A crystallographer colleague passes on this new paper, which I find very interesting and just a bit freaky. The authors (a collaboration between UCLA, HHMI, LBNL, and SLAC) are studying alpha-synuclein, the protein that aggregates in Parkinson’s disease pathology, and more particularly, they’re studying a short (11-amino acid) section of it that seems to be responsible for its bad behavior. Mutations and deletions in this region mostly abolish the aggregation behavior and toxicity of the protein, so it really seems to be the sticky anchor that weighs everything down.
The paper reports a very nice structure of the aggregated protein at 1.4 Angstrom resolution. What’s unnerving, though, is that the crystals themselves are invisible, since they’re smaller than optical wavelengths. The various crystallization conditions seemed to produce nothing or just amorphous material, but on inspection by electron microscopy, it became clear that this “amorphous” material was in fact made up of clusters of nanocrystals, with cross sections of only 50 to 300 nm, which would make them invisible in any visible wavelength. (I guess you might begin to detect the larger ones down in the short ultraviolet, although I don’t know if that would be of much help or not).
These crystals are far too tiny for x-ray beam equipment to deal with, so they were solved by cryo-electron microscopy, in what has to be considered a major milestone for that up-and-coming technique. These are, in fact, the first completely unknown protein structures to be determined by micro-electron-diffraction. The core of the structure is shown at right, and it’s the beta-sheet bad news that one would expect from what’s known about other aggregating proteins – everything is lined up perfectly for trouble. (Unusually, there are a couple of water molecules involved – the rest of these amyloid-type structures don’t have anything of the sort. The rest of the paper shows how the protein is likely to bend over and around this core, and how some of the known mutations are disrupting things.
And here’s some more on the technique used to solve them:
For amyloid crystals, our speculation is that the tiny size is a consequence of the natural twist of β-sheets that form the protofilaments of the fibrils. The crystal lattice restrains the twist, creating a strain in these crystals, which increases as crystals grow. Eventually this strain prevents further addition of β-strands, limiting the thickness of the needle crystals. In our experience, longer segments (for example, 11 residues compared to 9 residues) limit crystal growth even more; in the case of 11-residue NACore and 10-residue PreNAC, the strain produces nanocrystals, invisible by optical microscopy. These crystals are too small for mounting and conventional synchrotron data collection, but are ideally suited for analysis by MicroED. They are ~1010 times smaller than Perutz’s haemoglobin crystals and ~1012 times smaller than von Laue’s CuSO4 crystal, which yielded the first X-ray diffraction pattern. Our structures of NACore and PreNAC demonstrate that MicroED is capable of determining new and accurate structures of biological material at atomic resolutions.
How many other invisible crystals are out there, waiting to be solved? It’s quite a thought. . .