Membrane Biology
Structural insights into the main S-layer unit of Deinococcus radiodurans reveal a massive protein complex with porin-like features

https://doi.org/10.1074/jbc.RA119.012174Get rights and content
Under a Creative Commons license
open access

In the extremophile bacterium Deinococcus radiodurans, the outermost surface layer is tightly connected with the rest of the cell wall. This integrated organization provides a compact structure that shields the bacterium against environmental stresses. The fundamental unit of this surface layer (S-layer) is the S-layer deinoxanthin-binding complex (SDBC), which binds the carotenoid deinoxanthin and provides both, thermostability and UV radiation resistance. However, the structural organization of the SDBC awaits elucidation. Here, we report the isolation of the SDBC with a gentle procedure consisting of lysozyme treatment and solubilization with the nonionic detergent n-dodecyl-β-d-maltoside, which preserved both hydrophilic and hydrophobic components of the SDBC and allows the retention of several minor subunits. As observed by low-resolution single-particle analysis, we show that the complex possesses a porin-like structural organization, but is larger than other known porins. We also noted that the main SDBC component, the protein DR_2577, shares regions of similarity with known porins. Moreover, results from electrophysiological assays with membrane-reconstituted SDBC disclosed that it is a nonselective channel that has some peculiar gating properties, but also exhibits behavior typically observed in pore-forming proteins, such as porins and ionic transporters. The functional properties of this system and its porin-like organization provide information critical for understanding ion permeability through the outer cell surface of S-layer–carrying bacterial species.

electron microscopy (EM)
electrophysiology
mass spectrometry (MS)
structure–function
membrane protein
gating
protein structure
Deinococcus radiodurans
porin-like complex
S-layer
S-layer deinoxanthin–binding complex (SDBC)
stress resistance

Cited by (0)

This work was supported by National Science Center (Poland) Sonata BIS 7 Program (2017) Grant PRO-2017/26/E/NZ1/00344 (to D. P. and D. F.), L’Oréal-UNESCO Fellowship for Women in Science 2017, Italy (L’Oréal Italia Per le Donne e la Scienza) (to D. F.), Czech Science Foundation Grant 1825144Y (to S. K.), the core facility Cryo-Electron Microscopy and Tomography, CEITEC, Brno, Czech Republic, and National Science Center (Poland) Harmonia 10 Program (2018) Grant PRO-2018/30/M/NZ1/00284 (to D. P. and D. F.). The authors declare that they have no conflicts of interest with the contents of this article.

This article contains Figs. S1–S3 and Table S1.