The 1918 influenza pandemic resulted in about 20 million deaths. This enormous impact, coupled with renewed interest in emerging infections, makes characterization of the virus involved a priority. Receptor binding, the initial event in virus infection, is a major determinant of virus transmissibility that, for influenza viruses, is mediated by the hemagglutinin (HA) membrane glycoprotein. We have determined the crystal structures of the HA from the 1918 virus and two closely related HAs in complex with receptor analogs. They explain how the 1918 HA, while retaining receptor binding site amino acids characteristic of an avian precursor HA, is able to bind human receptors and how, as a consequence, the virus was able to spread in the human population.

Escherichia coli GlpG is an integral membrane protein that belongs to the widespread rhomboid protease family. Rhomboid proteases, like site-2 protease (S2P) and gamma-secretase, are unique in that they cleave the transmembrane domain of other membrane proteins. Here we describe the 2.1 A resolution crystal structure of the GlpG core domain. This structure contains six transmembrane segments. Residues previously shown to be involved in catalysis, including a Ser-His dyad, and several water molecules are found at the protein interior at a depth below the membrane surface. This putative active site is accessible by substrate through a large 'V-shaped' opening that faces laterally towards the lipid, but is blocked by a half-submerged loop structure. These observations indicate that, in intramembrane proteolysis, the scission of peptide bonds takes place within the hydrophobic environment of the membrane bilayer. The crystal structure also suggests a gating mechanism for GlpG that controls substrate access to its hydrophilic active site.

The three-dimensional structures of avian H5 and swine H9 influenza hemagglutinins (HAs) from viruses closely related to those that caused outbreaks of human disease in Hong Kong in 1997 and 1999 were determined bound to avian and human cell receptor analogs. Emerging influenza pandemics have been accompanied by the evolution of receptor-binding specificity from the preference of avian viruses for sialic acid receptors in ␣2,3 linkage to the preference of human viruses for ␣2,6 linkages. The four new structures show that HA binding sites specific for human receptors appear to be wider than those preferring avian receptors and how avian and human receptors are distinguished by atomic contacts at the glycosidic linkage. ␣2,3-Linked sialosides bind the avian HA in a trans conformation to form an ␣2,3 linkage-specific motif, made by the glycosidic oxygen and 4-OH of the penultimate galactose, that is complementary to the hydrogen-bonding capacity of Gln-226, an avian-specific residue. ␣2,6-Linked sialosides bind in a cis conformation, exposing the glycosidic oxygen to solution and nonpolar atoms of the receptor to Leu-226, a human-specific residue. The new structures are compared with previously reported crystal structures of HA͞sialoside complexes of the H3 subtype that caused the 1968 Hong Kong Influenza virus pandemic and analyzed in relation to HA sequences of all 15 subtypes and to receptor affinity data to make clearer how receptor-binding sites of HAs from avian viruses evolve as the virus adapts to humans.

Amyloid beta-peptide, which forms neuronal and vascular amyloid deposits in Alzheimer's disease, is derived from an integral membrane protein precursor. The biological function of the precursor is currently unclear. Here we describe the X-ray structure of E2, the largest of the three conserved domains of the precursor. The structure of E2 consists of two coiled-coil substructures connected through a continuous helix and bears an unexpected resemblance to the spectrin family of protein structures. E2 can reversibly dimerize in the solution, and the dimerization occurs along the longest dimension of the molecule in an antiparallel orientation, which enables the N-terminal substructure of one monomer to pack against the C-terminal substructure of a second monomer. Heparan sulfate proteoglycans, the putative ligand for the precursor present in extracellular matrix, bind to E2 at a conserved and positively charged site near the dimer interface.