Large biological macromolecules (up to 10,000 atoms) can catalyze chemical reactions in ways that are difficult to replicate inorganically. Their size allows for the complexity needed to perform such functions, but it also makes them more susceptible to misfolding and aggregating—thus, the importance of understanding the fundamental architectural principles that cause large proteins to favor specific conformations. At the nanoscale, where organic chemical groups interact with solvent water molecules, these principles are very different from the ones used to build houses and cars. To investigate these principles, a group of researchers designed a protein that would self-assemble into a hollow pyramid (or tetrahedron). Upon crystallizing the macromolecule, the group found that they had indeed been successful in creating the assembly, but it was unexpectedly warped and collapsed in an asymmetric manner, with some edges bent inward—an asymmetric tetrahedron. To ensure that this was not an artifact of crystallization, they investigated the protein’s behavior in solution using small-angle x-ray scattering (SAXS), which showed that the collapse could be controlled by adjusting the solution’s salt concentration; the structure was disassembled by varying the pH.
The flexibility of this macromolecule suggests that it could be useful for the controlled capture and release of smaller compounds. Overall, the researchers expect that, with the tools and techniques developed here, the combination of SAXS with crystallography or electron microscopy could be increasingly useful in analyzing and optimizing designed protein assemblies and understanding their behavior in solution.
Lai, Y.-T., et al. “Designing and Defining Dynamic Protein Cage Nanoassemblies in Solution.” Sci. Adv.2(12), e1501855 (2016). [DOI:10.1126/sciadv.1501855].
Instruments and Facilities Used: Small-angle X-ray scattering (SAXS) at Advanced Light Source (ALS) with SIBYLS Beamline 12.3.1 (a joint crystallography and SAXS beamline) at Lawrence Berkeley National Laboratory.