Influenza Hemagglutinin... A Protein that Springs into Action with Changing pH

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http://pubs.acs.org/cen/images/8221/8221cov1b.jpg
Influenza Hemagglutinin is a protein on the outer viral coat of influenza viruses.  

Influenza Hemagglutinin
http://ssrl.slac.stanford.edu/research/highlights_archive/1918flu_fig.gif

www.its.caltech.edu/~bi1/files_2011/lectures/lecture14/lecture14.ppt
The hemagglutinin structure is a trimer made up of three identical molecules each composed of two separate regions (HA1 and HA2) Low pH in the endosome results in  fusion of the influenza virus membrane with the endosomal membrane thus releasing viral RNA into the cytoplasm where it can replicated. 

The high and low pH forms of the Influenza Hemagglutinin

Summary Continued
Excellent Resources Describing the change Animation showing the HA Infection Process
http://cbm.msoe.edu/includes/swf/HAAnimation.swf

Jmol Animations Illustrating Conformational Changes
http://biology.kenyon.edu/BMB/Chime2/2005/Cerchiara-Holsberry/FRAMES/start.htm
 http://www.callutheran.edu/BioDev/omm/jmol/ha/ha.html

www.its.caltech.edu/~bi1/files_2011/lectures/lecture14/lecture14.ppt