Folding of the C-terminal bacterial binding domain in statherin upon adsorption onto hydroxyapatite crystals

Sum frequency generation and solid-state NMR study of the structure, orientation, and dynamics of polystyrene-adsorbed peptides

The power of combining sum frequency generation (SFG) vibrational spectroscopy and solid-state nuclear magnetic resonance (ssNMR) spectroscopy to quantify, with site specificity and atomic resolution, the orientation and dynamics of side chains in synthetic model peptides adsorbed onto polystyrene (PS) surfaces is demonstrated in this study. Although isotopic labeling has long been used in ssNMR studies to site-specifically probe the structure and dynamics of biomolecules, the potential of SFG to probe side chain orientation in isotopically labeled surface-adsorbed peptides and proteins remains largely unexplored. The 14 amino acid leucine-lysine peptide studied in this work is known to form an alpha-helical secondary structure at liquid-solid interfaces. Selective, individual deuteration of the isopropyl group in each leucine residue was used to probe the orientation and dynamics of each individual leucine side chain of LKalpha14 adsorbed onto PS. The selective isotopic labeling methods allowed SFG analysis to determine the orientations of individual side chains in adsorbed peptides. Side chain dynamics were obtained by fitting the deuterium ssNMR line shape to specific motional models. Through the combined use of SFG and ssNMR, the dynamic trends observed for individual side chains by ssNMR have been correlated with side chain orientation relative to the PS surface as determined by SFG. This combination provides a more complete and quantitative picture of the structure, orientation, and dynamics of these surface-adsorbed peptides than could be obtained if either technique were used separately.

Electron microscopic immunogold localization of statherin in human minor salivary glands

In this study, which supplements a recent article on the localization of statherin in human major salivary glands, we investigated the intracellular distribution of this peptide in minor salivary glands by immunogold cytochemistry at the electron microscopy level. In the lingual serous glands of von Ebner, gold particles were found in serous granules of all secreting cells, indicating that statherin is released through granule exocytosis. In buccal and labial glands, mostly composed of mucous tubuli, statherin reactivity was detected in the serous element, which represents only a small population of the glandular parenchyma. In these serous cells, however, statherin labeling was absent in secretory granules and restricted to small cytoplasmic vesicles near or partially fused with granules. Vesicle labeling could be related to the occurrence of an alternative secretory pathway for statherin in buccal and labial glands.

A (13)C{(31)P} REDOR NMR investigation of the role of glutamic acid residues in statherin- hydroxyapatite recognition

The side chain carboxyl groups of acidic proteins found in the extra-cellular matrix (ECM) of mineralized tissues play a key role in promoting or inhibiting the growth of minerals such as hydroxyapatite (HAP), the principal mineral component of bone and teeth. Among the acidic proteins found in the saliva is statherin, a 43-residue tyrosine-rich peptide that is a potent lubricant in the salivary pellicle and an inhibitor of both HAP crystal nucleation and growth. Three acidic amino acids-D1, E4, and E5-are located in the N-terminal 15 amino acid segment, with a fourth amino acid, E26, located outside the N-terminus. We have utilized (13)C{(31)P} REDOR NMR to analyze the role played by acidic amino acids in the binding mechanism of statherin to the HAP surface by measuring the distance between the delta-carboxyl (13)C spins of the three glutamic acid side chains of statherin (residues E4, E5, E26) and (31)P spins of the phosphate groups at the HAP surface. (13)C{(31)P} REDOR studies of glutamic-5-(13)C acid incorporated at positions E4 and E26 indicate a (13)C-(31)P distance of more than 6.5 A between the side chain carboxyl (13)C spin of E4 and the closest (31)P in the HAP surface. In contrast, the carboxyl (13)C spin at E5 has a much shorter (13)C-(31)P internuclear distance of 4.25 +/- 0.09 A, indicating that the carboxyl group of this side chain interacts directly with the surface. (13)C T(1rho) and slow-spinning MAS studies indicate that the motions of the side chains of E4 and E5 are more restricted than that of E26. Together, these results provide further insight into the molecular interactions of statherin with HAP surfaces.

Characterization of the dominant molecular step orientations on hydroxyapatite (100) surfaces

Hydroxyapatite (HAP) is the major inorganic component of bones and teeth. The characterization of HAP surfaces on the molecular level is important for achieving a fundamental understanding of bone remodeling and dental caries processes. On the microscopic level, hydroxyapatite growth and dissolution reactions mainly occur at steps. Therefore, this study focuses on individual molecular steps on HAP (100) facets under both static conditions and dynamic dissolution conditions using atomic force microscopy (AFM). We found that molecular steps parallel to the elongated axes of HAP crystals and those angled approximately 54 degrees against the elongated axis are not only energetically favorable but also kinetically dominant under dissolution conditions.

Interplay of sequence, conformation, and binding at the Peptide-titania interface as mediated by water

The initial stages of the adsorption of a hexapeptide at the aqueous titania interface are modeled using atomistic molecular dynamics simulations. This hexapeptide has been identified by experiment [Sano, K. I.; Shiba, K. J. Am. Chem. Soc. 2003, 125, 14234] to bind to Ti particles. We explore the current hypothesis presented by these authors that binding at this peptide-titania interface is the result of electrostatic interactions and find that contact with the surface appears to take place via a pair of oppositely charged groups in the peptide. Our data indicate that the peptide may initially recognize the water layers at the interface, not the titania surface itself, via these charged groups. We also report results of simulations for hexapeptide sequences with selected single-point mutations for alanine and compare these behaviors with those suggested from observed binding affinities from existing alanine scan experiments. Our results indicate that factors in addition to electrostatics also contribute, with the structural rigidity conferred by proline suggested to play a significant role. Finally, our findings suggest that intrapeptide interaction may provide mechanisms for surface detachment that could be detrimental to binding at the interface.

Solution- and adsorbed-state structural ensembles predicted for the statherin-hydroxyapatite system

We have developed a multiscale structure prediction technique to study solution- and adsorbed-state ensembles of biomineralization proteins. The algorithm employs a Metropolis Monte Carlo-plus-minimization strategy that varies all torsional and rigid-body protein degrees of freedom. We applied the technique to fold statherin, starting from a fully extended peptide chain in solution, in the presence of hydroxyapatite (HAp) (001), (010), and (100) monoclinic crystals. Blind (unbiased) predictions capture experimentally observed macroscopic and high-resolution structural features and show minimal statherin structural change upon adsorption. The dominant structural difference between solution and adsorbed states is an experimentally observed folding event in statherin's helical binding domain. Whereas predicted statherin conformers vary slightly at three different HAp crystal faces, geometric and chemical similarities of the surfaces allow structurally promiscuous binding. Finally, we compare blind predictions with those obtained from simulation biased to satisfy all previously published solid-state NMR (ssNMR) distance and angle measurements (acquired from HAp-adsorbed statherin). Atomic clashes in these structures suggest a plausible, alternative interpretation of some ssNMR measurements as intermolecular rather than intramolecular. This work demonstrates that a combination of ssNMR and structure prediction could effectively determine high-resolution protein structures at biomineral interfaces.

Adsorption of follicular dendritic cell-secreted protein (FDC-SP) onto mineral deposits. Application of a new stable gene expression system

Follicular dendritic cell-secreted protein (FDC-SP) is a small secretory protein having structural similarities to statherin, a protein in saliva thought to play a role in calcium retention in saliva. In contrast, FDC-SP is thought to play a role in the immune system associated with germinal centers. We report here the very specific expression of FDC-SP in junctional epithelium at the gingival crevice. This region is very important for the host defense against pathogens and for periodontal protection. To be able to better understand the function of FDC-SP, we developed a novel gene expression system that exploited gene trapping and site-specific gene integration to introduce the protein into a mammalian cell culture system. Using this system we were able to express FDC-SP as a fusion protein with green fluorescent protein in an osteogenic progenitor cell line with long term stability, which we then used to find that the fusion protein specifically adsorbs onto mineral deposits and the surface of hydroxyapatite particles exogenously added to the culture. This adsorption was highly dependent on the structural integrity of FDC-SP. These results suggest that FDC-SP may play an important role, adsorbing onto the surface of cementum and alveolar bone adjacent to periodontal ligament and onto tooth surface at the gingival crevice.

The structure and orientation of the C-terminus of LRAP

Amelogenin is the predominant protein found during enamel development and is thought to be the biomineralization protein controlling the unique elongated hydroxyapatite crystals that constitute enamel. The secondary structure of biomineralization proteins is thought to be important in the interaction with hydroxyapatite. Unfortunately, very little data are available on the structure or the orientation of amelogenin, either in solution or bound to hydroxyapatite. The C-terminus contains the majority of the charged residues and is predicted to interact with hydroxyapatite; thus, we used solid-state NMR dipolar recoupling techniques to investigate the structure and orientation of the C-terminus of LRAP, a naturally occurring splice variant of full-length amelogenin. Using (13)C{(15)N} Rotational Echo DOuble Resonance (REDOR), the structure of the C-terminus was found to be largely random coil, both on the surface of hydroxyapatite as well as lyophilized from solution. The orientation of the C-terminal region with respect to hydroxyapatite was investigated for two alanine residues (Ala(46) and Ala(49)) using (13)C{(31)P} REDOR and one lysine residue (Lys(52)) using (15)N{(31)P} REDOR. The residues examined were found to be 7.0, 5.7, and 5.8 A from the surface of hydroxyapatite for Ala(46), Ala(49), and Lys(52), respectively. This provides direct evidence that the charged C-terminus is interacting closely with hydroxyapatite, positioning the acidic amino acids to aid in controlling crystal growth. However, solid-state NMR dynamics measurements also revealed significant mobility in the C-terminal region of the protein, in both the side chains and the backbone, suggesting that this region alone is not responsible for binding.

Two and three-dimensional pattern recognition of organized surfaces by specific antibodies

Understanding molecular recognition of supramolecules for solid substrates is essential for designing chemical sensors and molecular devices. The rules of molecular recognition are well established at the level of single molecules. However, during the transition from molecular-scale devices to macroscopic devices, issues concerning control over recognition that are well-established at the molecular level become much more complex. Hopefully, the conceptual and practical considerations reported here will clarify some of these issues. The immune system uses antibodies to identify molecular surfaces through molecular recognition. Antibodies are thus appropriate tools to study the rules of macromolecule-surface interactions, and this was done using crystal surfaces as substrates. Crystals can be formed or introduced into organisms and should be thus treated by the organism as any other intruder, by eliciting antibodies specific to their surfaces. A structure-recognizing antibody is defined here as complementary to a certain ordered supramolecular organization. It can be considered as a mold bearing in its binding site memory of the organization against which it was elicited. On the surface of a crystal composed of relatively small organic molecules, an antibody binding site would encompass an array of 10-20 molecular moieties. The antibody binding site would not detect one molecule, but rather a two- or three-dimensional molecular arrangement on the surface, similar to a macromolecular surface. The complementarity between antibody binding site and surface is supported by stereoselective supramolecular interactions to the repetitive structural motifs that are exposed at the surface. A procedure was developed in order to isolate monoclonal antibodies that specifically recognize a certain crystalline surface. The procedure was applied in particular to crystals of cholesterol monohydrate, of 1,4-dinitrobenzene, and of the tripeptide (S)leucine-(S)leucine-(S)tyrosine (LLY). A series of antibodies were selected and studied, three of which provided reliable specific antibody-antigen structural models. The three docking models show an astounding geometrical and chemical match of the antibody binding sites on the respective crystal surfaces. We also showed that antibodies are intrinsically capable of recognition at the length scale necessary for detection of chirality. Once the structural parameters determining the antibody specificity to the target surfaces are characterized, the antibodies may be conceivably used as reporters of the existence and location of target domains with similar structure in biological milieus. In this context, we developed and characterized monoclonal antibodies specific to crystalline mixed monolayers of cholesterol and ceramide, fundamental building blocks of lipid microdomains in cellular membranes. When used on cells, one antibody indeed labels cell membrane domains composed of cholesterol and ceramide. The fundamental contribution of the approach developed here may be in the antibody ability to report on the structural organization of paracrystalline domains that cannot be determined by other means. Alternatively, structure-recognizing antibodies may be conceivably used to carry information or build connections to specific targets, which may offer interesting developments in medicine or electronics.

Salivary agglutinin/glycoprotein-340/DMBT1: a single molecule with variable composition and with different functions in infection, inflammation and cancer

Salivary agglutinin (SAG), lung glycoprotein-340 (gp-340) and Deleted in Malignant Brain Tumours 1 (DMBT1) are three names for identical proteins encoded by the dmbt1 gene. DMBT1/SAG/gp-340 belongs to the scavenger receptor cysteine-rich (SRCR) superfamily of proteins, a superfamily of secreted or membrane-bound proteins with SRCR domains that are highly conserved down to sponges, the most ancient metazoa. On the one hand, DMBT1 may represent an innate defence factor acting as a pattern recognition molecule. It interacts with a broad range of pathogens, including cariogenic streptococci and Helicobacter pylori, influenza viruses and HIV, but also with mucosal defence proteins, such as IgA, surfactant proteins and MUC5B. Stimulation of alveolar macrophage migration, suppression of neutrophil oxidative burst and activation of the complement cascade point further to an important role in the regulation of inflammatory responses. On the other hand, DMBT1 has been demonstrated to play a role in epithelial and stem cell differentiation. Inactivation of the gene coding for this protein may lead to disturbed differentiation, possibly resulting in tumour formation. These data strongly point to a role for DMBT1 as a molecule linking innate immune processes with regenerative processes.

The structure, dynamics, and energetics of protein adsorption-lessons learned from adsorption of statherin to hydroxyapatite

Proteins are found to be involved in interaction with solid surfaces in numerous natural events. Acidic proteins that adsorb to crystal faces of a biomineral to control the growth and morphology of hard tissue are only one example. Deducing the mechanisms of surface recognition exercised by proteins has implications to osteogenesis, pathological calcification and other proteins functions at their adsorbed state. Statherin is an enamel pellicle protein that inhibits hydroxyapatite nucleation and growth, lubricates the enamel surface, and is recognized by oral bacteria in periodontal diseases. Here, we highlight some of the insights we obtained recently using both thermodynamic and solid state NMR measurements to the adsorption process of statherin to hydroxyapatite. We combine macroscopic energy characterization with microscopic structural findings to present our views of protein adsorption mechanisms and the structural changes accompanying it and discuss the implications of these studies to understanding the functions of the protein adsorbed to the enamel surfaces.

Conformational changes in salivary proline-rich protein 1 upon adsorption to calcium phosphate crystals

Conformational analyses of PRP1, a proline-rich acidic salivary protein and major component of the acquired enamel pellicle, have been carried out in solution and upon binding to two enamel prototypes, hydroxyapatite (HA) and carbonated hydroxyapatite (CHA), using Fourier transform infrared spectroscopy (FTIR) in attenuated total reflection (ATR) mode. We have shown for the first time that, in solution, large portions of PRP1 adopt the hydrated polyproline type II (PPII) helical structure in addition to the random coil structure, with the maximum absorbance of the amide I band around 1620 cm(-1). Upon binding to HA or CHA, the protein undergoes significant conformational changes, loosing a considerable portion of hydrated PPII and random coil domains with a shift in the maximum absorbance to 1666 cm(-1), indicating that a large fraction of the protein is composed of beta turns. A small fraction of PPII in a calcium-bound or anhydrous form (approximately 1642 cm(-1)) was also observed in the HA- and CHA-bound proteins, which could play a role in protein-mineral interactions. The conformational changes in PRP1 adsorbed on CHA and HA were similar in nature; however, these changes were greater in the protein bound to HA. Interestingly, these results are in agreement with protein adsorption data that show that less protein is adsorbed onto CHA than onto HA. Our results demonstrate that binding to apatitic mineral surfaces leads to major conformational changes in PRP1, which might reflect the expulsion of water and the formation of protein-mineral and/or protein-protein interactions in the adsorbed layer.