Extended Data Fig. 6: Concentration-dependent assembly of GvpU yields anomalous spectroscopic features.
From: Phase transition of GvpU regulates gas vesicle clustering in bacteria
a) Far-UV circular dichroism (CD) spectra of three concentrations of GvpU. At the lowest concentration (1.27 μM = 0.02 mg/mL, lightest purple) of GvpU, noise below 220 nm precludes certainty in secondary structure assignment. Qualitatively, the local minimum at ~222 nm (denoted by vertical dashed line) may reflect some α-helical character at 1.27 μM. Notably, the spectra for the two higher concentrations do not trend as expected for a system of non-interacting chromophores71,72. Presented as molar ellipticity73,74, which is normalized to sample concentration and size, the spectra differ in shape and intensity. Specifically, as GvpU concentration increases, the minima of the spectra decrease in negative intensity and undergo a modest red shift (higher wavelength), despite a linear increase in absorbance signal with increasing concentration. b) Considering evidence from dynamic light scattering for the concentration-dependent formation of soluble oligomers of GvpU, the emergent CD spectral features at 12.7 and 22.9 μM likely reflect interactions between chromophores in separate GvpU subunits. In particular, the AF2 model for the GvpU homomeric assembly implicates hydrophobic packing at the subunit interface; drawing from exciton coupling theory, we speculate that the interactions between Phe and Tyr at the hydrophobic interface could be responsible for the observed hypochromic shift in the CD signal at concentrations that support self-assembly.
