Notch receptors bind their ligands with nearly undetectable monovalent affinities, and therefore rely upon clustering and avidity effects to form productive interactions in vivo. To overcome this obstacle and co-crystallize a Notch-ligand complex, we used directed evolution to affinity-mature interactions between Notch1 and the ligand Delta-like 4 (DLL4). The 2.3Å resolution structure of the interacting domains of Notch1 (magenta) bound to DLL4 (cyan) revealed a narrow, elongated interface that spans multiple domains of each protein. A defining feature of the Notch1-DLL4 interface is the direct contribution of Notch1 O-linked glycans (orange) to the binding interface, as very few proteins are known to utilize a composite protein/glycan recognition surface for ligand engagement.
Notch1 receptor bound to Delta-like 4 (DLL4) ligand
Japanese Encephalitis Virus Envelope (E) protein - prefusion dimer
Japanese Encephalitis Virus (JEV) is the leading global cause of viral encephalitis and a close relative to Zika Virus. The 2.1Å resolution structure of the JEV envelope protein (E) in the prefusion conformation revealed the 3-domain architecture characteristic of flavivirus E proteins (Domain I is red, Domain II is yellow, Domain III is blue). In this conformation, the membrane-penatrating fusion peptides (green) are "disarmed" because they are buried against DIII of an adjacent E subunit. The structure of JEV E exposed a remarkably small dimer interface relative to other flavivirus E proteins, and modeling experiments suggested that icosahedral JEV virions are stabilized by lateral contacts between E dimers, as opposed to contacts across the dimer interface. Our analyses of the JEV E structure highlighted how differences in E protein assembly could impact the reactivity of neutralizing antibodies, and has broad implications for rational vaccine design.
St. Louis Encephalitis Virus Envelope (E) protein - postfusion trimer
The 3.9Å resolution structure of the St. Louis Encephalitis Virus (SLEV) envelope protein (E) revealed a trimeric arrangement that assembles upon fusion of viral and host membranes. In this conformation, the hydrophobic SLEV fusion peptides (green) are fully exposed such that they may penetrate the host's lipid bilayer. Comparison of the SLEV E trimer with other flavivirus E trimers demonstrated that fusion peptides can adopt both open and closed arrangements and raised the possibility that the virus-specific residues surrounding the fusion peptides may may alter tropism by influencing lipid binding preferences. SLEV, like JEV, is also related to the epidemic Zika and Dengue viruses.
IL-2 in complex with Treg-potentiating antibody JES6-1 (in scFv format)
IL-2 is an immune cytokine that drives the proliferation of both effector (T-effs) and regulatory T-cells (T-regs). When in complex with the antibody JES6-1, IL-2 activity changes such that it potentiates the expansion of T-regs and does not stimulate T-effs. In collaboration with Dr. Jamie Spangler, I determined the 2.5Å resolution structure of the JES6-1 single-chain variable fragment (scFv) in complex with IL-2. The complex structure revealed that JES6-1 (blue/purple) completely blocks interactions with IL-2 receptors IL-2Rbeta and Common-gamma, but only partially blocks binding to a third IL-2 receptor, CD25. CD25 is enriched on T-regs, and partial overlap with the CD25-binding site of IL-2 (red) allows JES6-1 to be "kicked off" once IL-2 is fully engaged by CD25. This exchange mechanism facilitates the delivery of IL-2 to T-regs, which in turn potentiates their selective expansion.