Experimental support for the foldability–function tradeoff hypothesis: Segregation of the folding nucleus and functional regions in fibroblast growth factor‐1

The acquisition of function is often associated with destabilizing mutations, giving rise to the stability–function tradeoff hypothesis. To test whether function is also accommodated at the expense of foldability, fibroblast growth factor‐1 (FGF‐1) was subjected to a comprehensive φ‐value analysis at each of the 11 turn regions. FGF‐1, a β‐trefoil fold, represents an excellent model system with which to evaluate the influence of function on foldability: because of its threefold symmetric structure, analysis of FGF‐1 allows for direct comparisons between symmetry‐related regions of the protein that are associated with function to those that are not; thus, a structural basis for regions of foldability can potentially be identified. The resulting φ‐value distribution of FGF‐1 is highly polarized, with the majority of positions described as either folded‐like or denatured‐like in the folding transition state. Regions important for folding are shown to be asymmetrically distributed within the protein architecture; furthermore, regions associated with function (i.e., heparin‐binding affinity and receptor‐binding affinity) are localized to regions of the protein that fold after barrier crossing (late in the folding pathway). These results provide experimental support for the foldability–function tradeoff hypothesis in the evolution of FGF‐1. Notably, the results identify the potential for folding redundancy in symmetric protein architecture with important implications for protein evolution and design.     

Translation‐coupled protein folding assay using a protease to monitor the folding status

Protein folding is an essential prerequisite for proteins to execute nearly all cellular functions. There is a growing demand for a simple and robust method to investigate protein folding on a large‐scale under the same conditions. We previously developed a global folding assay system, in which proteins translated using an Escherichia coli‐based cell‐free translation system are centrifuged to quantitate the supernatant fractions. Although the assay is based on the assumption that the supernatants contain the folded native states, the supernatants also include nonnative unstructured proteins. In general, proteases recognize and degrade unstructured proteins, and thus we used a protease to digest the unstructured regions to monitor the folding status. The addition of Lon protease during the translation of proteins unmasked subfractions, not only in the soluble fractions but also in the aggregation‐prone fractions. We translated ～90 E. coli proteins in the protease‐inclusion assay, in the absence and presence of chaperones. The folding assay, which sheds light on the molecular mechanisms underlying the aggregate formation and the chaperone effects, can be applied to a large‐scale analysis.     

Oligomerization of a symmetric β‐trefoil protein in response to folding nucleus perturbation

Gene duplication and fusion events in protein evolution are postulated to be responsible for the common protein folds exhibiting internal rotational symmetry. Such evolutionary processes can also potentially yield regions of repetitive primary structure. Repetitive primary structure offers the potential for alternative definitions of critical regions, such as the folding nucleus (FN). In principle, more than one instance of the FN potentially enables an alternative folding pathway in the face of a subsequent deleterious mutation. We describe the targeted mutation of the carboxyl‐terminal region of the (internally located) FN of the de novo designed purely‐symmetric β‐trefoil protein Symfoil‐4P. This mutation involves wholesale replacement of a repeating trefoil‐fold motif with a “blade” motif from a β‐propeller protein, and postulated to trap that region of the Symfoil‐4P FN in a nonproductive folding intermediate. The resulting protein (termed “Bladefoil”) is shown to be cooperatively folding, but as a trimeric oligomer. The results illustrate how symmetric protein architectures have potentially diverse folding alternatives available to them, including oligomerization, when preferred pathways are perturbed.     

Kinetically trapped metastable intermediate of a disulfide‐deficient mutant of the starch‐binding domain of glucoamylase

Refolding of a thermally unfolded disulfide‐deficient mutant of the starch‐binding domain of glucoamylase was investigated using differential scanning calorimetry, isothermal titration calorimetry, CD, and ¹H NMR. When the protein solution was rapidly cooled from a higher temperature, a kinetic intermediate was formed during refolding. The intermediate was unexpectedly stable compared with typical folding intermediates that have short half‐lives. It was shown that this intermediate contained substantial secondary structure and tertiary packing and had the same binding ability with β‐cyclodextrin as the native state, suggesting that the intermediate is highly‐ordered and native‐like on the whole. These characteristics differ from those of partially folded intermediates such as molten globule states. Far‐UV CD spectra showed that the secondary structure was once disrupted during the transition from the intermediate to the native state. These results suggest that the intermediate could be an off‐pathway type, possibly a misfolded state, that has to undergo unfolding on its way to the native state.     

Phosphorylation of the FUS low‐complexity domain disrupts phase separation,aggregation, and toxicity

Neuronal inclusions of aggregated RNA‐binding protein fused in sarcoma (FUS) are hallmarks of ALS and frontotemporal dementia subtypes. Intriguingly, FUS's nearly uncharged, aggregation‐prone, yeast prion‐like, low sequence‐complexity domain (LC) is known to be targeted for phosphorylation. Here we map in vitro and in‐cell phosphorylation sites across FUS LC. We show that both phosphorylation and phosphomimetic variants reduce its aggregation‐prone/prion‐like character, disrupting FUS phase separation in the presence of RNA or salt and reducing FUS propensity to aggregate. Nuclear magnetic resonance spectroscopy demonstrates the intrinsically disordered structure of FUS LC is preserved after phosphorylation; however, transient domain collapse and self‐interaction are reduced by phosphomimetics. Moreover, we show that phosphomimetic FUS reduces aggregation in human and yeast cell models, and can ameliorate FUS‐associated cytotoxicity. Hence, post‐translational modification may be a mechanism by which cells control physiological assembly and prevent pathological protein aggregation, suggesting a potential treatment pathway amenable to pharmacologic modulation.     

Transient sampling of aggregation‐prone conformations causes pathogenic instability of a parkinsonian mutant of DJ‐1 at physiological temperature

Various missense mutations in the cytoprotective protein DJ‐1 cause rare forms of inherited parkinsonism. One mutation, M26I, diminishes DJ‐1 protein levels in the cell but does not result in large changes in the three‐dimensional structure or thermal stability of the protein. Therefore, the molecular defect that results in loss of M26I DJ‐1 protective function is unclear. Using NMR spectroscopy near physiological temperature, we found that the picosecond–nanosecond dynamics of wild‐type and M26I DJ‐1 are similar. In contrast, elevated amide hydrogen/deuterium exchange rates indicate that M26I DJ‐1 is more flexible than the wild‐type protein on longer timescales and that hydrophobic regions of M26I DJ‐1 are transiently exposed to solvent. Tryptophan fluorescence spectroscopy and thiol crosslinking analyzed by mass spectrometry also demonstrate that M26I DJ‐1 samples conformations that differ from the wild‐type protein at 37°C. These transiently sampled conformations are unstable and cause M26I DJ‐1 to aggregate in vitro at physiological temperature but not at lower temperatures. M26I DJ‐1 aggregation is correlated with pathogenicity, as the structurally similar but non‐pathogenic M26L mutation does not aggregate at 37°C. The onset of dynamically driven M26I DJ‐1 instability at physiological temperature resolves conflicting literature reports about the behavior of this disease‐associated mutant and illustrates the pitfalls of characterizing proteins exclusively at room temperature or below, as key aspects of their behavior may not be apparent.     

Direct prediction of profiles of sequences compatible with a protein structure by neural networks with fragment‐based local and energy‐based nonlocal profiles

Locating sequences compatible with a protein structural fold is the well‐known inverse protein‐folding problem. While significant progress has been made, the success rate of protein design remains low. As a result, a library of designed sequences or profile of sequences is currently employed for guiding experimental screening or directed evolution. Sequence profiles can be computationally predicted by iterative mutations of a random sequence to produce energy‐optimized sequences, or by combining sequences of structurally similar fragments in a template library. The latter approach is computationally more efficient but yields less accurate profiles than the former because of lacking tertiary structural information. Here we present a method called SPIN that predicts Sequence Profiles by Integrated Neural network based on fragment‐derived sequence profiles and structure‐derived energy profiles. SPIN improves over the fragment‐derived profile by 6.7% (from 23.6 to 30.3%) in sequence identity between predicted and wild‐type sequences. The method also reduces the number of residues in low complex regions by 15.7% and has a significantly better balance of hydrophilic and hydrophobic residues at protein surface. The accuracy of sequence profiles obtained is comparable to those generated from the protein design program RosettaDesign 3.5. This highly efficient method for predicting sequence profiles from structures will be useful as a single‐body scoring term for improving scoring functions used in protein design and fold recognition. It also complements protein design programs in guiding experimental design of the sequence library for screening and directed evolution of designed sequences. The SPIN server is available at http://sparks‐lab.org . Proteins 2014; 82:2565–2573. © 2014 Wiley Periodicals, Inc.     

Flexible connection of the N‐terminal domain in ClpB modulates substrate binding and the aggregate reactivation efficiency

ClpB reactivates aggregated proteins in cooperation with DnaK/J. The ClpB monomer contains two nucleotide‐binding domains (D1, D2), a coiled‐coil domain, and an N‐terminal domain attached to D1 with a 17‐residue‐long unstructured linker containing a Gly‐Gly motif. The ClpB‐mediated protein disaggregation is linked to translocation of substrates through the central channel in the hexameric ClpB, but the events preceding the translocation are poorly understood. The N‐terminal domains form a ring surrounding the entrance to the channel and contribute to the aggregate binding. It was suggested that the N‐terminal domain's mobility that is maintained by the unstructured linker might control the efficiency of aggregate reactivation. We produced seven variants of ClpB with modified sequence of the N‐terminal linker. To increase the linker's conformational flexibility, we inserted up to four Gly next to the GG motif. To decrease the linker's flexibility, we deleted the GG motif and converted it into GP and PP. We found that none of the linker modifications inhibited the basal ClpB ATPase activity or its capability to form oligomers. However, the modified linker ClpB variants showed lower reactivation rates for aggregated glucose‐6‐phosphate dehydrogenase and firefly luciferase and a lower aggregate‐binding efficiency than wt ClpB. We conclude that the linker does not merely connect the N‐terminal domain, but it supports the chaperone activity of ClpB by contributing to the efficiency of aggregate binding and disaggregation. Moreover, our results suggest that selective pressure on the linker sequence may be crucial for maintaining the optimal efficiency of aggregate reactivation by ClpB. Proteins 2012; © 2012 Wiley Periodicals, Inc.     

A tale of two tails: The importance of unstructured termini in the aggregation pathway of β2‐microglobulin

The identification of intermediate states for folding and aggregation is important from a fundamental standpoint and for the design of novel therapeutic strategies targeted at conformational disorders. Protein human β2‐microglobulin (HB2m) is classically associated with dialysis‐related amyloidosis, but the single point mutant D76N was recently identified as the causative agent of a hereditary systemic amyloidosis affecting visceral organs. Here, we use D76N as a model system to explore the early stage of the aggregation mechanism of HB2m by means of an integrative approach framed on molecular simulations. Discrete molecular dynamics simulations of a structured‐based model predict the existence of two intermediate states populating the folding landscape. The intermediate I₁ features an unstructured C‐terminus, while I₂, which is exclusively populated by the mutant, exhibits two unstructured termini. Docking simulations indicate that I₂ is the key species for aggregation at acidic and physiological pH contributing to rationalize the higher amyloidogenic potential of D76N relative to the wild‐type protein and the ΔN6 variant. The analysis carried out here recapitulates the importance of the DE‐loop in HB2m self‐association at a neutral pH and predicts a leading role of the C‐terminus and the adjacent G‐strand in the dimerization process under acidic conditions. The identification of aggregation hot‐spots is in line with experimental results that support the importance of Phe56, Asp59, Trp60, Phe62, Tyr63, and Tyr66 in HB2m amyloidogenesis. We further predict the involvement of new residues such as Lys94 and Trp95 in the aggregation process.     

Mechanism of formation of the C‐terminal β‐hairpin of the B3 domain of the immunoglobulin‐binding protein G from Streptococcus. IV. Implication for the mechanism of folding of the parent protein

A 34‐residue α/β peptide [IG(28–61)], derived from the C‐terminal part of the B3 domain of the immunoglobulin binding protein G from Streptoccocus, was studied using CD and NMR spectroscopy at various temperatures and by differential scanning calorimetry. It was found that the C‐terminal part (a 16‐residue‐long fragment) of this peptide, which corresponds to the sequence of the β‐hairpin in the native structure, forms structure similar to the β‐hairpin only at T = 313 K, and the structure is stabilized by non‐native long‐range hydrophobic interactions (Val47–Val59). On the other hand, the N‐terminal part of IG(28–61), which corresponds to the middle α‐helix in the native structure, is unstructured at low temperature (283 K) and forms an α‐helix‐like structure at 305 K, and only one helical turn is observed at 313 K. At all temperatures at which NMR experiments were performed (283, 305, and 313 K), we do not observe any long‐range connectivities which would have supported packing between the C‐terminal (β‐hairpin) and the N‐terminal (α‐helix) parts of the sequence. Such interactions are absent, in contrast to the folding pathway of the B domain of protein G, proposed recently by Kmiecik and Kolinski (Biophys J 2008, 94, 726–736), based on Monte‐Carlo dynamics studies. Alternative folding mechanisms are proposed and discussed. © 2010 Wiley Periodicals, Inc. Biopolymers 93: 469–480, 2010. This article was originally published online as an accepted preprint. The “Published Online” date corresponds to the preprint version. You can request a copy of the preprint by emailing the Biopolymers editorial office at biopolymers@wiley.com     

Role of the Disulfide Bond in Prion Protein Amyloid Formation: A Thermodynamic and Kinetic Analysis

Prion diseases are associated with the structural conversion of prion protein (PrP) to a β-sheet-rich aggregate, PrP^Sc. Previous studies have indicated that a reduction of the disulfide bond linking C179 and C214 of PrP yields an amyloidlike β-rich aggregate in vitro. To gain mechanistic insights into the reduction-induced aggregation, here I characterized how disulfide bond reduction modulates the protein folding/misfolding landscape of PrP, by examining 1) the equilibrium stabilities of the native (N) and aggregated states relative to the unfolded (U) state, 2) the transition barrier separating the U and aggregated states, and 3) the final structure of amyloidlike misfolded aggregates. Kinetic and thermodynamic experiments revealed that disulfide bond reduction decreases the equilibrium stabilities of both the N and aggregated states by ～3 kcal/mol, without changing either the amyloidlike aggregate structure, at least at the secondary structural level, or the transition barrier of aggregation. Therefore, disulfide bond reduction modulates the protein folding/misfolding landscape by entropically stabilizing disordered states, including the U and transition state of aggregation. This also indicates that the equilibrium stability of the N state, but not the transition barrier of aggregation, is the dominant factor determining the reduction-induced aggregation of PrP.     

Slow formation of aggregation‐resistant β‐sheet folding intermediates

Protein folding has been studied extensively for decades, yet our ability to predict how proteins reach their native state from a mechanistic perspective is still rudimentary at best, limiting our understanding of folding‐related processes in vivo and our ability to manipulate proteins in vitro. Here, we investigate the in vitro refolding mechanism of a large β‐helix protein, pertactin, which has an extended, elongated shape. At 55 kDa, this single domain, all‐β‐sheet protein allows detailed analysis of the formation of β‐sheet structure in larger proteins. Using a combination of fluorescence and far‐UV circular dichroism spectroscopy, we show that the pertactin β‐helix refolds remarkably slowly, with multiexponential kinetics. Surprisingly, despite the slow refolding rates, large size, and β‐sheet‐rich topology, pertactin refolding is reversible and not complicated by off‐pathway aggregation. The slow pertactin refolding rate is not limited by proline isomerization, and 30% of secondary structure formation occurs within the rate‐limiting step. Furthermore, site‐specific labeling experiments indicate that the β‐helix refolds in a multistep but concerted process involving the entire protein, rather than via initial formation of the stable core substructure observed in equilibrium titrations. Hence pertactin provides a valuable system for studying the refolding properties of larger, β‐sheet‐rich proteins, and raises intriguing questions regarding the prevention of aggregation during the prolonged population of partially folded, β‐sheet‐rich refolding intermediates. Proteins 2010. © 2009 Wiley‐Liss, Inc.     

β‐strand interactions at the domain interface critical for the stability of human lens γD‐crystallin

Human age‐onset cataracts are believed to be caused by the aggregation of partially unfolded or covalently damaged lens crystallin proteins; however, the exact molecular mechanism remains largely unknown. We have used microseconds of molecular dynamics simulations with explicit solvent to investigate the unfolding process of human lens γD‐crystallin protein and its isolated domains. A partially unfolded folding intermediate of γD‐crystallin is detected in simulations with its C‐terminal domain (C‐td) folded and N‐terminal domain (N‐td) unstructured, in excellent agreement with biochemical experiments. Our simulations strongly indicate that the stability and the folding mechanism of the N‐td are regulated by the interdomain interactions, consistent with experimental observations. A hydrophobic folding core was identified within the C‐td that is comprised of a and b strands from the Greek key motif 4, the one near the domain interface. Detailed analyses reveal a surprising non‐native surface salt‐bridge between Glu135 and Arg142 located at the end of the ab folded hairpin turn playing a critical role in stabilizing the folding core. On the other hand, an in silico single E135A substitution that disrupts this non‐native Glu135‐Arg142 salt‐bridge causes significant destabilization to the folding core of the isolated C‐td, which, in turn, induces unfolding of the N‐td interface. These findings indicate that certain highly conserved charged residues, that is, Glu135 and Arg142, of γD‐crystallin are crucial for stabilizing its hydrophobic domain interface in native conformation, and disruption of charges on the γD‐crystallin surface might lead to unfolding and subsequent aggregation.     

A Fluorescence‐Based Sensor Assay that Monitors General Protein Aggregation in Human Cells

Protein conformational disorders are characterized by disruption of protein folding and toxic accumulation of protein aggregates. Here we describe a sensitive and simple method to follow and monitor general protein aggregation in human cells. Heat shock protein 27 (HSP27) is an oligomeric small heat shock protein that binds and keeps unfolded proteins in a folding competent state. This high specificity of HSP27 for aggregated proteins can be explored to monitor aggregation in living cells by fusing it to a fluorescent protein as Green Fluorescent Protein (GFP). We have constructed a HeLa stable cell line expressing a HSP27:GFP chimeric reporter protein and after validation, this stable cell line is exposed to different agents that interfere with proteostasis, namely Arsenite, MG132, and Aβ‐peptide. Exposure to proteome destabilizers lead to re‐localization of HSP27:GFP fluorescence to foci, confirming that our reporter system is functional and can be used to detect and follow protein aggregation in living cells. This reporter is a valuable tool to setup wide‐genetic screens to identify genes and pathways involved in protein misfolding and aggregation.     

Specific protein homeostatic functions of small heat‐shock proteins increase lifespan

During aging, oxidized, misfolded, and aggregated proteins accumulate in cells, while the capacity to deal with protein damage declines severely. To cope with the toxicity of damaged proteins, cells rely on protein quality control networks, in particular proteins belonging to the family of heat‐shock proteins (HSPs). As safeguards of the cellular proteome, HSPs assist in protein folding and prevent accumulation of damaged, misfolded proteins. Here, we compared the capacity of all Drosophila melanogaster small HSP family members for their ability to assist in refolding stress‐denatured substrates and/or to prevent aggregation of disease‐associated misfolded proteins. We identified CG14207 as a novel and potent small HSP member that exclusively assisted in HSP70‐dependent refolding of stress‐denatured proteins. Furthermore, we report that HSP67BC, which has no role in protein refolding, was the most effective small HSP preventing toxic protein aggregation in an HSP70‐independent manner. Importantly, overexpression of both CG14207 and HSP67BC in Drosophila leads to a mild increase in lifespan, demonstrating that increased levels of functionally diverse small HSPs can promote longevity in vivo.     

Construct optimization for protein NMR structure analysis using amide hydrogen/deuterium exchange mass spectrometry

Disordered or unstructured regions of proteins, while often very important biologically, can pose significant challenges for resonance assignment and three‐dimensional structure determination of the ordered regions of proteins by NMR methods. In this article, we demonstrate the application of ¹H/²H exchange mass spectrometry (DXMS) for the rapid identification of disordered segments of proteins and design of protein constructs that are more suitable for structural analysis by NMR. In this benchmark study, DXMS is applied to five NMR protein targets chosen from the Northeast Structural Genomics project. These data were then used to design optimized constructs for three partially disordered proteins. Truncated proteins obtained by deletion of disordered N‐ and C‐terminal tails were evaluated using ¹H‐¹⁵N HSQC and ¹H‐¹⁵N heteronuclear NOE NMR experiments to assess their structural integrity. These constructs provide significantly improved NMR spectra, with minimal structural perturbations to the ordered regions of the protein structure. As a representative example, we compare the solution structures of the full length and DXMS‐based truncated construct for a 77‐residue partially disordered DUF896 family protein YnzC from Bacillus subtilis, where deletion of the disordered residues (ca. 40% of the protein) does not affect the native structure. In addition, we demonstrate that throughput of the DXMS process can be increased by analyzing mixtures of up to four proteins without reducing the sequence coverage for each protein. Our results demonstrate that DXMS can serve as a central component of a process for optimizing protein constructs for NMR structure determination. Proteins 2009. © 2009 Wiley‐Liss, Inc.     

Folding of Top7 in unbiased all‐atom Monte Carlo simulations

For computational studies of protein folding, proteins with both helical and β‐sheet secondary structure elements are very challenging, as they expose subtle biases of the physical models. Here, we present reproducible folding of a 92 residue α/β protein (residues 3–94 of Top7, PDB ID: 1QYS) in computer simulations starting from random initial conformations using a transferable physical model which has been previously shown to describe the folding and thermodynamic properties of about 20 other smaller proteins of different folds. Top7 is a de novo designed protein with two α‐helices and a five stranded β‐sheet. Experimentally, it is known to be unusually stable for its size, and its folding transition distinctly deviates from the two‐state behavior commonly seen in natural single domain proteins. In our all‐atom implicit solvent parallel tempering Monte Carlo simulations, Top7 shows a rapid transition to a group of states with high native‐like secondary structure, and a much slower subsequent transition to the native state with a root mean square deviation of about 3.5 Å from the experimentally determined structure. Consistent with experiments, we find Top7 to be thermally extremely stable, although the simulations also find a large number of very stable non‐native states with high native‐like secondary structure. Proteins 2013; 81:1446–1456. © 2013 Wiley Periodicals, Inc.     

The presence of non‐native helical structure in the unfolding of a beta‐sheet protein MPT63

MPT63, a major secreted protein from Mycobacterium tuberculosis, has been shown to have immunogenic properties and has been implicated in virulence. MPT63 is a β‐sandwich protein containing 11 β strands and a very short stretch of 3₁₀ helix. The detailed experimental and computational study reported here investigates the equilibrium unfolding transition of MPT63. It is shown that in spite of being a complete β‐sheet protein, MPT63 has a strong propensity toward helix structures in its early intermediates. Far UV‐CD and FTIR spectra clearly suggest that the low‐pH intermediate of MTP63 has enhanced helical content, while fluorescence correlation spectroscopy suggests a significant contraction. Molecular dynamics simulation complements the experimental results indicating that the unfolded state of MPT63 traverses through intermediate forms with increased helical characteristics. It is found that this early intermediate contains exposed hydrophobic surface, and is aggregation prone. Although MPT63 is a complete β‐sheet protein in its native form, the present findings suggest that the secondary structure preferences of the local interactions in early folding pathway may not always follow the native conformation. Furthermore, the Gly25Ala mutant supports the proposed hypothesis by increasing the non‐native helical propensity of the protein structure.     

Investigating the dynamics and polyanion binding sites of fibroblast growth factor‐1 using hydrogen‐deuterium exchange mass spectrometry

In this study, we examined the local dynamics of acidic fibroblast growth factor (FGF‐1) as well as the binding sites of various polyanions including poly‐sulfates (heparin and low MW heparin) and poly‐phosphates (phytic acid and ATP) using hydrogen‐deuterium exchange mass spectrometry (HX‐MS). For local dynamics, results are analyzed at the peptide level as well as in terms of buried amides employing crystallographic B‐factors and compared with a residue level heat map generated from HX‐MS results. Results show that strand 4 and 5 and the turn between them to be the most flexible regions as was previously seen by NMR. On the other hand, the C‐terminal strands 8, 9, and 10 appear to be more rigid which is also consistent with crystallographic B‐factors as well as local dynamics studies conducted by NMR. Crystal structures of FGF‐1 in complex with heparin have shown that heparin binds to N‐terminal Asn18 and to C‐terminal Lys105, Tryp107, Lys112, Lys113, Arg119, Pro121, Arg122, Gln127, and Lys128 indicating electrostatic forces as dominant interactions. Heparin binding as determined by HX‐MS is consistent with crystallography data. Previous studies have also shown that other polyanions including low MW heparin, phytic acid and ATP dramatically increase the thermal stability of FGF‐1. Using HX‐MS, we find other poly anions tested bind in a similar manner to heparin, primarily targeting the turns in the lysine rich C‐terminal region of FGF‐1 along with two distinct N‐terminal regions that contains lysines and arginines/histidines. This confirms the interactions between FGF‐1 and polyanions are primary directed by electrostatics.     

Structure‐Based Site‐Specific PEGylation of Fibroblast Growth Factor 2 Facilitates Rational Selection of Conjugate Sites

Polyethylene glycol modification (PEGylation) can enhance the pharmacokinetic properties of therapeutic proteins by the attachment of polyethylene glycol (PEG) to the surface of a protein to shield the protein surface from proteolytic degradation and limit aggregation. However, current PEGylation strategies often reduce biological activity, potentially as a result of steric hindrance of PEG. Overall, there are no structure‐based guidelines for selection of conjugate sites that retain optimal biological activity with improved pharmacokinetic properties. In this study, site‐specific PEGylation based on the FGF2‐FGFR1‐heparin complex structure is performed. The effects of the conjugate sites on protein function are investigated by measuring the receptor/heparin binding affinities of the modified proteins and performing assays to measure cell‐based bio‐activity and in vivo stability. Comprehensive analysis of these data demonstrates that PEGylation of FGF2 that avoids the binding sites for fibroblast growth factor receptor 1 (FGFR1) and heparin provides optimal pharmacokinetic enhancement with minimal losses to biological activity. Animal experiments demonstrate that PEGylated FGF2 exhibits greater efficacy in protecting against traumatic brain injury‐induced brain damage and neurological functions than the non‐modified FGF2. This rational structure‐based PEGylation strategy for protein modification is expected to have a major impact in the area of protein‐based therapeutics.