Membrane Protein Structures in Native Cellular Membranes Revealed by Solid-State NMR Spectroscopy

Chiral Drug Resolution Nanochannels Inspired by Mitochondrial Membranes

Fungicides have been widely used in agricultural production; however, their extensive use has caused serious environmental pollution. Because of its high efficiency, low toxicity, and high selectivity, chiral fungicides can effectively reduce the amount of fungicides and increase the efficiency. Hence, how to efficiently separate the enantiomers of chiral drugs with different structures is of significant research value. The multispecific recognition and selective control of the mitochondrial membrane during the transfer of substances allow us to isolate and enrich monochiral pesticide enantiomers. In this study, based on the conical nanochannel modification by L-alanine pillar[5]arene, combined with the "synergistic effect of double-layer membrane channel" of the mitochondrial membrane in living organisms, three different modes of double-layer serial biomimetic nanochannels were constructed. At the same time, the effect of three different modes of the tandem double nanochannel on hand selectivity is investigated. The results demonstrate that the SOD-In double nanochannels exhibit the optimal separation performance. In the experiment, using current as the detection signal, the selectivity ratio of R-propranolol/S-propranolol was determined to be 43.67. The transmembrane transport selectivity coefficient α(R-/S-PPL) was 13.19 in the single molecule transmission experiment. This study provides an effective method for highly selective enrichment of single configuration chiral pesticides, promoting green agriculture development.

Robust Heteronuclear Correlations for Sub-milligram Protein in Ultrafast Magic-Angle Spinning Solid-State NMR

Proton-detected solid-state nuclear magnetic resonance (ssNMR) under ultrafast magic-angle spinning (MAS) has become a powerful tool for elucidating the structures of proteins with sub-milligram quantities, where establishing 13C-15N correlations is essential. However, traditional 13C-15N cross-polarization (CP), effective at lower MAS frequencies, suffers diminished efficiency under ultrafast MAS conditions. To overcome this limitation, we developed a robust method for selective polarization between insensitive nuclei (SPINE). This approach significantly enhances the heteronuclear 13C-15N correlation efficiency over CP, with gain factors of 1.75 for 13CA-15N and 1.9 and 13CO-15N transfers. SPINE's efficacy was validated on four diverse proteins: the microcrystalline β1 immunoglobulin binding domain of protein G (GB1), the large-conductance mechanosensitive ion channel from Methanosarcina acetivorans (MaMscL), fibrillar septum-forming protein (SepF), and the vertex protein of the β-carboxysome shell (CcmL). This enhancement can reduce the duration of current multidimensional experiments to about one-third of that using a single 13C-15N CP and to about one-tenth with dual 13C-15N transfers. Our findings underscore the practical utility and versatility of SPINE in ssNMR spectroscopy, making it a valuable approach for advancing structural biology studies of sub-milligram protein.

Native Cellular Membranes Facilitate Channel Activity of MscL by Enhancing Slow Collective Motions of Its Transmembrane Helices

Mechanosensitive channels of large conductance (MscL) serve as a mechanoelectrical valve of cells in response to the membrane tension. The influence of membrane environments on the MscL channel activity and the underlying mechanism remains unclear. Herein, we developed a new sample preparation protocol that allows for the detection of high-quality 1H-detected solid-state NMR spectra of MscL in cellular membranes, enabling site-specific analysis of its dynamics. Dipolar order parameters and spin relaxation rates are measured for 51 residues of MscL in synthetic and native membranes. The dynamics data reveal that while MscL maintains a similar rigidity in both membrane environments, it exhibits enhanced slow collective motions in the native cellular membranes. Molecular dynamics simulations demonstrate the critical role of slow motions in the mechanosensitivity of MscL by promoting protein-membrane interactions. This study examines atomic-resolution dynamics of a membrane-protein in cellular membranes and provides novel insights into the functional significance of membrane-protein dynamics.

Selective correlations between aliphatic 13C nuclei in protein solid-state NMR

Solid-state nuclear magnetic resonance (NMR) is a potent tool for studying the structures and dynamics of insoluble proteins. It starts with signal assignment through multi-dimensional correlation experiments, where the aliphatic 13Cα-13Cβ correlation is indispensable for identifying specific residues. However, developing efficient methods for achieving this correlation is a challenge in solid-state NMR. We present a simple band-selective zero-quantum (ZQ) recoupling method, named POST-C4161 (PC4), which enhances 13Cα-13Cβ correlations under moderate magic-angle spinning (MAS) conditions. PC4 requires minimal 13C radio-frequency (RF) field and proton decoupling, exhibits high stability against RF variations, and achieves superior efficiency. Comparative tests on various samples, including the formyl-Met-Leu-Phe (fMLF) tripeptide, microcrystalline β1 immunoglobulin binding domain of protein G (GB1), and membrane protein of mechanosensitive channel of large conductance from Methanosarcina acetivorans (MaMscL), demonstrate that PC4 selectively enhances 13Cα-13Cβ correlations by up to 50 % while suppressing unwanted correlations, as compared to the popular dipolar-assisted rotational resonance (DARR). It has addressed the long-standing need for selective 13C-13C correlation methods. We anticipate that this simple but efficient PC4 method will have immediate applications in structural biology by solid-state NMR.

Background signal suppression by opposite polarity subtraction for targeted DNP NMR spectroscopy on mixture samples

A novel method for background signal suppression is introduced to improve the selectivity of dynamic nuclear polarization (DNP) NMR spectroscopy in the study of target molecules within complex mixtures. The method uses subtraction between positively and negatively enhanced DNP spectra, leading to an improved contrast factor, which is the ratio between the target and background signal intensities. The proposed approach was experimentally validated using a reverse-micelle system that confines the target molecules together with the polarizing agent, OX063 trityl. A substantial increase in the contrast factor was observed, and the contrast factor was optimized through careful selection of the DNP build-up time. A simulation study based on the experimental results provides insights into a strategy for choosing the appropriate DNP build-up time and the corresponding selectivity of the method. Further analysis revealed a broad applicability of the technique, encompassing studies from large biomolecules to surface-modified polymers, depending on the nuclear spin diffusion rate with a range of gyromagnetic ratios.

Progress, Challenges and Opportunities of NMR and XL-MS for Cellular Structural Biology

The validity of protein structures and interactions, whether determined under ideal laboratory conditions or predicted by AI tools such as Alphafold2, to precisely reflect those found in living cells remains to be examined. Moreover, understanding the changes in protein structures and interactions in response to stimuli within living cells, under both normal and disease conditions, is key to grasping proteins' functionality and cellular processes. Nevertheless, achieving high-resolution identification of these protein structures and interactions within living cells presents a technical challenge. In this Perspective, we summarize the recent advancements in in-cell nuclear magnetic resonance (NMR) and in vivo cross-linking mass spectrometry (XL-MS) for studying protein structures and interactions within a cellular context. Additionally, we discuss the challenges, opportunities, and potential benefits of integrating in-cell NMR and in vivo XL-MS in future research to offer an exhaustive approach to studying proteins in their natural habitat.