A class act

摘要

Membrane fusion occurs in many situations in living organisms -- when certain viruses enter host cells, for instance. Three crystal structures shed light on the protein rearrangements that bring about such fusion. Many viruses, such as the dengue fever, influenza and human immunodeficiency viruses, are encased in a lipid membrane. To reproduce, such 'enveloped' viruses must enter a host cell by fusing their own membrane coat with that of the cell. Fusion is caused by specific proteins in the viralmembrane, and at least two classes of these proteins have been identified. In both classes, tightly regulated conformational changes are involved in membrane fusion. But class i and class II fusion proteins have distinct structural features -- raising the expectation that the details of the two fusion mechanisms would also be unique. Yet the crystal structures of the post-fusion forms of three class II proteins, described elsewhere in this issue and in EMBO Journal, suggest an unanticipated mechanisticparallel between class I and class II fusion proteins. The new structures also provide the first view of a key region of the proteins (the fusion loop) in the conformation that is predicted to insert into the target membrane.