Protein solubility and folding monitored in vivo by structural complementation of a genetic marker protein

GATSol, an enhanced predictor of protein solubility through the synergy of 3D structure graph and large language modeling

BACKGROUND

Protein solubility is a critically important physicochemical property closely related to protein expression. For example, it is one of the main factors to be considered in the design and production of antibody drugs and a prerequisite for realizing various protein functions. Although several solubility prediction models have emerged in recent years, many of these models are limited to capturing information embedded in one-dimensional amino acid sequences, resulting in unsatisfactory predictive performance.

RESULTS

In this study, we introduce a novel Graph Attention network-based protein Solubility model, GATSol, which represents the 3D structure of proteins as a protein graph. In addition to the node features of amino acids extracted by the state-of-the-art protein large language model, GATSol utilizes amino acid distance maps generated using the latest AlphaFold technology. Rigorous testing on independent eSOL and the Saccharomyces cerevisiae test datasets has shown that GATSol outperforms most recently introduced models, especially with respect to the coefficient of determination R2, which reaches 0.517 and 0.424, respectively. It outperforms the current state-of-the-art GraphSol by 18.4% on the S. cerevisiae_test set.

CONCLUSIONS

GATSol captures 3D dimensional features of proteins by building protein graphs, which significantly improves the accuracy of protein solubility prediction. Recent advances in protein structure modeling allow our method to incorporate spatial structure features extracted from predicted structures into the model by relying only on the input of protein sequences, which simplifies the entire graph neural network prediction process, making it more user-friendly and efficient. As a result, GATSol may help prioritize highly soluble proteins, ultimately reducing the cost and effort of experimental work. The source code and data of the GATSol model are freely available at https://github.com/binbinbinv/GATSol .

Intra-FCY1: a novel system to identify mutations that cause protein misfolding

Protein misfolding is a common intracellular occurrence. Most mutations to coding sequences increase the propensity of the encoded protein to misfold. These misfolded molecules can have devastating effects on cells. Despite the importance of protein misfolding in human disease and protein evolution, there are fundamental questions that remain unanswered, such as, which mutations cause the most misfolding? These questions are difficult to answer partially because we lack high-throughput methods to compare the destabilizing effects of different mutations. Commonly used systems to assess the stability of mutant proteins in vivo often rely upon essential proteins as sensors, but misfolded proteins can disrupt the function of the essential protein enough to kill the cell. This makes it difficult to identify and compare mutations that cause protein misfolding using these systems. Here, we present a novel in vivo system named Intra-FCY1 that we use to identify mutations that cause misfolding of a model protein [yellow fluorescent protein (YFP)] in Saccharomyces cerevisiae. The Intra-FCY1 system utilizes two complementary fragments of the yeast cytosine deaminase Fcy1, a toxic protein, into which YFP is inserted. When YFP folds, the Fcy1 fragments associate together to reconstitute their function, conferring toxicity in media containing 5-fluorocytosine and hindering growth. But mutations that make YFP misfold abrogate Fcy1 toxicity, thus strains possessing misfolded YFP variants rise to high frequency in growth competition experiments. This makes such strains easier to study. The Intra-FCY1 system cancels localization of the protein of interest, thus can be applied to study the relative stability of mutant versions of diverse cellular proteins. Here, we confirm this method can identify novel mutations that cause misfolding, highlighting the potential for Intra-FCY1 to illuminate the relationship between protein sequence and stability.

The unfolded protein response of the endoplasmic reticulum supports mitochondrial biogenesis by buffering nonimported proteins

Almost all mitochondrial proteins are synthesized in the cytosol and subsequently targeted to mitochondria. The accumulation of nonimported precursor proteins occurring upon mitochondrial dysfunction can challenge cellular protein homeostasis. Here we show that blocking protein translocation into mitochondria results in the accumulation of mitochondrial membrane proteins at the endoplasmic reticulum, thereby triggering the unfolded protein response (UPRER). Moreover, we find that mitochondrial membrane proteins are also routed to the ER under physiological conditions. The level of ER-resident mitochondrial precursors is enhanced by import defects as well as metabolic stimuli that increase the expression of mitochondrial proteins. Under such conditions, the UPRER is crucial to maintain protein homeostasis and cellular fitness. We propose the ER serves as a physiological buffer zone for those mitochondrial precursors that cannot be immediately imported into mitochondria while engaging the UPRER to adjust the ER proteostasis capacity to the extent of precursor accumulation.

Biosensor Guided Polyketide Synthases Engineering for Optimization of Domain Exchange Boundaries

Type I modular polyketide synthases (PKSs) are multi-domain enzymes functioning like assembly lines. Many engineering attempts have been made for the last three decades to replace, delete and insert new functional domains into PKSs to produce novel molecules. However, inserting heterologous domains often destabilize PKSs, causing loss of activity and protein misfolding. To address this challenge, here we develop a fluorescence-based solubility biosensor that can quickly identify engineered PKSs variants with minimal structural disruptions. Using this biosensor, we screen a library of acyltransferase (AT)-exchanged PKS hybrids with randomly assigned domain boundaries, and we identify variants that maintain wild type production levels. We then probe each position in the AT linker region to determine how domain boundaries influence structural integrity and identify a set of optimized domain boundaries. Overall, we have successfully developed an experimentally validated, high-throughput method for making hybrid PKSs that produce novel molecules.

Rational Design of the Soluble Variant of l-Pipecolic Acid Hydroxylase using the α-Helix Rule and the Hydropathy Contradiction Rule

The production of recombinant proteins in Escherichia coli is an important application of biotechnology. 2-Oxoglutarate-dependent l-pipecolic acid hydroxylase derived from Xenorhabdus doucetiae (XdPH) is an excellent biocatalyst that catalyzes the hydroxylation of l-pipecolic acid to produce cis-5-hydroxy-l-pipecolic acid. However, the enzyme tends to form aggregates in the E. coli expression system. Our group established two rules, namely, the "α-helix rule" and the "hydropathy contradiction rule," to select residues to be altered for improving the heterologous recombinant production of proteins, by analyzing their primary structure. We rationally designed XdPH variants that are expressed in highly soluble and active forms in the E. coli expression system using these hotspot prediction methods, and the L142R variant showed a remarkably high soluble expression level compared to the wild-type XdPH. Further mutations were introduced into the L142R gene by site-directed mutagenesis. Moreover, the I28P/L142R and C76Y/L142R double variants displayed improved soluble expression levels compared to the single variants. These variants were also more thermostable than the wild-type XdPH. To analyze the effect of the alteration on one of the hotspots, L142 was replaced with various hydrophilic and positively charged residues. The remarkable increase in soluble protein expression caused by the alterations suggests that the decrease in the hydrophobicity of the protein surface and the enhancement of the interaction between nearby residues are important factors determining the solubility of the protein. Overall, this study demonstrated the effectiveness of our protocol in identifying aggregation hotspots for recombinant protein production and in basic biochemical research.

Ultraviolet optical horn antennas for label-free detection of single proteins

Single-molecule fluorescence techniques have revolutionized our ability to study proteins. However, the presence of a fluorescent label can alter the protein structure and/or modify its reaction with other species. To avoid the need for a fluorescent label, the intrinsic autofluorescence of proteins in the ultraviolet offers the benefits of fluorescence techniques without introducing the labelling drawbacks. Unfortunately, the low autofluorescence brightness of proteins has greatly challenged single molecule detection so far. Here we introduce optical horn antennas, a dedicated nanophotonic platform enabling the label-free detection of single proteins in the UV. This design combines fluorescence plasmonic enhancement, efficient collection up to 85° angle and background screening. We detect the UV autofluorescence from immobilized and diffusing single proteins, and monitor protein unfolding and dissociation upon denaturation. Optical horn antennas open up a unique and promising form of fluorescence spectroscopy to investigate single proteins in their native states in real time.

Identification of stabilizing point mutations through mutagenesis of destabilized protein libraries

Although there have been recent transformative advances in the area of protein structure prediction, prediction of point mutations that improve protein stability remains challenging. It is possible to construct and screen large mutant libraries for improved activity or ligand binding. However, reliable screens for mutants that improve protein stability do not yet exist, especially for proteins that are well folded and relatively stable. Here, we demonstrate that incorporation of a single, specific, destabilizing mutation termed parent inactivating mutation into each member of a single-site saturation mutagenesis library, followed by screening for suppressors, allows for robust and accurate identification of stabilizing mutations. We carried out fluorescence-activated cell sorting of such a yeast surface display, saturation suppressor library of the bacterial toxin CcdB, followed by deep sequencing of sorted populations. We found that multiple stabilizing mutations could be identified after a single round of sorting. In addition, multiple libraries with different parent inactivating mutations could be pooled and simultaneously screened to further enhance the accuracy of identification of stabilizing mutations. Finally, we show that individual stabilizing mutations could be combined to result in a multi-mutant that demonstrated an increase in thermal melting temperature of about 20 °C, and that displayed enhanced tolerance to high temperature exposure. We conclude that as this method is robust and employs small library sizes, it can be readily extended to other display and screening formats to rapidly isolate stabilized protein mutants.

Visualizing the Multistep Process of Protein Aggregation in Live Cells

Protein aggregation is a biological phenomenon in which aberrantly processed or mutant proteins misfold and assemble into a variety of insoluble aggregates. Decades of studies have delineated the structure, interaction, and activity of proteins in either their natively folded structures or insoluble aggregates such as amyloid fibrils. However, a variety of intermediate species exist between these two extreme states in the protein folding landscape. Herein, we collectively term these intermediate species as misfolded protein oligomers, including soluble oligomers and preamyloid oligomers that are formed by unfolded or misfolded proteins. While extensive tools have been developed to study folded proteins or amyloid fibrils, research to understand the properties and activities of misfolded protein oligomers has been limited by the lack of methods to detect and interrogate these species in live cells.In this Account, we describe our efforts in the development of chemical methods that allow for the characterization of the multistep protein aggregation process, in particular the misfolded protein oligomers, in living cells. As the start of this journey, we attempted to develop a fluorogenic method wherein the misfolded oligomers could turn on the fluorescence of chemical probes that are conjugated to the protein-of-interest (POI). To this end, we produced a series of destabilized HaloTag variants, formulating the primary component of the AgHalo sensor, which misfolds and aggregates when cells are subjected to stress. When AgHalo is covalently conjugated with a solvatochromic fluorophore, misfolding of the AgHalo conjugate would activate fluorescence, resulting in the observation of misfolded oligomers. Following this work, we extended the scope of detection from AgHalo to any protein-of-interest via the AggTag method, wherein the POIs are genetically fused to self-labeling protein tags (HaloTag or SNAP-tag). Focusing on the molecular rotor-based fluorophores, we applied the modulated fluorescent protein (FP) chromophore core as a prototype for the AggTag probes, to enable the fluorogenic detection of misfolded soluble oligomers of multiple proteins in live cells. Next, we further developed the AggTag method to distinguish insoluble aggregates from misfolded oligomers, using two classes of probes that activate different fluorescence emission toward these two conformations. To enable this goal, we applied physical organic chemistry and computational chemistry to discover a new category of triode-like fluorophores, wherein the π orbitals of either an electron density regulator or the donor-acceptor linkages are used to control the rotational barriers of fluorophores in the excited states. This mechanism allows us to rationally design molecular rotor-based fluorophores that have desired responses to viscosity, thus extending the application of the AggTag method.In summary, our work allows the direct monitoring of the misfolded protein oligomers and differentiation of insoluble aggregates from other conformations in live cells, thus enabling studies of many currently unanswered questions in protein aggregation. Future directions are to develop methods that enable quantitative analyses of the protein aggregation process. Further, new methods are needed to detect and to quantify the formation and maturation of protein or RNA condensates that form membraneless organelles.

Prediction of Residue-specific Contributions to Binding and Thermal Stability Using Yeast Surface Display

Accurate prediction of residue burial as well as quantitative prediction of residue-specific contributions to protein stability and activity is challenging, especially in the absence of experimental structural information. This is important for prediction and understanding of disease causing mutations, and for protein stabilization and design. Using yeast surface display of a saturation mutagenesis library of the bacterial toxin CcdB, we probe the relationship between ligand binding and expression level of displayed protein, with in vivo solubility in E. coli and in vitro thermal stability. We find that both the stability and solubility correlate well with the total amount of active protein on the yeast cell surface but not with total amount of expressed protein. We coupled FACS and deep sequencing to reconstruct the binding and expression mean fluorescent intensity of each mutant. The reconstructed mean fluorescence intensity (MFIseq) was used to differentiate between buried site, exposed non active-site and exposed active-site positions with high accuracy. The MFIseq was also used as a criterion to identify destabilized as well as stabilized mutants in the library, and to predict the melting temperatures of destabilized mutants. These predictions were experimentally validated and were more accurate than those of various computational predictors. The approach was extended to successfully identify buried and active-site residues in the receptor binding domain of the spike protein of SARS-CoV-2, suggesting it has general applicability.

A Luminescence-Based System for Identification of Genetically Encodable Inhibitors of Protein Aggregation

Molecules that disrupt protein aggregation represent potential tool compounds for the investigation of numerous human disease states. However, the identification of small molecules capable of disrupting protein aggregation has proven challenging. Larger biomolecules such as antibodies and proteins are promising alternatives due to their increased size. Despite the promise of protein-based inhibitors, generalizable assays are needed to more readily identify proteins capable of inhibiting aggregation. Herein, we utilize our previously reported self-assembling NanoLuc luciferase fragments to engineer a platform in which both detection reagents are expressed from the same plasmid, enabling facile co-transformation with a genetically encodable inhibitor. This streamlined system is capable of detecting changes in the solubility of amylin, huntingtin, and amyloid-β (Aβ) proteins in response to mutations, small-molecule inhibitors, and expression of genetically encodable inhibitors. This improved platform provides a means to begin to identify protein-based inhibitors with improved efficacy.

A luminescence-based assay for monitoring changes in alpha-synuclein aggregation in living cells

Parkinson's disease is characterized by the accumulation of protein aggregates in the brain, termed Lewy bodies. Lewy bodies are predominantly composed of α-synuclein and mutations that increase the aggregation potential of α-synuclein have been associated with early on-set disease. Assays capable of reporting on the solubility of α-synuclein in living cells could provide a means to interrogate the influence of mutations on aggregation as well as identify small molecules capable of modulating the aggregation of α-synuclein. Herein, we repurpose our previously reported self-assembling NanoLuc luciferase fragments to engineer a platform for detecting α-synuclein solubility in living cells. This new assay is capable of reporting on changes in α-synuclein solubility caused by disease-relevant mutations as well as inhibitors of aggregation. In the long term, this new assay platform provides a means to investigate the influence of mutations on α-synuclein solubility as well as identify potential tool compounds capable of modulating α-synuclein aggregation.

Hot CoFi Blot: A High-Throughput Colony-Based Screen for Identifying More Thermally Stable Protein Variants

Highly soluble and stable proteins are desirable for many different applications, from basic science to reaching a cancer patient in the form of a biological drug. For X-ray crystallography-where production of a protein crystal might take weeks and even months-a stable protein sample of high purity and concentration can greatly increase the chances of producing a well-diffracting crystal. For a patient receiving a specific protein drug, its safety, efficacy, and even cost are factors affected by its solubility and stability. Increased protein expression and protein stability can be achieved by randomly altering the coding sequence. As the number of mutants generated might be overwhelming, a powerful protein expression and stability screen is required. In this chapter, we describe a colony filtration technology, which allows us to screen random mutagenesis libraries for increased thermal stability-the Hot CoFi blot. We share how to create the random mutagenesis library, how to perform the Hot CoFi blot, and how to identify more thermally stable clones. We use the Tobacco Etch Virus protease as a target to exemplify the procedure.

Utilizing split-NanoLuc luciferase fragments as luminescent probes for protein solubility in living cells

Protein misfolding and aggregation is now recognized as a hallmark of numerous human diseases. Standard bioanalytical techniques for monitoring protein aggregation generally rely on small molecules that provide an optical readout of fibril formation. While these methods have been useful for mechanistic studies, additional approaches are required to probe the equilibrium between soluble and insoluble protein within living systems. Such approaches could provide platforms for the identification of inhibitors of protein aggregation as well as a means to investigate the effect of mutations on protein aggregation in model systems. In this chapter, we provide detailed protocols for employing split-NanoLuc luciferase (Nluc) fragments to monitor changes in protein solubility in bacterial and mammalian cells. This sensitive luminesce-based assay can report upon changes in protein solubility induced by inhibitors and disease-relevant mutations.

Quantification of Cellular Proteostasis in Live Cells by Fluorogenic Assay Using the AgHalo Sensor

Proper cellular proteostasis is essential to cellular fitness and viability. Exogenous stress conditions compromise proteostasis and cause aggregation of cellular proteins. We have developed a fluorogenic sensor (AgHalo) to quantify stress-induced proteostasis deficiency. The AgHalo sensor uses a destabilized HaloTag variant to represent aggregation-prone cellular proteins and is equipped with a series of fluorogenic probes that exhibit a fluorescence increase when the sensor forms either soluble oligomers or insoluble aggregates. Herein, we present protocols that describe how the AgHalo sensor can be employed to visualize and quantify proteome stress in live cells using a direct fluorescence read-out and visualization with a fluorescence microplate reader and a microscope. Additionally, protocols for using the AgHalo sensor in combination with fluorogenic probes and commercially available HaloTag probes to enable two-color imaging experiments are described. These protocols will enable use of the AgHalo sensor to visualize and quantify proteostasis in live cells, a task that is difficult to accomplish using previous, always-fluorescent methods. © 2018 by John Wiley & Sons, Inc.

A Split Transcriptional Repressor That Links Protein Solubility to an Orthogonal Genetic Circuit

Monitoring the aggregation of proteins within the cellular environment is key to investigating the molecular mechanisms underlying the formation of off-pathway protein assemblies associated with the development of disease and testing therapeutic strategies to prevent the accumulation of non-native conformations. It remains challenging, however, to couple protein aggregation events underlying the cellular pathogenesis of a disease to genetic circuits and monitor their progression in a quantitative fashion using synthetic biology tools. To link the aggregation propensity of a target protein to the expression of an easily detectable reporter, we investigated the use of a transcriptional AND gate system based on complementation of a split transcription factor. We first identified two-fragment tetracycline repressor (TetR) variants that can be regulated via ligand-dependent induction and demonstrated that split TetR variants can function as transcriptional AND gates in both bacteria and mammalian cells. We then adapted split TetR for use as an aggregation sensor. Protein aggregation was detected by monitoring complementation between a larger TetR fragment that serves as a "detector" and a smaller TetR fragment expressed as a fusion to an aggregation-prone protein that serves as a "sensor" of the target protein aggregation status. This split TetR represents a novel genetic component that can be used for a wide range of applications in bacterial as well as mammalian synthetic biology and a much needed cell-based sensor for monitoring a protein's conformational status in complex cellular environments.

Structural analysis reveals pathomechanisms associated with pseudoxanthoma elasticum-causing mutations in the ABCC6 transporter

Mutations in ABC subfamily C member 6 (ABCC6) transporter are associated with pseudoxanthoma elasticum (PXE), a disease resulting in ectopic mineralization and affecting multiple tissues. A growing number of mutations have been identified in individuals with PXE. For most of these variants, no mechanistic information is available regarding their role in normal and pathophysiologies. To assess how PXE-associated mutations alter ABCC6 biosynthesis and structure, we biophysically and biochemically evaluated the N-terminal nucleotide-binding domain. A high-resolution X-ray structure of nucleotide-binding domain 1 (NBD1) of human ABCC6 was obtained at 2.3 Å that provided a template on which to evaluate PXE-causing mutations. Biochemical analysis of mutations in this domain indicated that multiple PXE-causing mutations altered its structural properties. Analyses of the full-length protein revealed a strong correlation between the alterations in NBD properties and the processing and expression of ABCC6. These results suggest that a significant fraction of PXE-associated mutations located in NBD1 causes changes in its structural properties and that these mutation-induced alterations directly affect the maturation of the full-length ABCC6 protein.

ΔF508-CFTR Modulator Screen Based on Cell Surface Targeting of a Chimeric Nucleotide Binding Domain 1 Reporter

The most common cystic fibrosis-causing mutation in the cystic fibrosis transmembrane conductance regulator (CFTR) is deletion of phenylalanine at residue 508 (∆F508). The ∆F508 mutation impairs folding of nucleotide binding domain 1 (NBD1) and interfacial interactions of NBD1 and the membrane spanning domains. Here, we report a domain-targeted screen to identify ∆F508-CFTR modulators that act on NBD1. A biochemical screen for ΔF508-NBD1 cell surface expression was done in Madin-Darby canine kidney cells expressing a chimeric reporter consisting of ΔF508-NBD1, the CD4 transmembrane domain, and an extracellular horseradish peroxidase (HRP) reporter. Using a luminescence readout of HRP activity, the screen was robust with a Z' factor of 0.7. The screening of ~20,000 synthetic small molecules allowed the identification of compounds from four chemical classes that increased ∆F508-NBD1 cell surface expression by up to 4-fold; for comparison, a 12-fold increased cell surface expression was found for a wild-type NBD1 chimera. While the compounds were inactive as correctors of full-length ΔF508-CFTR, several carboxamide-benzothiophenes had potentiator activity with low micromolar EC₅₀. Interestingly, the potentiators did not activate G551D or wild-type CFTR. Our results provide a proof of concept for a cell-based NBD1 domain screen to identify ∆F508-CFTR modulators that target the NBD1 domain.

Directed evolution to improve protein folding in vivo

Recently, several innovative approaches have been developed that allow one to directly screen or select for improved protein folding in the cellular context. These methods have the potential of not just leading to a better understanding of the in vivo folding process, they may also allow for improved production of proteins of biotechnological interest.

AgHalo: A Facile Fluorogenic Sensor to Detect Drug-Induced Proteome Stress

Drug-induced proteome stress that involves protein aggregation may cause adverse effects and undermine the safety profile of a drug. Safety of drugs is regularly evaluated using cytotoxicity assays that measure cell death. However, these assays provide limited insights into the presence of proteome stress in live cells. A fluorogenic protein sensor is reported to detect drug-induced proteome stress prior to cell death. An aggregation prone Halo-tag mutant (AgHalo) was evolved to sense proteome stress through its aggregation. Detection of such conformational changes was enabled by a fluorogenic ligand that fluoresces upon AgHalo forming soluble aggregates. Using 5 common anticancer drugs, we exemplified detection of differential proteome stress before any cell death was observed. Thus, this sensor can be used to evaluate drug safety in a regime that the current cytotoxicity assays cannot cover and be generally applied to detect proteome stress induced by other toxins.

Stabilization of Nucleotide Binding Domain Dimers Rescues ABCC6 Mutants Associated with Pseudoxanthoma Elasticum

ABC transporters are polytopic membrane proteins that utilize ATP binding and hydrolysis to facilitate transport across biological membranes. Forty-eight human ABC transporters have been identified in the genome, and the majority of these are linked to heritable disease. Mutations in the ABCC6 (ATP binding cassette transporter C6) ABC transporter are associated with pseudoxanthoma elasticum, a disease of altered elastic properties in multiple tissues. Although ∼200 mutations have been identified in pseudoxanthoma elasticum patients, the underlying structural defects associated with the majority of these are poorly understood. To evaluate the structural consequences of these missense mutations, a combination of biophysical and cell biological approaches were applied to evaluate the local and global folding and assembly of the ABCC6 protein. Structural and bioinformatic analyses suggested that a cluster of mutations, representing roughly 20% of the patient population with identified missense mutations, are located in the interface between the transmembrane domain and the C-terminal nucleotide binding domain. Biochemical and cell biological analyses demonstrate these mutations influence multiple steps in the biosynthetic pathway, minimally altering local domain structure but adversely impacting ABCC6 assembly and trafficking. The differential impacts on local and global protein structure are consistent with hierarchical folding and assembly of ABCC6. Stabilization of specific domain-domain interactions via targeted amino acid substitution in the catalytic site of the C-terminal nucleotide binding domain restored proper protein trafficking and cell surface localization of multiple biosynthetic mutants. This rescue provides a specific mechanism by which chemical chaperones could be developed for the correction of ABCC6 biosynthetic defects.

[Construction of the pIRES2-ZsGreen1 eukaryotic expression vector of Factor Ⅸ gene and expression in HEK-293 cells]

Objective: To construct pIRES2-ZsGreen1/F Ⅸ expression vector, using the pcDNA/FⅨ plasmid containing FⅨ cDNA as template, and express in HEK-293 cells. Methods: The total ORF of F Ⅸ gene was amplified from pcDNA/F Ⅸ plasmid, then the amplified fragment was clonded into the pIRES2-ZsGreen1 vector using the Infusion enzyme. The positive clones of eukaryotic expression vector of pIRES2-ZsGreen1/F Ⅸ were screened and expanded after transfection, then were constructed and confirmed by PCR and sequencing. Transient expression experiments were performed using HEK-293 cells transfected with the expression vectors and observed the expression of ZsGreen1 protein by confocal laser microscope. The relative expression levels of FⅨ mRNA, protein and FⅨ activity (FⅨ∶C) were detected by real time PCR (RT-PCR), immunofluorescence microscopy, One-Stage method, respectively. Results: The expression vector, pIRES2-ZsGreen1/F Ⅸ, was successfully constructed and expressed in HEK-293 cells. RT-PCR detected the expression of F Ⅸ mRNA in HEK-293 cells and the immunofluorescence microscopy showed FⅨ protein distributed in the surrounding of nucleus. FⅨ∶C of cell lysates and cell culture fluid transfected with the expression vectors were (92.03 ± 0.29)% and (86.89 ± 8.78)%, respectively; while both F Ⅸ∶C of cell lysates and cell culture fluid transfected with or without the expression vectors were 0. Conclusion: The experimental results showed the expression vector, pIRES2-ZsGreen1/FⅨ, was successfully constructed , which provided experiment basement for the follow study on the location, function and molecular pathology of hemophilia B.

Integrated Modeling and Experimental Approaches to Control Silica Modification of Design Silk-Based Biomaterials

Mineralized polymeric biomaterials provide useful options toward mechanically robust systems for some tissue repairs. Silks as a mechanically robust protein-based material provide a starting point for biomaterial options, particularly when combined with silica toward organic-inorganic hybrid systems. To further understand the interplay between silk proteins and silica related to material properties, we systematically study the role of three key domains in bioengineered spider silk fusion proteins with respect to β-sheet formation and mineralization: (i) a core silk domain for materials assembly, (ii) a histidine tag for purification, and (iii) a silicification domain for mineralization. Computational simulations are used to identify the effect of each domain on the protein folding and accessibility of positively charged amino acids for silicification and predictions are then compared with experimental data. The results show that the addition of the silica and histidine domains reduces β-sheet structure in the materials, and increases solvent-accessible surface area to the positive charged amino acids, leading to higher levels of silica precipitation. Moreover, the simulations show that the location of the charged biomineralization domain has small effect on the protein folding and consequently surface exposure of charged amino acids. Those surfaces display correlation with the amount of silicification in experiments. The results demonstrate that the exposure of the positively charged amino acids impacts protein function related to mineralization. In addition, processing parameters (solvating agent, the method of β-sheet induction and temperature) affect protein secondary structure, folding and function. This integrated modeling and experimental approach provides insight into sequence-structure-function relationships for control of mineralized protein biomaterial structures.

Self-Assembling NanoLuc Luciferase Fragments as Probes for Protein Aggregation in Living Cells

Given the clear role of protein aggregation in human disease, there is a critical need for assays capable of quantifying protein aggregation in living systems. We hypothesized that the inherently low background and biocompatibility of luminescence signal readouts could provide a potential solution to this problem. Herein, we describe a set of self-assembling NanoLuc luciferase (Nluc) fragments that produce a tunable luminescence readout that is dependent upon the solubility of a target protein fused to the N-terminal Nluc fragment. To demonstrate this approach, we employed this assay in bacteria to assess mutations known to disrupt amyloid-beta (Aβ) aggregation as well as disease-relevant mutations associated with familial Alzheimer's diseases. The luminescence signal from these experiments correlates with the reported aggregation potential of these Aβ mutants and reinforces the increased aggregation potential of disease-relevant mutations in Aβ1-42. To further demonstrate the utility of this approach, we show that the effect of small molecule inhibitors on Aβ aggregation can be monitored using this system. In addition, we demonstrate that aggregation assays can be ported into mammalian cells. Taken together, these results indicate that this platform could be used to rapidly screen for mutations that influence protein aggregation as well as inhibitors of protein aggregation. This method offers a novel, genetically encodable luminescence readout of protein aggregation in living cells.

Thermostable artificial enzyme isolated by in vitro selection

Artificial enzymes hold the potential to catalyze valuable reactions not observed in nature. One approach to build artificial enzymes introduces mutations into an existing protein scaffold to enable a new catalytic activity. This process commonly results in a simultaneous reduction of protein stability as an undesired side effect. While protein stability can be increased through techniques like directed evolution, care needs to be taken that added stability, conversely, does not sacrifice the desired activity of the enzyme. Ideally, enzymatic activity and protein stability are engineered simultaneously to ensure that stable enzymes with the desired catalytic properties are isolated. Here, we present the use of the in vitro selection technique mRNA display to isolate enzymes with improved stability and activity in a single step. Starting with a library of artificial RNA ligase enzymes that were previously isolated at ambient temperature and were therefore mostly mesophilic, we selected for thermostable active enzyme variants by performing the selection step at 65°C. The most efficient enzyme, ligase 10C, was not only active at 65°C, but was also an order of magnitude more active at room temperature compared to related enzymes previously isolated at ambient temperature. Concurrently, the melting temperature of ligase 10C increased by 35 degrees compared to these related enzymes. While low stability and solubility of the previously selected enzymes prevented a structural characterization, the improved properties of the heat-stable ligase 10C finally allowed us to solve the three-dimensional structure by NMR. This artificial enzyme adopted an entirely novel fold that has not been seen in nature, which was published elsewhere. These results highlight the versatility of the in vitro selection technique mRNA display as a powerful method for the isolation of thermostable novel enzymes.

Heterologous fermentation of a diterpene from Alternaria brassisicola

A variety of different applications render terpenes and terpenoids attractive research targets. A promising but so far insufficiently explored family of terpenoids are the fusicoccanes that comprise a characteristic 5-8-5 fused tricyclic ring system. Besides herbicidal effects, these compounds also show apoptotic and anti-tumour effects on mammalian cells. The access to fusicoccanes from natural sources is scarce. Recently, we introduced a metabolically engineered Saccharomyces cerevisiae strain to enable the heterologous fermentation of the shared fusicoccane-diterpenoid precursor, fusicocca-2,10(14)-diene. Here, we show experiments towards the identification of bottlenecks in this process. The suppression of biosynthetic by-products via medium optimisation was found to be an important aspect. In addition, the fermentation process seems to be improved under oxygen limitation conditions. Under fed-batch conditions, the fermentation yield was reproducibly increased to approximately 20 mg/L. Furthermore, the impact of the properties of the terpene synthase on the fermentation yield is discussed, and the preliminary studies on the engineering of this key enzyme are presented.

Directed evolution of human heavy chain variable domain (VH) using in vivo protein fitness filter

Human immunoglobulin heavy chain variable domains (VH) are promising scaffolds for antigen binding. However, VH is an unstable and aggregation-prone protein, hindering its use for therapeutic purposes. To evolve the VH domain, we performed in vivo protein solubility selection that linked antibiotic resistance to the protein folding quality control mechanism of the twin-arginine translocation pathway of E. coli. After screening a human germ-line VH library, 95% of the VH proteins obtained were identified as VH3 family members; one VH protein, MG2x1, stood out among separate clones expressing individual VH variants. With further screening of combinatorial framework mutation library of MG2x1, we found a consistent bias toward substitution with tryptophan at the position of 50 and 58 in VH. Comparison of the crystal structures of the VH variants revealed that those substitutions with bulky side chain amino acids filled the cavity in the VH interface between heavy and light chains of the Fab arrangement along with the increased number of hydrogen bonds, decreased solvation energy, and increased negative charge. Accordingly, the engineered VH acquires an increased level of thermodynamic stability, reversible folding, and soluble expression. The library built with the VH variant as a scaffold was qualified as most of VH clones selected randomly were expressed as soluble form in E. coli regardless length of the combinatorial CDR. Furthermore, a non-aggregation feature of the selected VH conferred a free of humoral response in mice, even when administered together with adjuvant. As a result, this selection provides an alternative directed evolution pathway for unstable proteins, which are distinct from conventional methods based on the phage display.

Modulating protein stability - directed evolution strategies for improved protein function

Protein engineering is widely used to generate proteins with novel or enhanced function. However, manipulating protein function in the laboratory can prove laborious, protracted and challenging. Recent developments in the understanding of protein evolutionary dynamics have unveiled the full extent by which the evolution of function is limited by protein stability - a revelation that may be applied to protein engineering on a whole. Thus, strategies that modulate protein stability and reduce its constraining effects may facilitate the engineering of protein function. A combinatorial approach involving the introduction of compensatory mutations and manipulation of the stability threshold by chaperone buffering during directed evolution can improve the functional adaptation of a protein, thereby fostering our ability to attain ever-more ambitious protein functions in the laboratory.

Screening libraries for improved solubility: using E. coli dihydrofolate reductase as a reporter

Low protein solubility is a problem in many areas of protein science. Although chemical methods have been developed to solubilize proteins these are not always effective and add to the cost of producing the protein. One way of overcoming these difficulties is to evolve the protein to be more soluble. A major hurdle in this process is the ability to select mutant proteins with enhanced solubility from a large library of randomly mutated proteins. In this article, we describe such a method. The method relies on the fact that increasing the expression of dihydrofolate reductase (DHFR) makes Escherichia coli resistant to Trimethoprim (TMP). Proteins fused to DHFR will produce chimeras with altered levels of resistance to TMP. This variation in TMP resistance can be used to identify mutant proteins with enhanced solubility.

Chaperone discovery

Molecular chaperones assist de novo protein folding and facilitate the refolding of stress-denatured proteins. The molecular chaperone concept was coined nearly 35 years ago, and since then, tremendous strides have been made in understanding how these factors support protein folding. Here, we focus on how various chaperone proteins were first identified to play roles in protein folding. Examples are used to illustrate traditional routes of chaperone discovery and point out their advantages and limitations. Recent advances, including the development of folding biosensors and promising methods for the stabilization of proteins in vivo, provide new routes for chaperone discovery.

Rapid profiling of disease alleles using a tunable reporter of protein misfolding

Many human diseases are caused by genetic mutations that decrease protein stability. Such mutations may not specifically affect an active site, but can alter protein folding, abundance, or localization. Here we describe a high-throughput cell-based stability assay, IDESA (intra-DHFR enzyme stability assay), where stability is coupled to cell proliferation in the model yeast, Saccharomyces cerevisiae. The assay requires no prior knowledge of a protein's structure or activity, allowing the assessment of stability of proteins that have unknown or difficult to characterize activities, and we demonstrate use with a range of disease-relevant targets, including human alanine:glyoxylate aminotransferase (AGT), superoxide dismutase (SOD-1), DJ-1, p53, and SMN1. The assay can be carried out on hundreds of disease alleles in parallel or used to identify stabilizing small molecules (pharmacological chaperones) for unstable alleles. As demonstration of the general utility of this assay, we analyze stability of disease alleles of AGT, deficiency of which results in the kidney stone disease, primary hyperoxaluria type I, identifying mutations that specifically affect the protein-active site chemistry.

The ribosomal exit tunnel as a target for optimizing protein expression in Escherichia coli

The folding of many cellular proteins occurs co-translationally immediately outside the ribosome exit tunnel, where ribosomal proteins and other associated factors coordinate the synthesis and folding of newly translated polypeptides. Here, we show that the large subunit protein L29, which forms part of the exit tunnel in Escherichia coli, is required for the productive synthesis of an array of structurally diverse recombinant proteins including the green fluorescent protein (GFP) and an intracellular single-chain Fv antibody. Surprisingly, the corresponding mRNA transcript level of these proteins was markedly less abundant in cells lacking L29, suggesting an unexpected regulatory mechanism that links defects in the exit tunnel to the expression of genetic information. To further highlight the importance of L29 in maintaining protein expression, we used mutagenesis and selection to obtain L29 variants that enhanced GFP expression. Overall, our results suggest that the ribosomal exit tunnel proteins may be key targets for optimizing the overproduction of active, structurally complex recombinant proteins in bacterial cells.

Techniques for Monitoring Protein Misfolding and Aggregation in Vitro and in Living Cells

Protein misfolding and aggregation have been considered important in understanding many neurodegenerative diseases and recombinant biopharmaceutical production. Therefore, various traditional and modern techniques have been utilized to monitor protein aggregation in vitro and in living cells. Fibril formation, morphology and secondary structure content of amyloidogenic proteins in vitro have been monitored by molecular probes, TEM/AFM, and CD/FTIR analyses, respectively. Protein aggregation in living cells has been qualitatively or quantitatively monitored by numerous molecular folding reporters based on either fluorescent protein or enzyme. Aggregation of a target protein is directly correlated to the changes in fluorescence or enzyme activity of the folding reporter fused to the target protein, which allows non-invasive monitoring aggregation of the target protein in living cells. Advances in the techniques used to monitor protein aggregation in vitro and in living cells have greatly facilitated the understanding of the molecular mechanism of amyloidogenic protein aggregation associated with neurodegenerative diseases, optimizing culture conditions to reduce aggregation of biopharmaceuticals expressed in living cells, and screening of small molecule libraries in the search for protein aggregation inhibitors.

TDP-43 identified from a genome wide RNAi screen for SOD1 regulators

Amyotrophic Lateral Sclerosis (ALS) is a late-onset, progressive neurodegenerative disease affecting motor neurons in the brain stem and spinal cord leading to loss of voluntary muscular function and ultimately, death due to respiratory failure. A subset of ALS cases are familial and associated with mutations in superoxide dismutase 1 (SOD1) that destabilize the protein and predispose it to aggregation. In spite of the fact that sporadic and familial forms of ALS share many common patho-physiological features, the mechanistic relationship between SOD1-associated and sporadic forms of the disease if any, is not well understood. To better understand any molecular connections, a cell-based protein folding assay was employed to screen a whole genome RNAi library for genes that regulate levels of soluble SOD1. Statistically significant hits that modulate SOD1 levels, when analyzed by pathway analysis revealed a highly ranked network containing TAR DNA binging protein (TDP-43), a major component of aggregates characteristic of sporadic ALS. Biochemical experiments confirmed the action of TDP-43 on SOD1. These results highlight an unexpected relationship between TDP-43 and SOD1 which may have implications in disease pathogenesis.

Requirements for efficient correction of ΔF508 CFTR revealed by analyses of evolved sequences

Misfolding of ΔF508 cystic fibrosis (CF) transmembrane conductance regulator (CFTR) underlies pathology in most CF patients. F508 resides in the first nucleotide-binding domain (NBD1) of CFTR near a predicted interface with the fourth intracellular loop (ICL4). Efforts to identify small molecules that restore function by correcting the folding defect have revealed an apparent efficacy ceiling. To understand the mechanistic basis of this obstacle, positions statistically coupled to 508, in evolved sequences, were identified and assessed for their impact on both NBD1 and CFTR folding. The results indicate that both NBD1 folding and interaction with ICL4 are altered by the ΔF508 mutation and that correction of either individual process is only partially effective. By contrast, combination of mutations that counteract both defects restores ΔF508 maturation and function to wild-type levels. These results provide a mechanistic rationale for the limited efficacy of extant corrector compounds and suggest approaches for identifying compounds that correct both defective steps.

A screen to identify cellular modulators of soluble levels of an amyotrophic lateral sclerosis (ALS)-causing mutant SOD1

The molecular pathology of many protein misfolding, toxic gain-of-function diseases, such as amyotrophic lateral sclerosis (ALS), is not well understood. Although protein misfolding and aggregation are common themes in these diseases, efforts to identify cellular factors that regulate this process in an unbiased fashion and on a global scale have been lacking. Using an adapted version of an extant β-gal-based protein solubility assay, an expression screen for cellular modulators of solubility of an ALS-causing mutant SOD1 was carried out in mammalian cells. Following fluorescence-activated cell sorting enrichment of a mouse spinal cord cDNA library for gene products that increased SOD1 solubility, high-throughput screening of the cDNA pools from this enriched fraction was employed to identify pools containing relevant modulators. Positive pools, containing approximately 10 cDNA clones each, were diluted and rescreened iteratively until individual clones that improved SOD1 folding/solubility were identified. Genes with profound effects in the solubility assay were selected for validation by independent biochemical assays. Six of 10 validated genes had a significant effect on SOD1 solubility and folding in a SOD1 promoter-driven β-gal assay, indicating that global screening of cellular targets using such protein solubility/folding assay is viable and can be adapted for other misfolding diseases.

Bioprocess monitoring by marker gene analysis

The optimization and the scale up of industrial fermentation processes require an efficient and possibly comprehensive analysis of the physiology of the production system throughout the process development. Furthermore, to ensure a good quality control of established bioprocesses, on-line analysis techniques for the determination of marker gene expression are of interest to monitor the productivity and the safety of bioprocesses. A prerequisite for such analyses is the knowledge of genes, the expression of which is critical either for the productivity or for the performance of the bioprocess. This work reviews marker genes that are specific indicators for stress- and nutrient-limitation conditions or for the physiological status of the bacterial production hosts Bacillus subtilis, Bacillus licheniformis and Escherichia coli. The suitability of existing gene expression analysis techniques for bioprocess monitoring is discussed. Analytical approaches that enable a robust and sensitive determination of selected marker mRNAs or proteins are presented.

Large-scale experimental studies show unexpected amino acid effects on protein expression and solubility in vivo in E. coli

The biochemical and physical factors controlling protein expression level and solubility in vivo remain incompletely characterized. To gain insight into the primary sequence features influencing these outcomes, we performed statistical analyses of results from the high-throughput protein-production pipeline of the Northeast Structural Genomics Consortium. Proteins expressed in E. coli and consistently purified were scored independently for expression and solubility levels. These parameters nonetheless show a very strong positive correlation. We used logistic regressions to determine whether they are systematically influenced by fractional amino acid composition or several bulk sequence parameters including hydrophobicity, sidechain entropy, electrostatic charge, and predicted backbone disorder. Decreasing hydrophobicity correlates with higher expression and solubility levels, but this correlation apparently derives solely from the beneficial effect of three charged amino acids, at least for bacterial proteins. In fact, the three most hydrophobic residues showed very different correlations with solubility level. Leu showed the strongest negative correlation among amino acids, while Ile showed a slightly positive correlation in most data segments. Several other amino acids also had unexpected effects. Notably, Arg correlated with decreased expression and, most surprisingly, solubility of bacterial proteins, an effect only partially attributable to rare codons. However, rare codons did significantly reduce expression despite use of a codon-enhanced strain. Additional analyses suggest that positively but not negatively charged amino acids may reduce translation efficiency in E. coli irrespective of codon usage. While some observed effects may reflect indirect evolutionary correlations, others may reflect basic physicochemical phenomena. We used these results to construct and validate predictors of expression and solubility levels and overall protein usability, and we propose new strategies to be explored for engineering improved protein expression and solubility.

Filtering "genic" open reading frames from genomic DNA samples for advanced annotation

Background

In order to carry out experimental gene annotation, DNA encoding open reading frames (ORFs) derived from real genes (termed "genic") in the correct frame is required. When genes are correctly assigned, isolation of genic DNA for functional annotation can be carried out by PCR. However, not all genes are correctly assigned, and even when correctly assigned, gene products are often incorrectly folded when expressed in heterologous hosts. This is a problem that can sometimes be overcome by the expression of protein fragments encoding domains, rather than full-length proteins. One possible method to isolate DNA encoding such domains would to "filter" complex DNA (cDNA libraries, genomic and metagenomic DNA) for gene fragments that confer a selectable phenotype relying on correct folding, with all such domains present in a complex DNA sample, termed the “domainome”.

Results

In this paper we discuss the preparation of diverse genic ORF libraries from randomly fragmented genomic DNA using ß-lactamase to filter out the open reading frames. By cloning DNA fragments between leader sequences and the mature ß-lactamase gene, colonies can be selected for resistance to ampicillin, conferred by correct folding of the lactamase gene. Our experiments demonstrate that the majority of surviving colonies contain genic open reading frames, suggesting that ß-lactamase is acting as a selectable folding reporter. Furthermore, different leaders (Sec, TAT and SRP), normally translocating different protein classes, filter different genic fragment subsets, indicating that their use increases the fraction of the “domainone” that is accessible.

Conclusions

The availability of ORF libraries, obtained with the filtering method described here, combined with screening methods such as phage display and protein-protein interaction studies, or with protein structure determination projects, can lead to the identification and structural determination of functional genic ORFs. ORF libraries represent, moreover, a useful tool to proceed towards high-throughput functional annotation of newly sequenced genomes.

Strain engineering for improved expression of recombinant proteins in bacteria

Protein expression in Escherichia coli represents the most facile approach for the preparation of non-glycosylated proteins for analytical and preparative purposes. So far, the optimization of recombinant expression has largely remained a matter of trial and error and has relied upon varying parameters, such as expression vector, media composition, growth temperature and chaperone co-expression. Recently several new approaches for the genome-scale engineering of E. coli to enhance recombinant protein expression have been developed. These methodologies now enable the generation of optimized E. coli expression strains in a manner analogous to metabolic engineering for the synthesis of low-molecular-weight compounds. In this review, we provide an overview of strain engineering approaches useful for enhancing the expression of hard-to-produce proteins, including heterologous membrane proteins.

Scoring function to predict solubility mutagenesis

BACKGROUND

Mutagenesis is commonly used to engineer proteins with desirable properties not present in the wild type (WT) protein, such as increased or decreased stability, reactivity, or solubility. Experimentalists often have to choose a small subset of mutations from a large number of candidates to obtain the desired change, and computational techniques are invaluable to make the choices. While several such methods have been proposed to predict stability and reactivity mutagenesis, solubility has not received much attention.

RESULTS

We use concepts from computational geometry to define a three body scoring function that predicts the change in protein solubility due to mutations. The scoring function captures both sequence and structure information. By exploring the literature, we have assembled a substantial database of 137 single- and multiple-point solubility mutations. Our database is the largest such collection with structural information known so far. We optimize the scoring function using linear programming (LP) methods to derive its weights based on training. Starting with default values of 1, we find weights in the range [0,2] so that predictions of increase or decrease in solubility are optimized. We compare the LP method to the standard machine learning techniques of support vector machines (SVM) and the Lasso. Using statistics for leave-one-out (LOO), 10-fold, and 3-fold cross validations (CV) for training and prediction, we demonstrate that the LP method performs the best overall. For the LOOCV, the LP method has an overall accuracy of 81%.

AVAILABILITY

Executables of programs, tables of weights, and datasets of mutants are available from the following web page: http://www.wsu.edu/~kbala/OptSolMut.html.

Novel fluorescence-assisted whole-cell assay for engineering and characterization of proteases and their substrates

We have developed a sensitive and highly efficient whole-cell methodology for quantitative analysis and screening of protease activity in vivo. The method is based on the ability of a genetically encoded protease to rescue a coexpressed short-lived fluorescent substrate reporter from cytoplasmic degradation and thereby confer increased whole-cell fluorescence in proportion to the protease's apparent activity in the Escherichia coli cytoplasm. We demonstrated that this system can reveal differences in the efficiency with which tobacco etch virus (TEV) protease processes different substrate peptides. In addition, when analyzing E. coli cells expressing TEV protease variants that differed in terms of their in vivo solubility, cells containing the most-soluble protease variant exhibited the highest fluorescence intensity. Furthermore, flow cytometry screening allowed for enrichment and subsequent identification of an optimal substrate peptide and protease variant from a large excess of cells expressing suboptimal variants (1:100,000). Two rounds of cell sorting resulted in a 69,000-fold enrichment and a 22,000-fold enrichment of the superior substrate peptide and protease variant, respectively. Our approach presents a new promising path forward for high-throughput substrate profiling of proteases, engineering of novel protease variants with desired properties (e.g., altered substrate specificity and improved solubility and activity), and identification of protease inhibitors.

A rapid protein folding assay for the bacterial periplasm

An array of genetic screens and selections has been developed for reporting protein folding and solubility in the cytoplasm of living cells. However, there are currently no analogous folding assays for the bacterial periplasm, despite the significance of this compartment for the expression of recombinant proteins, especially those requiring important posttranslational modifications (e.g., disulfide bond formation). Here, we describe an engineered genetic selection for monitoring protein folding in the periplasmic compartment of Escherichia coli cells. In this approach, target proteins are sandwiched between an N-terminal signal recognition particle (SRP)-dependent signal peptide and a C-terminal selectable marker, TEM-1 beta-lactamase. The resulting chimeras are localized to the periplasmic space via the cotranslational SRP pathway. Using a panel of native and heterologous proteins, we demonstrate that the folding efficiency of various target proteins correlates directly with in vivo beta-lactamase activity and thus resistance to ampicillin. We also show that this reporter is useful for the discovery of extrinsic periplasmic factors (e.g., chaperones) that affect protein folding and for obtaining folding-enhanced proteins via directed evolution. Collectively, these data demonstrate that our periplasmic folding reporter is a powerful tool for screening and engineering protein folding in a manner that does not require any structural or functional information about the target protein.

The cystic fibrosis-causing mutation deltaF508 affects multiple steps in cystic fibrosis transmembrane conductance regulator biogenesis

The deletion of phenylalanine 508 in the first nucleotide binding domain of the cystic fibrosis transmembrane conductance regulator is directly associated with >90% of cystic fibrosis cases. This mutant protein fails to traffic out of the endoplasmic reticulum and is subsequently degraded by the proteasome. The effects of this mutation may be partially reversed by the application of exogenous osmolytes, expression at low temperature, and the introduction of second site suppressor mutations. However, the specific steps of folding and assembly of full-length cystic fibrosis transmembrane conductance regulator (CFTR) directly altered by the disease-causing mutation are unclear. To elucidate the effects of the ΔF508 mutation, on various steps in CFTR folding, a series of misfolding and suppressor mutations in the nucleotide binding and transmembrane domains were evaluated for effects on the folding and maturation of the protein. The results indicate that the isolated NBD1 responds to both the ΔF508 mutation and intradomain suppressors of this mutation. In addition, identification of a novel second site suppressor of the defect within the second transmembrane domain suggests that ΔF508 also effects interdomain interactions critical for later steps in the biosynthesis of CFTR.