Vasileios Kargas

Detergent solubilisation remains the most commonly used but potentially problematic method to extract membrane proteins from lipid bilayers for Cryo-EM studies. Although recent advances have introduced excellent alternatives—such as amphipols, nanodiscs and SMALPs—the use of detergents is often necessary for intermediate steps. In this paper, we share our experiences working with detergent-solubilised samples within the modern Cryo-EM structural pipeline from the perspective of an EM specialist. Our aim is to inform novice users about potential challenges they may encounter. Drawing on specific examples from a variety of biological membrane systems, including Magnesium channels, lipopolysaccharide biosynthesis, and the human major facilitator superfamily transporters, we describe how the intrinsic properties of detergent-extracted samples can affect protein purification, Cryo-EM grid preparation (including the formation of vitreous ice) and the reconstitution of proteins into micelles. We also discuss how these unique characteristics can impact different stages of structural analysis and lead to complications in single-particle averaging software analysis. For each case, we present our insights into the underlying causes and suggest possible mitigations or alternative approaches.

Detergent solubilisation remains the most commonly used but potentially problematic method to extract membrane proteins from lipid bilayers for Cryo-EM studies. Although recent advances have introduced excellent alternatives—such as amphipols, nano-discs, and SMALPs—the use of detergents is often necessary for intermediate steps. In this paper, we share our experiences working with detergent-solubilised samples within the modern Cryo-EM structural pipeline and from the perspective of an EM specialist. Our aim is to inform novice users about potential challenges they may encounter. Drawing on specific examples from a variety of biological membrane systems, including Magnesium channels, lipo-polysaccharide biosynthesis and the human major facilitator super-family, we describe how the intrinsic properties of detergent-extracted samples can affect protein purification, Cryo-EM grid preparation (including the formation of vitreous ice) and the reconstitution of proteins into micelles. We also discuss how these unique characteristics can impact different stages of structural analysis and lead to complications in single-particle averaging software analysis. For each case, we present our insights into the underlying causes and suggest possible mitigations or alternative approaches.

The endoplasmic reticulum (ER) unfolded protein response (UPR) is tuned by the balance between unfolded proteins and chaperones. While reserve chaperones are known to suppress the UPR transducers via their stress-sensing luminal domains, the underlying structural mechanisms remain unclear. Cellular and biophysical analyses established that the ER chaperone AGR2 forms a repressive complex with the luminal domain of the UPR transducer IRE1β. Structural prediction, X-ray crystallography and NMR spectroscopy identify critical interactions between an AGR2 monomer and a regulatory loop in IRE1β’s luminal domain. However, in the repressive complex it is an AGR2 dimer that binds IRE1β. Cryo-EM reconstruction reveals a mechanism of unanticipated simplicity: one AGR2 protomer engages the regulatory loop, while the second asymmetrically binds IRE1β’s luminal domain’s C-terminus, blocking IRE1β-activating dimerization. Molecular dynamic simulations indicate that the second, disruptive AGR2 protomer exploits rare fluctuations in the IRE1β dimer that expose its binding site. Thus, AGR2 actively disrupts IRE1β dimers to suppress the UPR, while chaperone clients compete for AGR2 to trigger UPR signalling.

During their final maturation in the cytoplasm, pre-60S ribosomal particles are converted to translation-competent large ribosomal subunits. Here, we present the mechanism of peptidyltransferase centre (PTC) completion that explains how integration of the last ribosomal proteins is coupled to release of the nuclear export adaptor Nmd3. Single-particle cryo-EM reveals that eL40 recruitment stabilises helix 89 to form the uL16 binding site. The loading of uL16 unhooks helix 38 from Nmd3 to adopt its mature conformation. In turn, partial retraction of the L1 stalk is coupled to a conformational switch in Nmd3 that allows the uL16 P-site loop to fully accommodate into the PTC where it competes with Nmd3 for an overlapping binding site (base A2971). Our data reveal how the central functional site of the ribosome is sculpted and suggest how the formation of translation-competent 60S subunits is disrupted in leukaemia-associated ribosomopathies.