of the RapGAP domain of plexin, which then catalyzes GTP hydrolysis for Rap by using a non-canonical mechanism. Our comprehensive structural analyses revealed how PlexinD1 binds the adaptor protein GIPC1, releasing its autoinhibition and promoting its interaction with the motor protein myosin VI. Surprisingly, GIPC and myosin VI form an alternating oligomeric complex, which may underlie their clustering in cells and the ability to mediate powerful and processive transport in processes such as endocytosis. We are currently working on the mechanisms that couple the extracellular ligand binding domains to the intracellular signaling domains of plexin through the transmembrane region. More recently, in collaboration with the labs of Dr. Xiao-chen Bai and James Chen at UTSW, we have determined the cryo-EM structures of full-length STING and its complex with TBK1. The presence of viral DNA in the cytosol is detected by the innate immunity sensor cGAS, which generates the second messenger cyclic-GMP-AMP (cGAMP). cGAMP binds and activates the innate immunity adaptor STING located on the ER membrane. Activated STING recruits and activates the serine/threonine kinase TBK1, which phosphorylates the transcription factor IRF3. Phosphorylated IRF3 dimerizes and enters the nucleus to promote the expression of type I interferons, leading to antiviral immune responses. Our structures of STING and the STING/TBK1 complex provide detailed molecular views of full-length STING in both the inactive and cGAMP-bound active states, as well as its interaction with TBK1, allowing us to propose a complete model of the activation of STING by cGAMP, and the subsequent recruitment and activation of TBK1.