All archaea, and many bacteria, possess a protein shell referred to as a surface layer (S-layer), which usually consist of a single protein that self-assembles into a two-dimensional (2D) crystal lattice. Studies have revealed the structural dynamics of this S-layer protein from the model bacterium Caulobacter crescentus, called RsaA. Using small angle scattering and diffraction (SAXS/D) techniques, multiple structural states of RsaA were successfully characterized including monomeric, aggregated, and crystalline states (see figure), with only monomeric Rssa forming 2D crystals. Enabling differentiation of the discrete states, these results rationalize physiological data implicating RsaA as a player in environmental adaptation of C. crescentus. The findings also provide a biochemical and physiological basis for RsaA’s calcium (Ca)-binding behavior, which extends far beyond Ca’s usual role in S-layer biology of aiding biogenesis or oligomerization, and demonstrate a connection to cellular fitness. Further characterization using slow and fast time-resolved SAXS/D methods is ongoing.Herrmann, J., et al. “Environmental Calcium Controls Alternate Physical States of the Caulobacter Surface Layer,” Biophys. J. 112(9), 1841–1851 (2017). [DOI:10.1016/j.bpj.2017.04.003].
Instruments and Facilities Used: Stanford ChEM-H Macromolecular Structure Knowledge Center, Stanford Department of Structural Biology Electron Microscopy Center, Stanford Synchrotron Radiation Lightsource (SSRL) at SLAC National Accelerator Laboratory (SLAC). Beamlines or instruments used: transmission electron microscopy (TEM) and small angle X-ray scattering and diffraction (SAXS/D) at SSRL beamline 4-2 at SLAC.