In silico prediction of structure and function for a large family of transmembrane proteins that includes human Tmem41b

Transmembrane proteins-Different anchoring systems

Transmembrane proteins are active in amphipathic environments. To stabilize the protein in such surrounding the exposure of hydrophobic residues on the protein surface is required. Transmembrane proteins are responsible for the transport of various molecules. Therefore, they often represent structures in the form of channels. This analysis focused on the stability and local flexibility of transmembrane proteins, particularly those related to their biological activity. Different forms of anchorage were identified using the fuzzy oil-drop model (FOD) and its modified form, FOD-M. The mainly helical as well as β-barrel structural forms are compared with respect to the mechanism of stabilization in the cell membrane. The different anchoring system was found to stabilize protein molecules with possible local fluctuation.

Deep Learning-based structure modelling illuminates structure and function in uncharted regions of β-solenoid fold space

Repeat proteins are common in all domains of life and exhibit a wide range of functions. One class of repeat protein contains solenoid folds where the repeating unit consists of β-strands separated by tight turns. β-solenoids have distinguishing structural features such as handedness, twist, oligomerisation state, coil shape and size which give rise to their diversity. Characterised β-solenoid repeat proteins are known to form regions in bacterial and viral virulence factors, antifreeze proteins and functional amyloids. For many of these proteins, the experimental structure has not been solved, as they are difficult to crystallise or model. Here we use various deep learning-based structure-modelling methods to discover novel predicted β-solenoids, perform structural database searches to mine further structural neighbours and relate their predicted structure to possible functions. We find both eukaryotic and prokaryotic adhesins, confirming a known functional linkage between adhesin function and the β-solenoid fold. We further identify exceptionally long, flat β-solenoid folds as possible structures of mucin tandem repeat regions and unprecedentedly small β-solenoid structures. Additionally, we characterise a novel β-solenoid coil shape, the FapC Greek key β-solenoid as well as plausible complexes between it and other proteins involved in Pseudomonas functional amyloid fibres.

Cyanobacterial membrane dynamics in the light of eukaryotic principles

Intracellular compartmentalization is a hallmark of eukaryotic cells. Dynamic membrane remodeling, involving membrane fission/fusion events, clearly is crucial for cell viability and function, as well as membrane stabilization and/or repair, e.g., during or after injury. In recent decades, several proteins involved in membrane stabilization and/or dynamic membrane remodeling have been identified and described in eukaryotes. Yet, while typically not having a cellular organization as complex as eukaryotes, also bacteria can contain extra internal membrane systems besides the cytoplasmic membranes (CMs). Thus, also in bacteria mechanisms must have evolved to stabilize membranes and/or trigger dynamic membrane remodeling processes. In fact, in recent years proteins, which were initially defined being eukaryotic inventions, have been recognized also in bacteria, and likely these proteins shape membranes also in these organisms. One example of a complex prokaryotic inner membrane system is the thylakoid membrane (TM) of cyanobacteria, which contains the complexes of the photosynthesis light reaction. Cyanobacteria are evolutionary closely related to chloroplasts, and extensive remodeling of the internal membrane systems has been observed in chloroplasts and cyanobacteria during membrane biogenesis and/or at changing light conditions. We here discuss common principles guiding eukaryotic and prokaryotic membrane dynamics and the proteins involved, with a special focus on the dynamics of the cyanobacterial TMs and CMs.

Systematic in silico discovery of novel solute carrier-like proteins from proteomes

Solute carrier (SLC) proteins represent the largest superfamily of transmembrane transporters. While many of them play key biological roles, their systematic analysis has been hampered by their functional and structural heterogeneity. Based on available nomenclature systems, we hypothesized that many as yet unidentified SLC transporters exist in the human genome, which await further systematic analysis. Here, we present criteria for defining "SLC-likeness" to curate a set of "SLC-like" protein families from the Transporter Classification Database (TCDB) and Protein families (Pfam) databases. Computational sequence similarity searches surprisingly identified ~120 more proteins in human with potential SLC-like properties compared to previous annotations. Interestingly, several of these have documented transport activity in the scientific literature. To complete the overview of the "SLC-ome", we present an algorithm to classify SLC-like proteins into protein families, investigating their known functions and evolutionary relationships to similar proteins from 6 other clinically relevant experimental organisms, and pinpoint structural orphans. We envision that our work will serve as a stepping stone for future studies of the biological function and the identification of the natural substrates of the many under-explored SLC transporters, as well as for the development of new therapeutic applications, including strategies for personalized medicine and drug delivery.

Regulation of ER-derived membrane dynamics by the DedA domain-containing proteins VMP1 and TMEM41B

The endoplasmic reticulum (ER) is a central hub for the biogenesis of various organelles and lipid-containing structures. Recent studies suggest that vacuole membrane protein 1 (VMP1) and transmembrane protein 41B (TMEM41B), multispanning ER membrane proteins, regulate the formation of many of these ER-derived structures, including autophagosomes, lipid droplets, lipoproteins, and double-membrane structures for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) replication. VMP1 and TMEM41B possess a DedA domain that is widely distributed not only in eukaryotes but also in prokaryotes and predicted to adopt a characteristic structure containing two reentrant loops. Furthermore, recent studies show that both proteins have lipid scrambling activity. Based on these findings, the potential roles of VMP1 and TMEM41B in the dynamic remodeling of ER membranes and the biogenesis of ER-derived structures are discussed.

An alliance between lipid transfer proteins and scramblases for membrane expansion

Membrane growth requires lipid supply, which is usually accomplished by lipid synthesis or vesicular trafficking. In the case of autophagosomes, these principles do not apply. Ghanbarpour et al. postulate that autophagosome expansion relies on non-vesicular lipid delivery from the ER, whereby the activity of a lipid transfer protein (LTP) is directly coupled to scramblase activities in the donor and acceptor bilayers¹. This new concept opens the possibility that lipid traffic is controlled by scramblases that provide not only specific docking sites for LTPs, thereby directing lipid flow, but also support their activity by overcoming barriers for lipid extraction and deposition.

A Klebsiella pneumoniae DedA family membrane protein is required for colistin resistance and for virulence in wax moth larvae

Ineffectiveness of carbapenems against multidrug resistant pathogens led to the increased use of colistin (polymyxin E) as a last resort antibiotic. A gene belonging to the DedA family encoding conserved membrane proteins was previously identified by screening a transposon library of K. pneumoniae ST258 for sensitivity to colistin. We have renamed this gene dkcA (dedA of Klebsiella required for colistin resistance). DedA family proteins are likely membrane transporters required for viability of Escherichia coli and Burkholderia spp. at alkaline pH and for resistance to colistin in a number of bacterial species. Colistin resistance is often conferred via modification of the lipid A component of bacterial lipopolysaccharide with aminoarabinose (Ara4N) and/or phosphoethanolamine. Mass spectrometry analysis of lipid A of the ∆dkcA mutant shows a near absence of Ara4N in the lipid A, suggesting a requirement for DkcA for lipid A modification with Ara4N. Mutation of K. pneumoniae dkcA resulted in a reduction of the colistin minimal inhibitory concentration to approximately what is found with a ΔarnT strain. We also identify a requirement of DkcA for colistin resistance that is independent of lipid A modification, instead requiring maintenance of optimal membrane potential. K. pneumoniae ΔdkcA displays reduced virulence in Galleria mellonella suggesting colistin sensitivity can cause loss of virulence.

Topological analysis of a bacterial DedA protein associated with alkaline tolerance and antimicrobial resistance

Maintaining membrane integrity is of paramount importance to the survival of bacteria as the membrane is the site of multiple crucial cellular processes including energy generation, nutrient uptake and antimicrobial efflux. The DedA family of integral membrane proteins are widespread in bacteria and are associated with maintaining the integrity of the membrane. In addition, DedA proteins have been linked to resistance to multiple classes of antimicrobials in various microorganisms. Therefore, the DedA family are attractive targets for the development of new antibiotics. Despite DedA family members playing a key physiological role in many bacteria, their structure, function and physiological role remain unclear. To help illuminate the structure of the bacterial DedA proteins, we performed substituted cysteine accessibility method (SCAM) analysis on the most comprehensively characterized bacterial DedA protein, YqjA from Escherichia coli. By probing the accessibility of 15 cysteine residues across the length of YqjA using thiol reactive reagents, we mapped the topology of the protein. Using these data, we experimentally validated a structural model of YqjA generated using evolutionary covariance, which consists of an α-helical bundle with two re-entrant hairpin loops reminiscent of several secondary active transporters. In addition, our cysteine accessibility data suggest that YqjA forms an oligomer wherein the protomers are arranged in a parallel fashion. This experimentally verified model of YqjA lays the foundation for future work in understanding the function and mechanism of this interesting and important family.

The evolutionary history of topological variations in the CPA/AT transporters

CPA/AT transporters are made up of scaffold and a core domain. The core domain contains two non-canonical helices (broken or reentrant) that mediate the transport of ions, amino acids or other charged compounds. During evolution, these transporters have undergone substantial changes in structure, topology and function. To shed light on these structural transitions, we create models for all families using an integrated topology annotation method. We find that the CPA/AT transporters can be classified into four fold-types based on their structure; (1) the CPA-broken fold-type, (2) the CPA-reentrant fold-type, (3) the BART fold-type, and (4) a previously not described fold-type, the Reentrant-Helix-Reentrant fold-type. Several topological transitions are identified, including the transition between a broken and reentrant helix, one transition between a loop and a reentrant helix, complete changes of orientation, and changes in the number of scaffold helices. These transitions are mainly caused by gene duplication and shuffling events. Structural models, topology information and other details are presented in a searchable database, CPAfold (cpafold.bioinfo.se).