Hemoglobin crystals immersed in liquid oxygen reveal diffusion channels.

Human hemoglobin (HbA) transports molecular oxygen (O2) from the lung to tissues where the partial pressure of O2 is lower. O2 binds to HbA at the heme cofactor and is stabilized by a distal histidine (HisE7). HisE7 has been observed to occupy opened and closed conformations, and is postulated to act as a gate controlling the binding/release of O2. However, it has been suggested that HbA also contains intraprotein oxygen channels for entrances/exits far from the heme. In this study, we developed a novel method of crystal immersion in liquid oxygen prior to X-ray data collection. In the crystals immersed in liquid oxygen, the heme center was oxidized to generate aquomethemoglobin. Increases of structural flexibility were also observed in regions that are synonymous with previously postulated oxygen channels. These regions also correspond to medically relevant mutations which affect O2 affinity. The way HbA utilizes these O2 channels could have a profound impact on understanding the relationship of HbA O2 transport within these disease conditions. Finally, the liquid oxygen immersion technique can be utilized as a new tool to crystallographically examine proteins and protein complexes which utilize O2 for enzyme catalysis or transport.

Structural studies of antitumor compounds that target the RING domain of MDM2.

Mouse double minute 2 homolog (MDM2) is an E3 ubiquitin-protein ligase that is involved in the transfer of ubiquitin to p53 and other protein substrates. The expression of MDM2 is elevated in cancer cells and inhibitors of MDM2 showed potent anticancer activities. Many inhibitors target the p53 binding domain of MDM2. However, inhibitors such as Inulanolide A and MA242 are found to bind the RING domain of MDM2 to block ubiquitin transfer. In this report, crystal structures of MDM2 RING domain in complex with Inulanolide A and MA242 were solved. These inhibitors primarily bind in a hydrophobic site centered at the sidechain of Tyr489 at the C-terminus of MDM2 RING domain. The C-terminus of MDM2 RING domain, especially residue Tyr489, is required for ubiquitin discharge induced by MDM2. The binding of these inhibitors at Tyr489 may interrupt interactions between the MDM2 RING domain and the E2-Ubiquitin complex to inhibit ubiquitin transfer, regardless of what the substrate is. Our results suggest a new mechanism of inhibition of MDM2 E3 activity for a broad spectrum of substrates.

Nucleocapsid Structure of Negative Strand RNA Virus.

Negative strand RNA viruses (NSVs) include many important human pathogens, such as influenza virus, Ebola virus, and rabies virus. One of the unique characteristics that NSVs share is the assembly of the nucleocapsid and its role in viral RNA synthesis. In NSVs, the single strand RNA genome is encapsidated in the linear nucleocapsid throughout the viral replication cycle. Subunits of the nucleocapsid protein are parallelly aligned along the RNA genome that is sandwiched between two domains composed of conserved helix motifs. The viral RNA-dependent-RNA polymerase (vRdRp) must recognize the protein-RNA complex of the nucleocapsid and unveil the protected genomic RNA in order to initiate viral RNA synthesis. In addition, vRdRp must continuously translocate along the protein-RNA complex during elongation in viral RNA synthesis. This unique mechanism of viral RNA synthesis suggests that the nucleocapsid may play a regulatory role during NSV replication.