Alphavirus structure: activation for entry at the target cell surface

• Structural studies reveal novel details on the organization of the alphavirus virion.
• Viral protein structures provide molecular mechanistic information for fusion process.
• The structures suggest an active role of alphavirus receptor to trigger viral fusion.

A wealth of new data about the 3D organization of alphavirus particles was obtained in the last few years. This includes the crystal structures of the envelope glycoprotein complexes at neutral and at acid pH, as well as electron microscopy reconstructions of intact virions at neutral pH to resolutions between 7 Å and 4 Å. The combination has provided unprecedented detail in the description of the alphavirus virion. This review surveys the main features discovered and the implications for the biology of the virus, in particular for the process of disassembly of the glycoprotein shell during entry. The major outstanding questions in this area are also identified and discussed.