Cryo-electron microscopy of vitrified specimens was just emerging as a practical method when Richard Henderson proposed that we should teach an EMBO course on the new technique. The request seemed to come too early because at that moment the method looked more like a laboratory game than a useful tool. However, during the months which ellapsed before the start of the course, several of the major difficulties associated with electron microscopy of vitrified specimens found surprisingly elegant solutions or simply became non-existent. The course could therefore take place under favourable circumstances in the summer of 1983. It was repeated the following years and cryo-electron microscopy spread rapidly. Since that time, water, which was once the arch enemy of all electronmicroscopists, became what it always was in nature – an integral part of biological matter and a beautiful substance.

Thin vitrified layers of unfixed, unstained and unsupported virus suspensions can be prepared for observation by cryo-electron microscopy in easily controlled conditions. The viral particles appear free from the kind of damage caused by dehydration, freezing or adsorption to a support that is encountered in preparing biological samples for conventional electron microscopy. Cryo-electron microscopy of vitrified specimens offers possibilities for high resolution observations that compare favourably with any other electron microscopical method.

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The coronavirus SARS-CoV is the primary cause of the life-threatening severe acute respiratory syndrome (SARS). With the aim of developing therapeutic agents, we have tested peptides derived from the membrane-proximal (HR2) and membrane-distal (HR1) heptad repeat region of the spike protein as inhibitors of SARS-CoV infection of Vero cells. It appeared that HR2 peptides, but not HR1 peptides, were inhibitory. Their efficacy was, however, significantly lower than that of corresponding HR2 peptides of the murine coronavirus mouse hepatitis virus (MHV) in inhibiting MHV infection. Biochemical and electron microscopical analyses showed that, when mixed, SARS-CoV HR1 and HR2 peptides assemble into a six-helix bundle consisting of HR1 as a central triple-stranded coiled coil in association with three HR2 ␣-helices oriented in an antiparallel manner. The stability of this complex, as measured by its resistance to heat dissociation, appeared to be much lower than that of the corresponding MHV complex, which may explain the different inhibitory potencies of the HR2 peptides. Analogous to other class I viral fusion proteins, the six-helix complex supposedly represents a postfusion conformation that is formed after insertion of the fusion peptide, proposed here for coronaviruses to be located immediately upstream of HR1, into the target membrane. The resulting close apposition of fusion peptide and spike transmembrane domain facilitates membrane fusion. The inhibitory potency of the SARS-CoV HR2-peptides provides an attractive basis for the development of a therapeutic drug for SARS.