Maxwell W. Berkow, Hosu Gwak, Matthew N. Idso, Michael B. Schmithorst, Bailey Rhodes, Brad D. Price, Daniel S. Gianola, Songi Han, Bradley F. Chmelka, “Co-assembly of functionally-active proteorhodopsin membrane protein molecules in mesostructured silica-surfactant films,” Chemistry of Materials, 35, 8502–8516 (2023).
Proteorhodopsin is a light-activated H+ ion pump that is technologically promising for solar-to-electrochemical energy conversion. The significance of this paper is that it reported the incorporation of a functionally-active transmembrane protein, proteorhodopsin, into robust transparent non-biological host materials with the highest protein loadings yet reported (Fig. 2A, up to 44 wt%), high extents of mesostructural order, with high thermal stabilities (to 110 oC), and in >100-μm-thick films with high mechanical strengths. Such combined properties had not previously been possible to obtain, because typical materials synthesis conditions for the preparation of inorganic or hybrid host materials were incompatible with the stability of highly hydrophobic membrane proteins (Fig. 2B).
These challenges were overcome by using a novel combination of non-ionic, cationic, and zwitterionic surfactants, which simultaneously both stabilized the transmembrane protein proteorhodopsin guests, as well as directed their co-assembly into robust transparent mesostructured silica-surfactant films. This progress was enabled by multiscale understanding of the compositions, structures, and dynamics of the protein guests, surfactants, and silica species in these films, as established by small-angle X-ray scattering, solid-state 2D NMR, optical spectroscopy, and nanoindentation analyses. Multivariate fits to transient UV-visible spectroscopy data (Fig. 2C) enabled apparent rate coefficients to be obtained for the interconversion of different PR photo-conformers of PR and the rate-limiting step of the photocycle to be established in different abiotic host materials. Proteorhodopsin maintains its functional activity to >100 oC in well-ordered rectangular mesostructured silica-surfactant films (Fig. 2D), manifesting high thermal stability. The resulting insights establish design criteria for syntheses of co-assembled silica-surfactant-membrane protein materials that are expected to be applicable to other transmembrane proteins with diverse and highly selective transport properties.
Chung-Ta Han, Khanh D. Q. Nguyen, Maxwell W. Berkow, Sunyia Hussain, Ahmad Kiani, Maia Kinnebrew, Matthew N. Idso, Naomi Baxter, Evelyn Chang, Emily Aye, Elsa Winslow, Mohammad Rahman, Susanna Seppala, Michelle O’Malley, Bradley F. Chmelka, Blake Mertz, Songi Han, “Lipid membrane mimetics and oligomerization tune functional properties of proteorhodopsin,” Biophysical Journal, 122, 1–12 (2023).
Transmembrane proteins play crucial roles in diverse biological processes at interfaces between cells and their surrounding environments. The significance of this paper is that it demonstrated that the functional properties of a transmembrane protein, specifically the light-activated H+-ion pump proteorhodopsin, depend heavily on the extent of PR oligomerization and the surfactants that stabilize the protein and its local abiotic environment. Among me and my group’s important contributions to this study were transient UV-visible light spectroscopy analyses (analogous to those in Fig. 2C), which established the oligomerization- and surfactant-dependences of the photo-cycle kinetics associated with the light-activated interconversion of different photo-conformers of proteorhodopsin. In particular, an optimized combination of zwitterionic and cationic lipids reduced the pKa value of a key ion-channel amino acid moiety by 2 units, leading to much higher functional activity of PR at neutral pH compared to previous work (Fig. 3A). In addition, cryo-electron microscopy measurements of membrane proteins in concentrated micellar solutions provided single-particle analyses of PR pentamers (Fig. 3B). The findings from this study established composition-structure-function relationships for proteorhodopsin in different non-biological host materials and the different mechanisms by which oligomerization and surfactant composition mediate protein function.
