Hemagglutinin fusion protein

FIGURE 11-24 Fusion induced by the hemagglutinin (FIA) protein during viral infection. HA protein is exposed on the membrane surface of the influenza virus. When the virus moves from the neutral pH of the interstitial fluid to the low-pH compartment (endosome) in the host cell, HA undergoes dramatic shape changes that mediate fusion of the viral and endosomal membranes, releasing the viral contents into the cytoplasm. 

Lambrecht, B., and Schmidt, M. F. 1986. Membrane fusion induced by influenza virus hemagglutinin requires protein bound fatty acids. FEBS Lett. 202 127-132. 

The most striking feature of influenzavirus is the layer of spikes projecting outward from the surface (Fig. I). These spikes are the hemagglutinin (FIA) proteins of which there are estimated to be a few hundred copies per virion (Ruigrok et al, 1984 Amano and Hosaka, 1992). The HA spikes carry out both receptor binding and membrane fusion during infection. ELA of influenza A virus was the first enveloped virus surface protein to be studied by X-ray crystallography (Wilson et al, 1981),... 

Negative-strand RNA viruses, 158-163 ebola virus matrix protein/ glycoprotein, 162-163 hemagglutinin-neuraminidase and, 162 influenza A and, 158-163 Ml and, 161 Ms and, 161-162 neuraminidase and, 161 paramyxovirus fusion protein and, 162 Neuraminidase, negative-strand RNA viruses and, 161... 

Figure 3 HPSEC of a Triton X-100 extract ot purified Sendai and Newcastle disease virions. Virus was grown in 10-day-old embryonated chicken eggs and purified and extracted as described [5]. A TSK 4000SW (600 x 7.5 mm ID) column was eiuted with 50 mM sodium phosphate, pH 6.5, containing 0.1% SDS. Samples were boiled for 2 min in 4% SDS prior to chromatography. The flow rate was 1 mL/min and the absorbance was monitored at 280 nm. 1, Tetramer of the hemagglutinin neuraminidase (HN) protein of Sendai virus 2, dimer of HN 3, Sendai virus fusion protein F , Triton X-100.

Fig. 1 Sialic acid binding sites of the hemagglutinin (a) and the neuraminidase (b) of influenza A virus and the hemagglutinin-esterase-fusion protein of influenza C virus (c). Molecular surfaces of HA and HEF trimers and the NA tetramer are shown. Receptor-binding sites of HA, HEF and the hemadsorption site of NA are colored ye/tow. The catalytic sites of NA and HEF are colored green. Sialic acid moieties in the binding sites of HA and NA are shown as stick models. The figtffe is based on crystal structures IMQM, 1W20, and IFLC from Protein Data Bank...

In addition to binding to sialic acid residues of the carbohydrate side chains of cellular proteins that the virus exploits as receptors, hemagglutinin has a second function in the infection of host cells. Viruses, bound to the plasma membrane via their membrane receptors, are taken into the cells by endocytosis. Proton pumps in the membrane of endocytic vesicles that now contain the bound viruses cause an accumulation of protons and a consequent lowering of the pH inside the vesicles. The acidic pH (below pH 6) allows hemagglutinin to fulfill its second role, namely, to act as a membrane fusogen by inducing the fusion of the viral envelope membrane with the membrane of the endosome. This expels the viral RNA into the cytoplasm, where it can begin to replicate. 

Chapman and Liljas, Fig. 8. The structure of influenza hemagglutinin (Wilson et al., 1981). The strands of the jelly-roll domain (top) are denoted 1 through 8. The color scheme is blue to red from the N terminus of chain 1 (Nt 1) to the C terminus of chain 2 (Ct 2), which is cleaved from the membrane anchor. The fusion peptide is at the N terminus of chain 2 (Nt 2). In the virus, the protein forms a trimer where the long helices are parallel to the 3-fold axis and form a stem. 

A FIGURE 17-13 Model for membrane fusion directed by hemagglutinin (HA). A number of low pH-activated HA spikes, possibly in concert with host-cell membrane proteins, form a scaffold that connects a small region of the viral envelope and the endosomal membrane. By unknown mechanisms, the exoplasmic leaflets of the two membranes fuse and then the cytosolic leaflets fuse, forming a pore that widens until the two membranes are completely joined. A similar interaction between membrane bilayers may be brought about during SNARE-mediated vesicle fusion. [Adapted from J. R. Monck and J. M. Fernandez, 1992,... 

Triton X-100 extracts of Sendai virus and several strains of the closely related Newcastle disease virus (NDV) were subjected to SEC. The elution patterns are shown in Fig. 3. SDS-polyacrylamide gel electrophoresis (SDS-PAGE) showed that the tetramer and the dimer of the Sendai virus hemagglutinin-neuraminidase (HN) protein (272 and 136 kDa, respectively) and the monomeric fusion (F) protein (65 kDa) were present in peaks 1, 2, and 3, respectively. These peaks are followed by an extremely large peak that contains Triton X-100 micelles. The elution patterns of the other viruses show that there are notable differences when they are compared with that of Sendai virus. In some cases, for example, the strains La Sota and Texas, the most prominent protein eluting from the column is an HN monomer. In other cases, multimeric forms of the HN protein that differ from Sendai HN protein were eluted (strains Mukteswar, Florida, and Herts). 

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