There is a common notion that “If Nature can find a use for something, She will.” and this story has been gradually playing out for the cross-beta protein fold. Known generally as “amyloid”, the cross-beta fold was first identified in pathologies including neurodegenerative disorders such as Alzheimer’s and systemic amyloidoses such as amyotrophic lateral sclerosis (often referred to as “Lou Gehrig’s Disease”). This happenstance initially pegged the fold as a feature unique to abnormal proteins. However, it subsequently became clear that normal proteins subjected to abnormal conditions would also assume the cross-beta fold. Still, it seemed that the fold was a sign of proteins gone awry. Then came discoveries of cross-beta folds in native, functional proteins. Some are primarily extracellular bacterial proteins involved in negotiating air-water interfaces at sporulation. But some involve the intracellular packaging of proteins in humans. Now Brandeis investigators Eugenio Daviso, Marina Belenky and Judith Herzfeld, with MIT collaborators Marvin Bayro and Robert Griffin, have found that an entire organelle is assembled with the cross-beta fold.1 Gas vesicles, the pressure-resistant floatation organelles of aquatic micro-organisms, comprise a protein-encased gas bubble. Assembly and disassembly of these bodies allows the cells to navigate up and down the water column and the cross-beta fold of the protein shell lends the vesicles the strength and interfacial stability that is critical for their function.
(Left) Schematic of the architecture of a gas vesicle. The gas vesicle is a bipolar cylinder with conical end caps. The ribs of the vesicle comprise GvpA monomers assembled in a low pitch helix. The horizontal lines shown within one of these ribs illustrate the orientation of the β-strands of GvpA as determined previously by x-ray diffraction.2 The expanded view of this rib shows the contacts between β-strands that have now been detected with solid-state NMR, thus establishing the presence of a continuous cross-beta sheet.1
1. Bayro M, Daviso E, Belenky M, Griffin RG and Herzfeld J*. An Amyloid Organelle: Solid State NMR Evidence for Cross-Beta Assembly of Gas Vesicles. J. Biol. Chem., DOI 10.1074/jbc.M111.313049.
2. Blaurock AE and Walsby AE (1976) Crystalline structure of the gas vesicle wall from Anabaena flos-aquae, J. Mol. Biol. 105, 183-199.
