Structural and functional characterization of minimal membrane fusion machinery of influenza hemagglutinin.

摘要

Membrane fusion is an essential step in the life cycle of many viruses. This energetically costly process is facilitated by membrane associated proteins. 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 in both liposome and cell-cell fusion assays, have provided evidence for an alternative scenario for HA-induced membrane fusion and perhaps that of other viral systems. In developing a model for membrane fusion, we have begun to dissect the domains of FHA2 into their various functional roles. Here, the kinked loop (amino acids 105--115) is shown to serve as an essential pH regulator. In addition, electron paramagnetic resonance (EPR) studies of the linker region (amino acids 20--40) suggest that the region is helical, perhaps allowing for energy coupling between molecular conformational changes and the process of membrane apposition.