Insight into structural diversity of influenza virus haemagglutinin.

Influenza virus infects host cells through membrane fusion, a process mediated by the low pH-induced conformational change of the viral surface glycoprotein haemagglutinin (HA). We determined the structures and biochemical properties of the HA proteins from A/Korea/01/2009 (KR01), a 2009 pandemic strain, and A/Thailand/CU44/2006 (CU44), a seasonal strain. The crystal structure of KR01 HA revealed a V-shaped head-to-head arrangement, which is not seen in other HA proteins including CU44 HA. We isolated a broadly neutralizing H1-specific monoclonal antibody GC0757. The KR01 HA-Fab0757 complex structure also exhibited a head-to-head arrangement of HA. Both native and Fab complex structures reveal a different spatial orientation of HA1 relative to HA2, indicating that HA is flexible and dynamic at neutral pH. Further, the KR01 HA exhibited significantly lower protein stability and increased susceptibility to proteolytic cleavage compared with other HAs. Our structures provide important insights into the conformational flexibility of HA.

Crystal structures of aprotinin and its complex with sucrose octasulfate reveal multiple modes of interactions with implications for heparin binding.

The crystal structures of aprotinin and its complex with sucrose octasulfate (SOS), a polysulfated heparin analog, were determined at 1.7-2.6A resolutions. Aprotinin is monomeric in solution, which associates into a decamer at high salt concentrations. Sulfate ions serve to neutralize the basic amino acid residues of aprotinin to stabilize the decameric aprotinin. Whereas SOS interacts with heparin binding proteins at 1:1 molar ratio, SOS was surprisingly found to induce strong agglutination of aprotinins. Five molecules of aprotinin interact with one molecule of the sulfated sugar, which is stabilized by electrostatic interactions between the positively charged residues of aprotinin and sulfate groups of SOS. The multiple binding modes of SOS with five individual aprotinin molecules may represent the diverse patterns of potential heparin binding to aprotinin, reflecting the interactions of densely packed protein molecules along the heparin polymer.

Epidemiologic study of human influenza virus infection in South Korea from 1999 to 2007: origin and evolution of A/Fujian/411/2002-like strains.

Influenza epidemics arise through the accumulation of viral genetic changes, culminating in a novel antigenic type that is able to escape host immunity. Following an outbreak of the A/Fujian/411/2002-like strains in Asia, including China, Japan, and South Korea, in 2002, Australia and New Zealand experienced substantial outbreaks of the same strains in 2003, and subsequently worldwide outbreaks occurred in the 2003-2004 season. The emergence of A/Fujian/411/2002-like strains coincided with a higher level of influenza-like illness in South Korea than what is seen at the peak of a normal season, and there was at least a year's difference between South Korea and the United States. Genetic evolution of human influenza A/H3N2 viruses was monitored by sequence analysis of hemagglutinin (HA) genes collected in Asia, including 269 (164 new) HA genes isolated in South Korea from 1999 to 2007. The Fujian-like influenza strains were disseminated with rapid sequence variation across the antigenic sites of the HA1 domain, which sharply distinguished between the A/Moscow/10/1999-like and A/Fujian/411/2002-like strains. This fast variation, equivalent to approximately 10 amino acid changes within a year, occurred in Asia and would be the main cause of the disappearance of the reassortants, although the reassortant and nonreassortant Fujian-like strains circulated simultaneously in Asia.

Structural Basis of Novel Iron-Uptake Route and Reaction Intermediates in Ferritins from Gram-Negative Bacteria.

Iron and oxygen chemistry is mediated by iron proteins for many biological functions. Carboxylate-bridged diiron enzymes including ferritin have the common mechanism of oxygen activation via peroxodiferric intermediates. However, the route for iron uptake and the structural identification of intermediates still remain incomplete. The 4-fold symmetry channel of Helicobacter pylori ferritin was previously proposed as the iron-uptake route in eubacteria, but the amino acid residues at the 4-fold channel are not highly conserved. Here, we show evidence for a short path for iron uptake from His93 on the surface to the ferroxidase center in H. pylori ferritin and Escherichia coli ferritin. The amino acid residues along this path are highly conserved in Gram-negative bacteria and some archaea, and the mutants containing S20A and H93L showed significantly decreased iron oxidation. Surprisingly, the E. coli ferritin S20A crystal structure showed oxygen binding and side-on, symmetric µ-η2:η2 peroxodiferric and oxodiferric intermediates. The results provide the structural basis for understanding the chemical nature of intermediates in iron oxidation in bacteria and some of archaea.