A number of different viral spike proteins, responsible for membrane fusion, show striking similarities in their core structures. The prospect of developing a general structure-based mechanism seems plausible in light of these newly determined structures. Influenza hemagglutinin (HA) is the best-studied fusion machine, whose action has previously been described by a hypothetical “spring-loaded” model. This model has recently been extended to explain the mechanism of other systems, such as HIV gp120–gp41. However, evidence supporting this idea is insufficient, requiring re-examination of the mechanism of HA-induced membrane fusion. Recent experiments with a shortened construct of HA, which is able to induce lipid mixing, have provided evidence for an alternative scenario for HA-induced membrane fusion and perhaps that of other viral systems.

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