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Crystal Structure of Unliganded Influenza B Virus Hemagglutinin ^,

Department of Biochemistry and Molecular Biology, Baylor College of Medicine, One Baylor Plaza, BCM-125, Houston, Texas 77030,¹ Department of Structural Biology and Computational Biology, University of Virginia, Charlottesville, Virginia 22908,² Department of Bioengineering, Rice University, 6100 Main Street, Houston, Texas 77005³

Received 17 November 2007/ Accepted 2 January 2008

Here we report the crystal structure of hemagglutinin (HA) from influenza B/Hong Kong/8/73 (B/HK) virus determined to 2.8 Å. At a sequence identity of 25% to influenza A virus HAs, B/HK HA shares a similar overall structure and domain organization. More than two dozen amino acid substitutions on influenza B virus HAs have been identified to cause antigenicity alteration in site-specific mutants, monoclonal antibody escape mutants, or field isolates. Mapping these substitutions on the structure of B/HK HA reveals four major epitopes, the 120 loop, the 150 loop, the 160 loop, and the 190 helix, that are located close in space to form a large, continuous antigenic site. Moreover, a systematic comparison of known HA structures across the entire influenza virus family reveals evolutionarily conserved ionizable residues at all regions along the chain and subunit interfaces. These ionizable residues are likely the structural basis for the pH dependence and sensitivity to ionic strength of influenza HA and hemagglutinin-esterase fusion proteins.

Published ahead of print on 9 January 2008.

Dedicated to the memory of Don C. Wiley for his enlightenment and guidance.