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None of the HIV vaccines tested so far has come close to providing adequate protection thanks to the challenges posed by the envelope protein, known as Env.

Env's structure is so complex and delicate, scientists have found it difficult to get the protein in a form that is suitable for the atomic-resolution imaging necessary to understand it.

Professor Andrew Ward, of The Scripps Research Institute (TSRI), explained: "It tends to fall apart, for example, even when it's on the surface of the virus, so to study it we have to engineer it to be more stable."

In the study, researchers created a version of the Env three-component structure that has the stability and other properties needed for atomic-resolution imaging, but that retains virtually all the structures found on Env.

Using cutting-edge imaging methods, electron microscopy and X-ray crystallography the researchers were able to look at the new Env.

It was the first ever study of Env, and both methods resolved the structure to a finer level of detail than has been reported before.

The study for Science Express revealed how Env assembles and undergoes shape changes during infection, and showed how it compares to envelope proteins on other dangerous viruses – such as flu and Ebola.

Professor Ian Wilson, of TSRI, said: "Most of the prior structural studies of this envelope complex focused on individual subunits; but we've needed the structure of the full complex to properly define the sites of vulnerability that could be targeted, for example with a vaccine."

Professor John Moore, of Cornell University who helped with the study, said: "Now we all need to harness this new knowledge to design and test next-generation trimers and see if we can induce the broadly active neutralising antibodies an effective vaccine is going to need."