Mechanosensitive channels respond to mechanical forces exerted on the cell membrane and play vital roles in regulating the chemical equilibrium within cells and their environment. High-resolution structural information is required to understand the gating mechanisms of mechanosensitive channels. Protein-lipid interactions are essential for the structural and functional integrity of mechanosensitive channels, but detergents cannot maintain the crucial native lipid environment for purified mechanosensitive channels. Recently, detergent-free systems have emerged as alternatives for membrane protein structural biology. This report shows that while membrane-active polymer, SMA2000, could retain some native cell membrane lipids on the transmembrane domain of the mechanosensitive-like YnaI channel, the complete structure of the transmembrane domain of YnaI was not resolved. This reveals a significant limitation of SMA2000 or similar membrane-active copolymers. This limitation may come from the heterogeneity of the polymers and nonspecific interactions between the polymers and the relatively large hydrophobic pockets within the transmembrane domain of YnaI. However, this limitation offers development opportunities for detergent-free technology for challenging membrane proteins.

Human serum albumin (HSA) is the most prevalent plasma protein in the human body, accounting for 60 % of the total plasma protein. HSA plays a major pharmacokinetic function, serving as a facilitator in the distribution of endobiotics and xenobiotics within the organism. In this paper we report the cryoEM structures of HSA in the apo form and in complex with two ligands (salicylic acid and teniposide) at a resolution of 3.5, 3.7 and 3.4 Å, respectively. We expand upon previously published work and further demonstrate that sub-4 Å maps of ∼60 kDa proteins can be routinely obtained using a 200 kV microscope, employing standard workflows. Most importantly, these maps allowed for the identification of small molecule ligands, emphasizing the practical applicability of this methodology and providing a starting point for subsequent computational modeling and in silico optimization.

Proteoliposomes mimic the cell membrane environment allowing for structural and functional membrane protein analyses as well as antigen presenting and drug delivery devices. To make proteoliposomes, purified functional membrane proteins are required. Detergents have traditionally been used for the first step in this process However, they can irreversibly denature or render membrane proteins unstable, and the necessary removal of detergents after reconstitution can decrease proteoliposome yields. The recently developed native cell membrane nanoparticles (NCMN) system has provided a variety of detergent-free alternatives for membrane protein preparation for structural biology research. Here we attempt to employ the MCMN system for the functional reconstitution of channels into proteoliposomes. NCMN polymers NCMNP1–1 and NCMNP7–1, members of a NCMN polymer library that have been successful in extraction and affinity purification of a number of intrinsic membrane proteins, were selected for the purification and subsequent reconstitution of three bacterial channels: KcsA and the mechanosensitive channels of large and small conductance (MscL and MscS). We found that channels in NCMN particles, which appeared to be remarkably stable when stored at 4 °C, can be reconstituted into bilayers by simply incubating with lipids. We show that the resulting proteoliposomes can be patched for electrophysiological studies or used for the generation of liposome-based nanodevices. In sum, the findings demonstrate that the NCMN system is a simple and robust membrane protein extraction and reconstitution approach for making high-quality functional proteoliposomes that could significantly impact membrane protein research and the development of nanodevices.

PTPN2 (protein tyrosine phosphatase non-receptor type 2, or TC-PTP) and PTPN1 are attractive immuno-oncology targets, with the deletion of Ptpn1 and Ptpn2 improving response to immunotherapy in disease models. Targeted protein degradation has emerged as a promising approach to drug challenging targets including phosphatases. We developed potent PTPN2/N1 dual heterobifunctional degraders (Cmpd-1 and Cmpd-2) which facilitate efficient complex assembly with E3 ubiquitin ligase CRL4^CRBN, and mediate potent PTPN2/N1 degradation in cells and mice. To provide mechanistic insights into the cooperative complex formation introduced by degraders, we employed a combination of structural approaches. Our crystal structure reveals how PTPN2 is recognized by the tri-substituted thiophene moiety of the degrader. We further determined a high-resolution structure of DDB1-CRBN/Cmpd-1/PTPN2 using single-particle cryo-electron microscopy (cryo-EM). This structure reveals that the degrader induces proximity between CRBN and PTPN2, albeit the large conformational heterogeneity of this ternary complex. The molecular dynamic (MD)-simulations constructed based on the cryo-EM structure exhibited a large rigid body movement of PTPN2 and illustrated the dynamic interactions between PTPN2 and CRBN. Together, our study demonstrates the development of PTPN2/N1 heterobifunctional degraders with potential applications in cancer immunotherapy. Furthermore, the developed structural workflow could help to understand the dynamic nature of degrader-induced cooperative ternary complexes.

Antimicrobial resistance poses a severe threat to human health and Pseudomonas aeruginosa stands out among the pathogens responsible for this emergency. The SOS response to DNA damage is crucial in bacterial evolution, influencing resistance development and adaptability in challenging environments, especially under antibiotic exposure. Recombinase A (RecA) and the transcriptional repressor LexA are the key players that orchestrate this process, determining either the silencing or the active transcription of the genes under their control. By integrating state-of-the-art structural approaches with in vitro binding and functional assays, we elucidated the molecular events activating the SOS response in P. aeruginosa, focusing on the RecA-LexA interaction. Our findings identify the conserved determinants and strength of the interactions that allow RecA to trigger LexA autocleavage and inactivation. These results provide the groundwork for designing novel antimicrobial strategies and exploring the potential translation of Escherichia coli-derived approaches, to address the implications of P. aeruginosa infections.

Membrane proteins are a ubiquitous group of bio-macromolecules responsible for many crucial biological processes and serve as drug targets for a wide range of modern drugs. Detergent-free technologies such as styrene-maleic acid lipid particles (SMALP), diisobutylene-maleic acid lipid particles (DIBMALP), and native cell membrane nanoparticles (NCMN) systems have recently emerged as revolutionary alternatives to the traditional detergent-based approaches for membrane protein research. NCMN systems aim to create a membrane-active polymer library suitable for high-resolution structure determination. Herein, we report our design, synthesis, characterization and comparative application analyses of three novel classes of NCMN polymers, NCMNP13-x, NCMNP21-x and NCMNP21b-x. Although each NCMN polymer can solubilize various model membrane proteins and conserve native lipids into NCMN particles, only the NCMNP21b-x series reveals lipid-protein particles with good buffer compatibility and high homogeneity suitable for single-particle cryo-EM analysis. Consequently, the NCMNP21b-x polymers that bring out high-quality NCMN particles are particularly attractive for membrane protein structural biology. Graphical Highlights: Demonstrate straightforward strategies for tailoring styrene-maleic co-polymer (SMA) that revealed novel buffer compatible polymers, NCMNP13-50, NCMNP21-20 and NCMNP21b-x. Elucidate how modification factors alter the membrane-active properties of these polymers, i.e., membrane protein extraction efficiency, morphology, etc. Provide valuable insights into the rational design of membrane-active polymers for membrane protein structural biology. NCMNP21b-x polymers are highly compatible with high-resolution structure determination using an emerging technique, cryo-EM.