Truncated titin is structurally integrated into the human dilated cardiomyopathic sarcomere

Heterozygous (HET) truncating variant mutations in the TTN gene (TTNtvs), encoding the giant titin protein, are the most common genetic cause of dilated cardiomyopathy (DCM). However, the molecular mechanisms by which TTNtv mutations induce DCM are controversial. Here, we studied 127 clinically identified DCM human cardiac samples with next-generation sequencing (NGS), high-resolution gel electrophoresis, Western blot analysis, and super-resolution microscopy in order to dissect the structural and functional consequences of TTNtv mutations. The occurrence of TTNtv was found to be 15% in the DCM cohort. Truncated titin proteins matching, by molecular weight, the gene sequence predictions were detected in the majority of the TTNtv+ samples. Full-length titin was reduced in TTNtv+ compared with TTNtv- samples. Proteomics analysis of washed myofibrils and stimulated emission depletion (STED) super-resolution microscopy of myocardial sarcomeres labeled with sequence-specific anti-titin antibodies revealed that truncated titin was structurally integrated into the sarcomere. Sarcomere length-dependent anti-titin epitope position, shape, and intensity analyses pointed at possible structural defects in the I/A junction and the M-band of TTNtv+ sarcomeres, which probably contribute, possibly via faulty mechanosensor function, to the development of manifest DCM.

Molecular imaging of bacterial outer membrane vesicles based on bacterial surface display

The important roles of bacterial outer membrane vesicles (OMVs) in various diseases and their emergence as a promising platform for vaccine development and targeted drug delivery necessitates the development of imaging techniques suitable for quantifying their biodistribution with high precision. To address this requirement, we aimed to develop an OMV specific radiolabeling technique for positron emission tomography (PET). A novel bacterial strain (E. coli BL21(DE3) ΔnlpI, ΔlpxM) was created for efficient OMV production, and OMVs were characterized using various methods. SpyCatcher was anchored to the OMV outer membrane using autotransporter-based surface display systems. Synthetic SpyTag-NODAGA conjugates were tested for OMV surface binding and 64Cu labeling efficiency. The final labeling protocol shows a radiochemical purity of 100% with a ~ 29% radiolabeling efficiency and excellent serum stability. The in vivo biodistribution of OMVs labeled with 64Cu was determined in mice using PET/MRI imaging which revealed that the biodistribution of radiolabeled OMVs in mice is characteristic of previously reported data with the highest organ uptakes corresponding to the liver and spleen 3, 6, and 12 h following intravenous administration. This novel method can serve as a basis for a general OMV radiolabeling scheme and could be used in vaccine- and drug-carrier development based on bioengineered OMVs.

Comprehensive Collection and Prediction of ABC Transmembrane Protein Structures in the AI Era of Structural Biology

The number of unique transmembrane (TM) protein structures doubled in the last four years, which can be attributed to the revolution of cryo-electron microscopy. In addition, AlphaFold2 (AF2) also provided a large number of predicted structures with high quality. However, if a specific protein family is the subject of a study, collecting the structures of the family members is highly challenging in spite of existing general and protein domain-specific databases. Here, we demonstrate this and assess the applicability and usability of automatic collection and presentation of protein structures via the ABC protein superfamily. Our pipeline identifies and classifies transmembrane ABC protein structures using the PFAM search and also aims to determine their conformational states based on special geometric measures, conftors. Since the AlphaFold database contains structure predictions only for single polypeptide chains, we performed AF2-Multimer predictions for human ABC half transporters functioning as dimers. Our AF2 predictions warn of possibly ambiguous interpretation of some biochemical data regarding interaction partners and call for further experiments and experimental structure determination. We made our predicted ABC protein structures available through a web application, and we joined the 3D-Beacons Network to reach the broader scientific community through platforms such as PDBe-KB.

Nanomechanics combined with HDX reveals allosteric drug binding sites of CFTR NBD1

Cystic fibrosis (CF) is a frequent genetic disease in Caucasians that is caused by the deletion of F508 (ΔF508) in the nucleotide binding domain 1 (NBD1) of the CF transmembrane conductance regulator (CFTR). The ΔF508 compromises the folding energetics of the NBD1, as well as the folding of three other CFTR domains. Combination of FDA approved corrector molecules can efficiently but incompletely rescue the ΔF508-CFTR folding and stability defect. Thus, new pharmacophores that would reinstate the wild-type-like conformational stability of the ΔF508-NBD1 would be highly beneficial. The most prominent molecule, 5-bromoindole-3-acetic acid (BIA) that can thermally stabilize the NBD1 has low potency and efficacy. To gain insights into the NBD1 (un)folding dynamics and BIA binding site localization, we combined molecular dynamics (MD) simulations, atomic force spectroscopy (AFM) and hydrogen-deuterium exchange (HDX) experiments. We found that the NBD1 α-subdomain with three adjacent strands from the β-subdomain plays an important role in early folding steps, when crucial non-native interactions are formed via residue F508. Our AFM and HDX experiments showed that BIA associates with this α-core region and increases the resistance of the ΔF508-NBD1 against mechanical unfolding, a phenomenon that could be exploited in future developments of folding correctors.

Development, structure and mechanics of a synthetic E. coli outer membrane model

The outer membrane (OM) of Gram-negative bacteria is a complex asymmetric bilayer containing lipids, lipopolysaccharides (LPS) and proteins. While it is a mechanical and chemical barrier, it is also the primary surface of bacterial recognition processes that involve infection by and of the bacterium. Uncovering the mechanisms of these biological functions has been hampered by the lack of suitable model systems. Here we report the step-by-step assembly of a synthetic OM model from its fundamental components. To enable the efficient formation of a supported lipid bilayer at room temperature, dimyristoyl-phosphocholine (DMPC) was used as the lipid component to which we progressively added LPS and OM proteins. The assembled system enabled us to explore the contribution of the molecular components to the topographical structure and stability of the OM. We found that LPS prefers solid-state membrane regions and forms stable vesicles in the presence of divalent cations. LPS can gradually separate from DMPC membranes to form independent vesicles, pointing at the dynamic nature of the lipid-LPS system. The addition of OM proteins from E. coli and saturating levels of LPS to DMPC liposomes resulted in a thicker and more stable bilayer the surface of which displayed a nanoscale texture formed of parallel, curved, long (>500 nm) stripes spaced apart with a 15 nm periodicity. The synthetic membrane may facilitate the investigation of binding and recognition processes on the surface of Gram-negative bacteria.

The MemMoRF database for recognizing disordered protein regions interacting with cellular membranes

Protein and lipid membrane interactions play fundamental roles in a large number of cellular processes (e.g. signalling, vesicle trafficking, or viral invasion). A growing number of examples indicate that such interactions can also rely on intrinsically disordered protein regions (IDRs), which can form specific reversible interactions not only with proteins but also with lipids. We named IDRs involved in such membrane lipid-induced disorder-to-order transition as MemMoRFs, in an analogy to IDRs exhibiting disorder-to-order transition upon interaction with protein partners termed Molecular Recognition Features (MoRFs). Currently, both the experimental detection and computational characterization of MemMoRFs are challenging, and information about these regions are scattered in the literature. To facilitate the related investigations we generated a comprehensive database of experimentally validated MemMoRFs based on manual curation of literature and structural data. To characterize the dynamics of MemMoRFs, secondary structure propensity and flexibility calculated from nuclear magnetic resonance chemical shifts were incorporated into the database. These data were supplemented by inclusion of sentences from papers, functional data and disease-related information. The MemMoRF database can be accessed via a user-friendly interface at https://memmorf.hegelab.org, potentially providing a central resource for the characterization of disordered regions in transmembrane and membrane-associated proteins.

The transport pathway in the ABCG2 protein and its regulation revealed by molecular dynamics simulations

Atomic-level structural insight on the human ABCG2 membrane protein, a pharmacologically important transporter, has been recently revealed by several key papers. In spite of the wealth of structural data, the pathway of transmembrane movement for the large variety of structurally different ABCG2 substrates and the physiological lipid regulation of the transporter has not been elucidated. The complex molecular dynamics simulations presented here may provide a breakthrough in understanding the steps of the substrate transport process and its regulation by cholesterol. Our analysis revealed drug binding cavities other than the central binding site and delineated a putative dynamic transport pathway for substrates with variable structures. We found that membrane cholesterol accelerated drug transport by promoting the closure of cytoplasmic protein regions. Since ABCG2 is present in all major biological barriers and drug-metabolizing organs, influences the pharmacokinetics of numerous clinically applied drugs, and plays a key role in uric acid extrusion, this information may significantly promote a reliable prediction of clinically important substrate characteristics and drug-drug interactions.

Discovering the chloride pathway in the CFTR channel

Cystic fibrosis (CF), a lethal monogenic disease, is caused by pathogenic variants of the CFTR chloride channel. The majority of CF mutations affect protein folding and stability leading overall to diminished apical anion conductance of epithelial cells. The recently published cryo-EM structures of full-length human and zebrafish CFTR provide a good model to gain insight into structure–function relationships of CFTR variants. Although, some of the structures were determined in the phosphorylated and ATP-bound active state, none of the static structures showed an open pathway for chloride permeation. Therefore, we performed molecular dynamics simulations to generate a conformational ensemble of the protein and used channel detecting algorithms to identify conformations with an opened channel. Our simulations indicate a main intracellular entry at TM4/6, a secondary pore at TM10/12, and a bottleneck region involving numerous amino acids from TM1, TM6, and TM12 in accordance with experiments. Since chloride ions entered the pathway in our equilibrium simulations, but did not traverse the bottleneck region, we performed metadynamics simulations, which revealed two possible exits. One of the chloride ions exits includes hydrophobic lipid tails that may explain the lipid-dependency of CFTR function. In summary, our in silico study provides a detailed description of a potential chloride channel pathway based on a recent cryo-EM structure and may help to understand the gating of the CFTR chloride channel, thus contributing to novel strategies to rescue dysfunctional mutants.

Quantitative comparison of ABC membrane protein type I exporter structures in a standardized way

An increasing number of ABC membrane protein structures are determined by cryo-electron microscopy and X-ray crystallography, consequently identifying differences between their conformations has become an arising issue. Therefore, we propose to define standardized measures for ABC Type I exporter structure characterization. We set conformational vectors, conftors, which describe the relative orientation of domains and can highlight structural differences. In addition, continuum electrostatics calculations were performed to characterize the energetics of membrane insertion illuminating functionally crucial regions. In summary, the proposed metrics contribute to deeper understanding of ABC membrane proteins' structural features, structure validation, and analysis of movements observed in a molecular dynamics trajectory. Moreover, the concept of standardized metrics can be applied not only to ABC membrane protein structures (http://conftors.hegelab.org).

ABCMdb reloaded: updates on mutations in ATP binding cassette proteins

ABC (ATP-Binding Cassette) proteins with altered function are responsible for numerous human diseases. To aid the selection of positions and amino acids for ABC structure/function studies we have generated a database, ABCMdb (Gyimesi et al., ABCMdb: a database for the comparative analysis of protein mutations in ABC transporters, and a potential framework for a general application. Hum Mutat 2012; 33:1547–1556.), with interactive tools. The database has been populated with mentions of mutations extracted from full text papers, alignments and structural models. In the new version of the database we aimed to collect the effect of mutations from databases including ClinVar. Because of the low number of available data, even in the case of the widely studied disease-causing ABC proteins, we also included the possible effects of mutations based on SNAP2 and PROVEAN predictions. To aid the interpretation of variations in non-coding regions, the database was supplemented with related DNA level information. Our results emphasize the importance of in silico predictions because of the sparse information available on variants and suggest that mutations at analogous positions in homologous ABC proteins have a strong predictive power for the effects of mutations. Our improved ABCMdb advances the design of both experimental studies and meta-analyses in order to understand drug interactions of ABC proteins and the effects of mutations on functional expression.

Database URL:http://abcm2.hegelab.org

Effects of the gout-causing Q141K polymorphism and a CFTR ΔF508 mimicking mutation on the processing and stability of the ABCG2 protein

ABCG2 is an important multidrug transporter involved also in urate transport, thus its mutations can lead to the development of gout and may also alter general drug absorption, distribution and excretion. The frequent ABCG2 polymorphism, Q141K, is associated with an elevated risk of gout and has been controversially reported to reduce the plasma membrane expression and/or the transport function of the protein. In the present work we examined the stability and cellular processing of the Q141K ABCG2 variant, as well as that of the ΔF142 ABCG2, corresponding to the ΔF508 mutation in the CFTR (ABCC7) protein, causing cystic fibrosis. The processing and localization of full length ABCG2 variants were investigated in mammalian cells, followed by Western blotting and confocal microscopy, respectively. Folding and stability were examined by limited proteolysis of Sf9 insect cell membranes expressing these ABCG2 constructs. Stability of isolated nucleotide binding domains, expressed in and purified from bacteria, was studied by CD spectroscopy. We find that the Q141K variant has a mild processing defect which can be rescued by low temperature, a slightly reduced activity, and a mild folding defect, especially affecting the NBD. In contrast, the ΔF142 mutant has major processing and folding defects, and no ATPase function. We suggest that although these mutations are both localized within the NBD, based on molecular modeling their contribution to the ABCG2 structure and function is different, thus rescue strategies may be devised accordingly.

ABCMdb: a database for the comparative analysis of protein mutations in ABC transporters, and a potential framework for a general application

To overcome the pathological phenomena caused by altered function of ABC (ATP Binding Cassette) proteins, their mechanisms of action are extensively investigated, often involving the design of mutant constructs for experiments. Designing mutagenetic constructs, interpreting the result of mutagenetic experiments, and finding individual genetic variants require an extensive knowledge of previously published mutations. To aid the recapitulation of mutations described in the literature, we set up a database of ABC protein mutations (ABCMdb) extracted from full-text papers using an automatic mining approach. We have also developed a Web application interface to compare mutations in different ABC proteins using sequence alignments and to interactively map the mutations to 3D structural models. Currently our database contains protein mutations published for ABCB1, ABCB11, ABCC1, ABCC6, ABCC7, and the proteins of the ABCG subfamily. The database will be extended to include other members and subfamilies, and to provide information on whether or not a mutation is disease causing, represents a high-incidence polymorphism, or was generated only in vitro. The ABCMdb database should already help to compare the effects of mutations at homologous positions in different ABC proteins, and its interactive tools aim to advance the design of experiments for a wider range of proteins.

Identification and correction of abnormal, incomplete and mispredicted proteins in public databases

Background

Despite significant improvements in computational annotation of genomes, sequences of abnormal, incomplete or incorrectly predicted genes and proteins remain abundant in public databases. Since the majority of incomplete, abnormal or mispredicted entries are not annotated as such, these errors seriously affect the reliability of these databases. Here we describe the MisPred approach that may provide an efficient means for the quality control of databases. The current version of the MisPred approach uses five distinct routines for identifying abnormal, incomplete or mispredicted entries based on the principle that a sequence is likely to be incorrect if some of its features conflict with our current knowledge about protein-coding genes and proteins: (i) conflict between the predicted subcellular localization of proteins and the absence of the corresponding sequence signals; (ii) presence of extracellular and cytoplasmic domains and the absence of transmembrane segments; (iii) co-occurrence of extracellular and nuclear domains; (iv) violation of domain integrity; (v) chimeras encoded by two or more genes located on different chromosomes.

Results

Analyses of predicted EnsEMBL protein sequences of nine deuterostome (Homo sapiens, Mus musculus, Rattus norvegicus, Monodelphis domestica, Gallus gallus, Xenopus tropicalis, Fugu rubripes, Danio rerio and Ciona intestinalis) and two protostome species (Caenorhabditis elegans and Drosophila melanogaster) have revealed that the absence of expected signal peptides and violation of domain integrity account for the majority of mispredictions. Analyses of sequences predicted by NCBI's GNOMON annotation pipeline show that the rates of mispredictions are comparable to those of EnsEMBL. Interestingly, even the manually curated UniProtKB/Swiss-Prot dataset is contaminated with mispredicted or abnormal proteins, although to a much lesser extent than UniProtKB/TrEMBL or the EnsEMBL or GNOMON-predicted entries.

Conclusion

MisPred works efficiently in identifying errors in predictions generated by the most reliable gene prediction tools such as the EnsEMBL and NCBI's GNOMON pipelines and also guides the correction of errors. We suggest that application of the MisPred approach will significantly improve the quality of gene predictions and the associated databases.

Modules, multidomain proteins and organismic complexity

Originally the term 'protein module' was coined to distinguish mobile domains that frequently occur as building blocks of diverse multidomain proteins from 'static' domains that usually exist only as stand-alone units of single-domain proteins. Despite the widespread use of the term 'mobile domain', the distinction between static and mobile domains is rather vague as it is not easy to quantify the mobility of domains. In the present work we show that the most appropriate measure of the mobility of domains is the number of types of local environments in which a given domain is present. Ranking of domains with respect to this parameter in different evolutionary lineages highlighted marked differences in the propensity of domains to form multidomain proteins. Our analyses have also shown that there is a correlation between domain size and domain mobility: smaller domains are more likely to be used in the construction of multidomain proteins, whereas larger domains are more likely to be static, stand-alone domains. It is also shown that shuffling of a limited set of modules was facilitated by intronic recombination in the metazoan lineage and this has contributed significantly to the emergence of novel complex multidomain proteins, novel functions and increased organismic complexity of metazoa.

Hydroxamate-based peptide inhibitors of matrix metalloprotease 2

There is major interest in designing inhibitors for matrix metalloproteinase 2 (MMP-2, gelatinase A) since this enzyme is known to be involved in pathological processes such as tumor invasion or rheumatoid arthritis. The majority of MMP-2 inhibitor candidate drugs block the active site of MMP-2 by binding to its catalytic Zn2+ ion through a chelating (hydroxamate, sulphonate etc.) group. Despite the general interest in designing MMP-2 inhibitors, the results with many of the drug candidates were disappointing, their failure was usually explained by cross-reactions with other MMPs. One way to enhance MMP-2 selectivity is to design inhibitors that interact with both the active site and exosites such as the fibronectin type II (FN2) domains of the enzyme. In the present work, we have examined the inhibitory potential and MMP-2 selectivity of hydroxamates of three groups of peptides known to bind to the collagen-binding FN2 domains of MMP-2. The first type of peptides consisted of collagen-like (Pro-Pro-Gly)(n) repeats, peptides of the second group were identified from a random 15-mer phage display library based on their binding to immobilized FN2 domains of MMP-2. A hydroxamate of peptide p33-42, known to bind to the third FN2 domain of MMP-2 has also been tested. Our studies have shown that these compounds inhibited MMP-2 with IC50 values of 10-100 microM. The fact that their inhibitory potential was nearly identical for MMP-2del, a recombinant version of MMP-2 that lacks the FN2 domains, suggests that inhibition is not mediated by their binding to FN2 domains. It seems likely that the failure to exploit interaction with the FN2 domains is due to the fact that the FN2 domains and the catalytic domain of MMP-2 tumble independently, therefore only a tiny fraction of the conformational isomers can bind peptide hydroxamates via both the active site and the FN2 domain(s).

Insertion of spliceosomal introns in proto-splice sites: the case of secretory signal peptides

Analysis of the exon-intron structures of 2208 human genes has revealed that there is a statistically highly significant excess of phase 1 introns in the vicinity of the signal peptide cleavage sites. It is suggested that amino acid sequences surrounding signal peptide cleavage sites are significantly enriched in phase 1 proto-splice sites and this has favored insertion of spliceosomal introns in these sites.

Modular Autonomy, Ligand Specificity, and Functional Cooperativity of the Three In-tandem Fibronectin Type II Repeats from Human Matrix Metalloproteinase 2

Matrix metalloproteinase 2 (MMP-2) contains three fibronectin type II (col) modules that contribute to its collagen specificity. We observed that the CD spectra of the separate col modules account for the CD and temperature profiles of the in-tandem col-123 construct. Thus, to the extent of not significantly perturbing the secondary structure and thermal stability characteristics of the neighboring units, the domains within col-123 do not interact. Via NMR, we investigated ligand binding properties of the three repeats within col-123: col-123/1 (the col-1 domain within col-123), col-123/2, and col-123/3. Interactions of col-123 with the collagen mimic peptide (Pro-Pro-Gly)6 (PPG6) and propeptide segment PIIKFPGDVA (p33-42) were studied. While col-123/1 and col-123/2 bound PPG6, they interacted more weakly with p33-42. In contrast, col-123/3 exhibited a higher affinity for p33-42 than for PPG6. Thus, despite their structural homology, the col repeats of MMP-2 differ in substrate specificity. Furthermore the binding affinities toward the three in-tandem col repeats were close to those determined for the individual isolated domains or for col-12/1, indicating that vis-a-vis these ligands each module interacts essentially as an autonomous unit. Interestingly the domains within col-123 exhibited enhanced affinities for Hel3, a construct that contains ((Gly-Pro-Pro)12)3 in triple helical configuration. Nevertheless the affinities were significantly higher for col-123/1 and col-123/2 relative to col-123/3 in line with their behaviors toward PPG6. This hints at a cooperative participation toward Hel3, which is a closer mimic of collagen, a hypothesis that is supported by the detected lower affinities of col-12/1, col-12/2, col-2, col-23/2, col-3, and col-23/3 for Hel3.

Gelatin-binding region of human matrix metalloproteinase-2: solution structure, dynamics, and function of the COL-23 two-domain construct

Human matrix metalloproteinase-2 (MMP-2) contains an array of three fibronectin type II (FII) modules postulated to interact with gelatin (denatured collagen). Here, we verify that the NMR solution structure of the third FII repeat (COL-3) is similar to that of the second FII repeat (COL-2); characterize its ligand-binding properties; and derive dynamics properties and relative orientation in solution for the two domains of the COL-23 fragment, a construct comprising COL-2 and COL-3 in tandem, with each domain possessing a putative collagen-binding site. Interaction of the synthetic gelatin-like octadecapeptide (Pro-Pro-Gly)(6) (PPG6) with COL-3 is weaker than with COL-2. We found that a synthetic peptide comprising segment 33-42 (peptide 33-42) from the MMP-2 prodomain interacts with COL-3 and, albeit with lower affinity, with COL-2 in a way that mimics PPG6 binding. COL-3 strongly prefers peptide 33-42 over PPG6, which suggests that intramolecular interactions with the prodomain could modulate binding of pro-MMP-2 to its gelatin substrate. In COL-23, the two modules retain their structural individuality and tumble independently. Overall, the NMR data indicate that the relative orientation of the modules in COL-23 is not fixed in solution, that the modules do not interact with one another, and that COL-23 is rather flexible. The binding sites face opposite each other, and their responses to, and normalized affinities for, the longer ligand PPG12 are virtually identical to those of the individual domains for PPG6, thus precluding co- operativity, although they may interact simultaneously with multiple sites of the extracellular matrix.

The PAN module: the N-terminal domains of plasminogen and hepatocyte growth factor are homologous with the apple domains of the prekallikrein family and with a novel domain found in numerous nematode proteins

Based on homology search and structure prediction methods we show that (1) the N-terminal N domains of members of the plasminogen/hepatocyte growth factor family, (2) the apple domains of the plasma prekallikrein/coagulation factor XI family, and (3) domains of various nematode proteins belong to the same module superfamily, hereafter referred to as the PAN module. The patterns of conserved residues correspond to secondary structural elements of the known three-dimensional structure of hepatocyte growth factor N domain, therefore we predict a similar fold for all members of this superfamily. Based on available functional informations on apple domains and N domains, it is clear that PAN modules have significant functional versatility, they fulfill diverse biological functions by mediating protein-protein or protein-carbohydrate interactions.

The second type II module from human matrix metalloproteinase 2: structure, function and dynamics

BACKGROUND

Matrix metalloproteinase 2 (MMP-2, gelatinase A, 72 kDa type IV collagenase) has an important role in extracellular matrix degradation during cell migration and tissue remodeling. It is involved in development, inflammation, wound healing, tumor invasion, metastasis and other physiological and pathological processes. The enzyme cleaves several types of collagen, elastin, fibronectin and laminin. Binding to collagen is mediated by three repeats homologous to fibronectin type II modules, which are inserted in the catalytic domain in proximity to the active site.

RESULTS

We have determined the NMR solution structure of the second type II module from human MMP-2 (col-2). The module exhibits a typical type II fold with two short double-stranded antiparallel beta sheets and three large loops packed around a cluster of conserved aromatic residues. Backbone amide dynamics, derived from (15)N relaxation experiments, correlate well with solvent accessibility and intramolecular hydrogen bonding. A synthetic peptide with the collagen consensus sequence, (Pro-Pro-Gly)(6), is shown to interact with the module.

CONCLUSIONS

Spectral perturbations induced by (Pro-Pro-Gly)(6) binding reveal the region involved in the interaction of col-2 with collagen. The binding surface comprises exposed aromatic residues Phe21, Tyr38, Trp40, Tyr47, Tyr53 and Phe55, and the neighboring Gly33-Gly37 segment.

The gelatin-binding site of the second type-II domain of gelatinase A/MMP-2

We have shown previously that all three fibronectin type-II modules of gelatinase A contribute to its gelatin affinity. In the present work the second type-II module was subjected to site-directed mutagenesis in order to localize its gelatin-binding site. The functional integrity of mutant proteins was assessed by their affinity for gelatin using gelatin-Sepharose affinity chromatography. The structural integrity of the mutant proteins, i.e. their resistance to thermal and chaotropic agent-induced denaturation, was characterized by CD spectroscopy. Our studies show that, in the case of mutants R19L, R38L, K50G, K50R and R19L/R38L, the mutations had no significant effect on the structure and gelatin affinity of the type-II module, excluding the direct involvement of these residues in ligand binding. In the case of mutants Y25A, Y46A, D49A and Y52A, the mutations yielded proteins that were devoid of gelatin affinity. Structural characterization of these proteins, however, indicated that they had also lost their ability to fold into the native structure characteristic of the wild-type domain. In the case of mutant Y37A, the structure and stability of the mutant protein is similar to the wild-type module. However, its gelatin affinity was severely impaired compared with the wild-type protein. The fact that the Y37A mutation impairs ligand binding without detectable distortion of the module's architecture suggests that Y37 is directly involved in ligand binding. Homology modeling based on the three-dimensional structure of the second type-II module of PDC-109 places Y37 on the right-hand rim of a hydrophobic pocket that includes residues F20, W39, Y46, Y52 and F54, and thus provides proof for the involvement of this pocket in ligand binding.

Structure and domain-domain interactions of the gelatin binding site of human 72-kilodalton type IV collagenase (gelatinase A, matrix metalloproteinase 2)

We have shown previously that all three fibronectin type II modules of gelatinase A contribute to its gelatin affinity. In the present investigation we have studied the structure and module-module interactions of this gelatin-binding domain by circular dichroism spectroscopy and differential scanning calorimetry. Comparison of the Tm values of the thermal transitions of isolated type II modules with those of bimodular or trimodular proteins has shown that the second type II module is significantly more stable in the trimodular protein coll 123 (Tm = 54 degrees C) than in the single-module protein coll 2 (Tm = 44 degrees C) or in the bimodular proteins coll 23 (Tm = 47 degrees C) and coll 12 (Tm = 48 degrees C). Analysis of the enthalpy changes associated with thermal unfolding of the second type II module suggests that it is stabilized by domain-domain interactions in coll 123. We propose that intimate contacts exist between the three tandem type 11 units and they form a single gelatin-binding site. Based on the three-dimensional structures of homologous metalloproteases and type II modules, a model is proposed in which the three type II units form an extension of the substrate binding cleft of gelatinase A.

The gelatin-binding site of human 72 kDa type IV collagenase (gelatinase A)

To identify structures critical for gelatin-binding of 72 kDa type IV collagenase (gelatinase A), fragments of this metalloproteinase have been expressed in Escherichia coli and assayed for their gelatin affinity. Each of the three fibronectin-related type II domains was found to have affinity for gelatin. Fragments containing all three tandem type II domains had significantly stronger affinity than any of the constituent units, indicating that they co-operate to form the high-affinity gelatin-binding site. Competition experiments have also shown that gelatinase A binds more tightly to gelatin than fibronectin and can displace the latter from denatured collagen.