  The mode of action of proteins is to a large extent given by their ability to
adopt different conformations. This is why imaging single biomolecules at
atomic resolution is one of the ultimate goals of biophysics and structural
biology. The existing protein database has emerged from X-ray crystallography,
NMR or cryo-TEM investigations. However, these tools all require averaging over
a large number of proteins and thus over different conformations. This of
course results in the loss of structural information. Likewise it has been
shown that even the emergent X-FEL technique will not get away without
averaging over a large quantity of molecules. Here we report the first
recordings of a protein at sub-nanometer resolution obtained from one
individual ferritin by means of low-energy electron holography. One single
protein could be imaged for an extended period of time without any sign of
radiation damage. Since ferritin exhibits an iron core, the holographic
reconstructions could also be cross-validated against TEM images of the very
same molecule by imaging the iron cluster inside the molecule while the protein
shell is decomposed.
