Studies on the Structure of the Influenza Virus Haemagglutinin at the pH of Membrane Fusion

Abstract

SUMMARYAt the pH required to trigger the membrane fusion activity of the influenza virus haemagglutinin (HA) the soluble ectodomain of the molecule, BHA, which is released from virus by bromelain digestion, aggregates into rosettes. Analyses of soluble proteolytic fragments derived from the rosettes indicated that aggregation is mediated by association of the conserved hydrophobic amino-terminal region of BHA2, the smaller glycopolypeptide component of each BHA subunit. Further analyses of the structure of the soluble fragments and of HA in its low pH conformation by electron microscopy, spectroscopy and in crosslinking experiments showed that, although the membrane distal globular domains lose their trimer structure at the pH of fusion, the central fibrous stem of the molecule remains trimeric and assumes a more stable conformation. The increase in length of BHA2 at low pH observed microscopically appears to result from movement of the amino-terminal region to the membrane proximal end of the molecule and in virus incubated at low pH the amino terminus may insert into the virus membrane. The consequences of these possibilities for the mechanism of membrane fusion are discussed.

The interaction of influenza H5N1 viral hemagglutinin with sialic acid receptors leads to the activation of human γδ T cells

“…This finding supports the hypothesis that HA proteins may play an essential role in the initiation of innate immunity against viral influenza infection. 49,50 We also found that recombinant H5N1 HA proteins could specifically bind to the surface of human cd T cells. However, the binding activity was not dependent on TCRcd and NKG2D, suggesting that H5N1 HA-induced activation of cd T cells may not be mediated via the TCRcd or NKG2D signaling pathway.…”

“…Previous studies demonstrated that the HA from influenza A virus interacts with sialic acid receptors on the cell surface and mediates the entry of viral particles. [50][51][52] In this study, we found that human cd T cells expressed high levels of sialic acid receptors that were required for the binding of H5N1 HA to the surface of human cd T cells. These findings indicate that H5N1 virus-induced activation of cd T cells may be mediated by the interaction of H5N1 virus HA with sialic acid receptors on the surface of cd T cells.…”

Fusion Characteristics of Influenza C Viruses

“…A similar conformational change of the HA of influenza A viruses has been described (Skehel et aL, 1982). Morphological (Ruigrok et al, 1986(Ruigrok et al, , 1988 and spectroscopic studies (Skehel et al, 1982;Wharton et al, 1988a) of the HA revealed a relative movement of structural domains during the conformational change rather than a loss of secondary structure.…”

Irreversible thermal denaturation ofTorpedo californicaacetylcholinesterase

“…Such a situation has been claimed to exist in the case of influenza virus hemagglutinin, where, as discussed by Baker and Agard (1994), the native state is kinetically trapped at neutral pH (see also Hinds & Levitt, 1995); upon acidification, it goes to a lower energy state, which is more thermostable than the native form even at neutral pH (Ruigrok et al, 1988). We earlier argued, based on our chemical modification studies using both organomercurials and disulfides (Kreimer et al, 1994a), that the MG state of AChE produced by chemical modification, whether or not it is subsequently demodified, is energetically favored relative to the N state.…”