Solution structure of a Hck SH3 domain ligand complex reveals novel interaction modes

We studied the interaction of hematopoietic cell kinase SH3 domain (HckSH3) with an artificial 12-residue proline-rich peptide PD1 (HSKYPLPPLPSL) identified as high affinity ligand (K(D)=0.2 muM). PD1 shows an unusual ligand sequence for SH3 binding in type I orientation because it lacks the typical basic anchor residue at position P(-3), but instead has a tyrosine residue at this position. A basic lysine residue, however, is present at position P(-4). The solution structure of the HckSH3:PD1 complex, which is the first HckSH3 complex structure available, clearly reveals that the P(-3) tyrosine residue of PD1 does not take the position of the typical anchor residue but rather forms additional van der Waals interactions with the HckSH3 RT loop. Instead, lysine at position P(-4) of PD1 substitutes the function of the P(-3) anchor residue. This finding expands the well known ligand consensus sequence +xxPpxP by +xxxPpxP. Thus, software tools like iSPOT fail to identify PD1 as a high-affinity HckSH3 ligand so far. In addition, a short antiparallel beta-sheet in the RT loop of HckSH3 is observed upon PD1 binding. The structure of the HckSH3:PD1 complex reveals novel features of SH3 ligand binding and yields new insights into the structural basics of SH3-ligand interactions. Consequences for computational prediction tools adressing SH3-ligand interactions as well as the biological relevance of our findings are discussed.     

Identification of a collapsed intermediate with non-native long-range interactions on the folding pathway of a pair of Fyn SH3 domain mutants by NMR relaxation dispersion spectroscopy

Recent 15N and 13C spin-relaxation dispersion studies of fast-folding mutants of the Fyn SH3 domain have established that folding proceeds through a low-populated on-pathway intermediate (I) where the central beta-sheet is at least partially formed, but without interactions between the NH2- and COOH-terminal beta-strands that exist in the folded state (F). Initial studies focused on mutants where Gly48 is replaced; in an effort to establish whether this intermediate is a general feature of Fyn SH3 folding a series of 15N relaxation experiments monitoring the folding of Fyn SH3 mutants N53P/V55L and A39V/N53P/V55L are reported here. For these mutants as well, folding proceeds through an on-pathway intermediate with similar features to those observed for G48M and G48V Fyn SH3 domains. However, the 15N chemical shifts extracted for the intermediate indicate pronounced non-native contacts between the NH2 and COOH-terminal regions not observed previously. The kinetic parameters extracted for the folding of A39V/N53P/V55L Fyn SH3 from the three-state folding model F<-->I<-->U are in good agreement with folding and unfolding rates extrapolated to zero denaturant obtained from stopped-flow experiments analyzed in terms of a simplified two-state folding reaction. The folding of the triple mutant was studied over a wide range of temperatures, establishing that there is no difference in heat capacities between F and I states. This confirms a compact folding intermediate structure, which is supported by the 15N chemical shifts of the I state extracted from the dispersion data. The temperature-dependent relaxation data simplifies data analysis because at low temperatures (< 25 degrees C) the unfolded state (U) is negligibly populated relative to I and F. A comparison between parameters extracted at low temperatures where the F<-->I exchange model is appropriate with those from the more complex, three-state model at higher temperatures has been used to validate the protocol for analysis of three-site exchange relaxation data.     

Proline Isomerization Preorganizes the Itk SH2 Domain for Binding to the Itk SH3 Domain

We report here the NMR-derived structure of the binary complex formed by the interleukin-2 tyrosine kinase (Itk) Src homology 3 (SH3) and Src homology 2 (SH2) domains. The interaction is independent of both a phosphotyrosine motif and a proline-rich sequence, the classical targets of the SH2 and SH3 domains, respectively. The Itk SH3/SH2 structure reveals the molecular details of this nonclassical interaction and provides a clear picture for how the previously described prolyl cis/trans isomerization present in the Itk SH2 domain mediates SH3 binding. The higher-affinity cis SH2 conformer is preorganized to form a hydrophobic interface with the SH3 domain. The structure also provides insight into how autophosphorylation in the Itk SH3 domain might increase the affinity of the intermolecular SH3/SH2 interaction. Finally, we can compare this Itk complex with other examples of SH3 and SH2 domains engaging their ligands in a nonclassical manner. These small binding domains exhibit a surprising level of diversity in their binding repertoires.     

Solution structure and dynamics of human metallothionein-3 (MT-3)

Alzheimer's disease is characterized by progressive loss of neurons accompanied by the formation of intraneural neurofibrillary tangles and extracellular amyloid plaques. Human neuronal growth inhibitory factor, classified as metallothionein-3 (MT-3), was found to be related to the neurotrophic activity promoting cortical neuron survival and dendrite outgrowth in the cell culture studies. We have determined the solution structure of the alpha-domain of human MT-3 (residues 32-68) by multinuclear and multidimensional NMR spectroscopy in combination with the molecular dynamic simulated annealing approach. The human MT-3 shows two metal-thiolate clusters, one in the N-terminus (beta-domain) and one in the C-terminus (alpha-domain). The overall fold of the alpha-domain is similar to that of mouse MT-3. However, human MT-3 has a longer loop in the acidic hexapeptide insertion than that of mouse MT-3. Surprisingly, the backbone dynamics of the protein revealed that the beta-domain exhibits similar internal motion to the alpha-domain, although the N-terminal residues are more flexible. Our results may provide useful information for understanding the structure-function relationship of human MT-3.     

Molecular basis of Bcl-xL's target recognition versatility revealed by the structure of Bcl-xL in complex with the BH3 domain of Beclin-1

Beclin-1, originally identified as a Bcl-2 binding protein, is an evolutionarily conserved protein required for autophagy. The direct interaction between Beclin-1 and Bcl-2 or Bcl-xL provides a potential convergence point for apoptosis and autophagy, two programmed cell death processes. Given the functional significance of the interaction between Beclin-1 and Bcl-2/Bcl-xL, we performed detailed biochemical and structural characterizations of this interaction. We demonstrated that the Bcl-xL-binding domain of Beclin-1 contains a BH3 domain. Therefore, Beclin-1 is a new member of the BH3-only family proteins. The structure of Bcl-xL in complex with the Beclin-1 BH3 domain was determined at high resolution by NMR spectroscopy. Although similar to other known BH3 domains, the Beclin-1 BH3 domain displays its own distinct features in the complex with Bcl-xL. Systematic analysis of all known Bcl-xL/BH3 domain complexes helped us to identify the molecular basis underlying the capacity of Bcl-xL to recognize diverse target sequences.     

High-resolution structural and thermodynamic analysis of extreme stabilization of human procarboxypeptidase by computational protein design

Recent efforts to design de novo or redesign the sequence and structure of proteins using computational techniques have met with significant success. Most, if not all, of these computational methodologies attempt to model atomic-level interactions, and hence high-resolution structural characterization of the designed proteins is critical for evaluating the atomic-level accuracy of the underlying design force-fields. We previously used our computational protein design protocol RosettaDesign to completely redesign the sequence of the activation domain of human procarboxypeptidase A2. With 68% of the wild-type sequence changed, the designed protein, AYEdesign, is over 10 kcal/mol more stable than the wild-type protein. Here, we describe the high-resolution crystal structure and solution NMR structure of AYEdesign, which show that the experimentally determined backbone and side-chains conformations are effectively superimposable with the computational model at atomic resolution. To isolate the origins of the remarkable stabilization, we have designed and characterized a new series of procarboxypeptidase mutants that gain significant thermodynamic stability with a minimal number of mutations; one mutant gains more than 5 kcal/mol of stability over the wild-type protein with only four amino acid changes. We explore the relationship between force-field smoothing and conformational sampling by comparing the experimentally determined free energies of the overall design and these focused subsets of mutations to those predicted using modified force-fields, and both fixed and flexible backbone sampling protocols.     

Solution structure of the BRK domains from CHD7

CHD7 is a member of the chromodomain helicase DNA binding domain (CHD) family of ATP-dependent chromatin remodelling enzymes. It is mutated in CHARGE syndrome, a multiple congenital anomaly condition. CHD7 is one of a subset of CHD proteins, unique to metazoans that contain the BRK domain, a protein module also found in the Brahma/BRG1 family of helicases. We describe here the NMR solution structure of the two BRK domains of CHD7. Each domain has a compact betabetaalphabeta fold. The second domain has a C-terminal extension consisting of two additional helices. The structure differs from those of other domains present in chromatin-associated proteins.     

Crystallographic structure of the SH3 domain of the human c-Yes tyrosine kinase: loop flexibility and amyloid aggregation

SH3 domains from the Src family of tyrosine kinases represent an interesting example of the delicate balance between promiscuity and specificity characteristic of proline-rich ligand recognition by SH3 domains. The development of inhibitors of therapeutic potential requires a good understanding of the molecular determinants of binding affinity and specificity and relies on the availability of high quality structural information. Here, we present the first high-resolution crystal structure of the SH3 domain of the c-Yes oncogen. Comparison with other SH3 domains from the Src family revealed significant deviations in the loop regions. In particular, the n-Src loop, highly flexible and partially disordered, is stabilized in an unusual conformation by the establishment of several intramolecular hydrogen bonds. Additionally, we present here the first report of amyloid aggregation by an SH3 domain from the Src family.     

Solution NMR structure of the junction between tropomyosin molecules: implications for actin binding and regulation

Tropomyosin is a coiled-coil protein that binds head-to-tail along the length of actin filaments in eukaryotic cells, stabilizing them and providing protection from severing proteins. Tropomyosin cooperatively regulates actin's interaction with myosin and mediates the Ca2+ -dependent regulation of contraction by troponin in striated muscles. The N-terminal and C-terminal ends are critical functional determinants that form an "overlap complex". Here we report the solution NMR structure of an overlap complex formed of model peptides. In the complex, the chains of the C-terminal coiled coil spread apart to allow insertion of 11 residues of the N-terminal coiled coil into the resulting cleft. The plane of the N-terminal coiled coil is rotated 90 degrees relative to the plane of the C terminus. A consequence of the geometry is that the orientation of postulated periodic actin binding sites on the coiled-coil surface is retained from one molecule to the next along the actin filament when the overlap complex is modeled into the X-ray structure of tropomyosin determined at 7 Angstroms. Nuclear relaxation NMR data reveal flexibility of the junction, which may function to optimize binding along the helical actin filament and to allow mobility of tropomyosin on the filament surface as it switches between regulatory states.     

NMR structure of the Escherichia coli type 1 pilus subunit FimF and its interactions with other pilus subunits

Type 1 pili from uropathogenic Escherichia coli strains mediate bacterial attachment to target receptors on the host tissue. They are composed of up to 3000 copies of the subunit FimA, which form the stiff, helical pilus rod, and the subunits FimF, FimG, and FimH, which form the linear tip fibrillum. All subunits in the pilus interact via donor strand complementation, in which the incomplete immunoglobulin-like fold of each subunit is complemented by insertion of an N-terminal extension from the following subunit. We determined the NMR structure of a monomeric, self-complemented variant of FimF, FimF_F, which has a second FimF donor strand segment fused to its C-terminus that enables intramolecular complementation of the FimF fold. NMR studies on bimolecular complexes between FimF_F and donor strand-depleted variants of FimF and FimG revealed that the relative orientations of neighboring domains in the tip fibrillum cover a wide range. The data provide strong support for the intrinsic flexibility of the tip fibrillum. They lend further support to the hypothesis that this flexibility would significantly increase the probability that the adhesin at the distal end of the fibrillum successfully targets host cell receptors.     

Dynamic conformational equilibria in the physiological function of the Bombyx mori pheromone-binding protein

The Bombyx mori pheromone-binding protein (BmorPBP) undergoes a pH-dependent conformational transition from a form at basic pH, which contains an open cavity suitable for ligand binding (BmorPBP^B), to a form at pH 4.5, where this cavity is occupied by an additional helix (BmorPBP^A). This helix α7 is formed by the C-terminal dodecapeptide 131-142, which is flexibly disordered on the protein surface in BmorPBP^B and in its complex with the pheromone bombykol. Previous work showed that the ligand-binding cavity cannot accommodate both bombykol and helix α7. Here we further investigated mechanistic aspects of the physiologically crucial ejection of the ligand at lower pH values by solution NMR studies of the variant protein BmorPBP(1-128), where the C-terminal helix-forming tetradecapeptide is removed. The NMR structure of the truncated protein at pH 6.5 corresponds closely to BmorPBP^B. At pH 4.5, BmorPBP(1-128) maintains a B-type structure that is in a slow equilibrium, on the NMR chemical shift timescale, with a low-pH conformation for which a discrete set of ¹⁵N-¹H correlation peaks is NMR unobservable. The full NMR spectrum was recovered upon readjusting the pH of the protein solution to 6.5. These data reveal dual roles for the C-terminal tetradecapeptide of BmorPBP in the mechanism of reversible pheromone binding and transport, where it governs dynamic equilibria between two locally different protein conformations at acidic pH and competes with the ligand for binding to the interior cavity.     

Solution structure of CEH-37 homeodomain of the nematode Caenorhabditis elegans

The nematode Caenorhabditis elegans protein CEH-37 belongs to the paired OTD/OTX family of homeobox-containing homeodomain proteins. CEH-37 shares sequence similarity with homeodomain proteins, although it specifically binds to double-stranded C. elegans telomeric DNA, which is unusual to homeodomain proteins. Here, we report the solution structure of CEH-37 homeodomain and molecular interaction with double-stranded C. elegans telomeric DNA using nuclear magnetic resonance (NMR) spectroscopy. NMR structure shows that CEH-37 homeodomain is composed of a flexible N-terminal region and three α-helices with a helix-turn-helix (HTH) DNA binding motif. Data from size-exclusion chromatography and fluorescence spectroscopy reveal that CEH-37 homeodomain interacts strongly with double-stranded C. elegans telomeric DNA. NMR titration experiments identified residues responsible for specific binding to nematode double-stranded telomeric DNA. These results suggest that C. elegans homeodomain protein, CEH-37 could play an important role in telomere function via DNA binding.     

Solution structure of the inner DysF domain of myoferlin and implications for limb girdle muscular dystrophy type 2b

Mutations in the protein dysferlin, a member of the ferlin family, lead to limb girdle muscular dystrophy type 2B and Myoshi myopathy. The ferlins are large proteins characterised by multiple C2 domains and a single C-terminal membrane-spanning helix. However, there is sequence conservation in some of the ferlin family in regions outside the C2 domains. In one annotation of the domain structure of these proteins, an unusual internal duplication event has been noted where a putative domain is inserted in between the N- and C-terminal parts of a homologous domain. This domain is known as the DysF domain. Here, we present the solution structure of the inner DysF domain of the dysferlin paralogue myoferlin, which has a unique fold held together by stacking of arginine and tryptophans, mutations that lead to clinical disease in dysferlin.     

Allochromatium vinosum DsrC: solution-state NMR structure, redox properties, and interaction with DsrEFH, a protein essential for purple sulfur bacterial sulfur oxidation

Sequenced genomes of dissimilatory sulfur-oxidizing and sulfate-reducing bacteria containing genes coding for DsrAB, the enzyme dissimilatory sulfite reductase, inevitably also contain the gene coding for the 12-kDa DsrC protein. DsrC is thought to have a yet unidentified role associated with the activity of DsrAB. Here we report the solution structure of DsrC from the sulfur-oxidizing purple sulfur bacterium Allochromatium vinosum determined with NMR spectroscopy in reducing conditions, and we describe the redox behavior of two conserved cysteine residues upon transfer to an oxidizing environment. In reducing conditions, the DsrC structure is disordered in the highly conserved carboxy-terminus. We present multiple lines of evidence that, in oxidizing conditions, a strictly conserved cysteine (Cys111) at the penultimate position in the sequence forms an intramolecular disulfide bond with Cys100, which is conserved in DsrC in all organisms with DsrAB. While an intermolecular Cys111-Cys111 disulfide-bonded dimer is rapidly formed under oxidizing conditions, the intramolecularly disulfide-bonded species (Cys100-Cys111) is the thermodynamically stable form of the protein under these conditions. Treatment of the disulfidic forms with reducing agent regenerates the monomeric species that was structurally characterized. Using a band-shift technique under nondenaturing conditions, we obtained evidence for the interaction of DsrC with heterohexameric DsrEFH, a protein encoded in the same operon. Mutation of Cys100 to serine prevented formation of the DsrC species assigned as an intramolecular disulfide in oxidizing conditions, while still allowing formation of the intermolecular Cys111-Cys111 dimer. In the reduced form, this mutant protein still interacted with DsrEFH. This was not the case for the Cys111Ser and Cys100Ser/Cys111Ser mutants, both of which also did not form protein dimers. Our observations highlight the central importance of the carboxy-terminal DsrC cysteine residues and are consistent with a role as a sulfur-substrate binding/transferring protein, as well as with an electron-transfer function via thiol-disulfide interchanges.     

Solution structure and characterization of the DNA-binding activity of the B3BP-Smr domain

The MutS1 protein recognizes unpaired bases and initiates mismatch repair, which are essential for high-fidelity DNA replication. The homologous MutS2 protein does not contribute to mismatch repair, but suppresses homologous recombination. MutS2 lacks the damage-recognition domain of MutS1, but contains an additional C-terminal extension: the small MutS-related (Smr) domain. This domain, which is present in both prokaryotes and eukaryotes, has previously been reported to bind to DNA and to possess nicking endonuclease activity. We determine here the solution structure of the functionally active Smr domain of the Bcl3-binding protein (also known as Nedd4-binding protein 2), a protein with unknown function that lacks other domains present in MutS proteins. The Smr domain adopts a two-layer α-β sandwich fold, which has a structural similarity to the C-terminal domain of IF3, the R3H domain, and the N-terminal domain of DNase I. The most conserved residues are located in three loops that form a contiguous, exposed, and positively charged surface with distinct sequence identity for prokaryotic and eukaryotic Smr domains. NMR titration experiments and DNA binding studies using Bcl3-binding protein-Smr domain mutants suggested that these most conserved loop regions participate in DNA binding to single-stranded/double-stranded DNA junctions. Based on the observed DNA-binding-induced multimerization, the structural similarity with both subdomains of DNase I, and the experimentally identified DNA-binding surface, we propose a model for DNA recognition by the Smr domain.     

Solution structure of NOD1 CARD and mutational analysis of its interaction with the CARD of downstream kinase RICK

NOD1 is a cytosolic signalling host pattern-recognition receptor composed of a caspase-activating and recruitment domain (CARD), a nucleotide-binding and oligomerization domain (NOD) and leucine-rich repeats. It plays a crucial role in innate immunity by activating the NF-kappaB pathway via its downstream effector the kinase RICK (RIP2) following the recognition of a specific bacterial ligand. RICK is recruited by NOD1 through interaction of their respective CARDs. Here we present the high resolution NMR structure of the NOD1 CARD. It is generally similar to other CARDs of known structure, consisting of six tightly packed helices, although the length and orientation of the last helix is unusual. Mutations in both the NOD1 and RICK CARD domains were assayed by immuno-precipitation of cell lysates and in vivo NF-kappaB activation in order to define residues important for CARD-CARD interaction and downstream signalling. The results show that the interaction is critically dependent on three acidic residues on NOD1 CARD and three basic residues on RICK CARD and thus is likely to have a strong electrostatic component, similar to other characterised CARD-CARD interactions.     

Solution structure and dynamics of the N-terminal cytosolic domain of rhomboid intramembrane protease from Pseudomonas aeruginosa: insights into a functional role in intramembrane proteolysis

Rhomboids are ubiquitous integral membrane proteases that release cellular signals from membrane-bound substrates through a general signal transduction mechanism known as regulated intramembrane proteolysis (RIP). We present the NMR structure of the cytosolic N-terminal domain (NRho) of P. aeruginosa Rhomboid. NRho consists of a novel alpha/beta fold and represents the first detailed structural insight into this class of intramembrane proteases. We find evidence that NRho is capable of strong and specific association with detergent micelles that mimic the membrane/water interface. Relaxation measurements on NRho reveal structural fluctuations on the microseconds-milliseconds timescale in regions including and contiguous to those implicated in membrane interaction. This structural plasticity may facilitate the ability of NRho to recognize and associate with membranes. We suggest that NRho plays a role in scissile peptide bond selectivity by optimally positioning the Rhomboid active site relative to the membrane plane.     

Improved structural characterizations of the drkN SH3 domain unfolded state suggest a compact ensemble with native-like and non-native structure

Due to their dynamic ensemble nature and a deficiency of experimental restraints, disordered states of proteins are difficult to characterize structurally. Here, we have expanded upon our previous work on the unfolded state of the Drosophila drk N-terminal (drkN) SH3 domain with our program ENSEMBLE, which assigns population weights to pregenerated conformers in order to calculate ensembles of structures whose properties are collectively consistent with experimental measurements. The experimental restraint set has been enlarged with newly measured paramagnetic relaxation enhancements from Cu(2+) bound to an amino terminal Cu(2+)-Ni(2+) binding (ATCUN) motif as well as nuclear Overhauser effect (NOE) and hydrogen exchange data from recent studies. In addition, two new pseudo-energy minimization algorithms have been implemented that have dramatically improved the speed of ENSEMBLE population weight assignment. Finally, we have greatly improved our conformational sampling by utilizing a variety of techniques to generate both random structures and structures that are biased to contain elements of native-like or non-native structure. Although it is not possible to uniquely define a representative structural ensemble, we have been able to assess various properties of the drkN SH3 domain unfolded state by performing ENSEMBLE minimizations of different conformer pools. Specifically, we have found that the experimental restraint set enforces a compact structural distribution that is not consistent with an overall native-like topology but shows preference for local non-native structure in the regions corresponding to the diverging turn and the beta5 strand of the folded state and for local native-like structure in the region corresponding to the beta6 and beta7 strands. We suggest that this approach could be generally useful for the structural characterization of disordered states.