The Structure of the Cys-rich Terminal Domain of Hydra Minicollagen, Which Is Involved in Disulfide Networks of the Nematocyst Wall

The minicollagens found in the nematocysts of Hydra constitute a family of invertebrate collagens with unusual properties. They share a common modular architecture with a central collagen sequence ranging from 14 to 16 Gly-X-Y repeats flanked by polyproline/hydroxyproline stretches and short terminal domains that show a conserved cysteine pattern (CXXXCXXXCXXX-CXXXCC). The minicollagen cysteine-rich domains are believed to function in a switch of the disulfide connectivity from intra- to intermolecular bonds during maturation of the capsule wall. The solution structure of the C-terminal fragment including a minicollagen cysteine-rich domain of minicollagen-1 was determined in two independent groups by 1H NMR. The corresponding peptide comprising the last 24 residues of the molecule was produced synthetically and refolded by oxidation under low protein concentrations. Both presented structures are identical in their fold and disulfide connections (Cys2-Cys18, Cys6-Cys14, and Cys10-Cys19) revealing a robust structural motif that is supposed to serve as the polymerization module of the nematocyst capsule.

Hydroxylation-induced stabilization of the collagen triple helix. Acetyl-(glycyl-4(R)-hydroxyprolyl-4(R)-hydroxyprolyl)(10)-NH(2) forms a highly stable triple helix

The collagen triple helix is one of the most abundant protein motifs in animals. The structural motif of collagen is the triple helix formed by the repeated sequence of -Gly-Xaa-Yaa-. Previous reports showed that H-(Pro-4(R)Hyp-Gly)(10)-OH (where '4(R)Hyp' is (2S,4R)-4-hydroxyproline) forms a trimeric structure, whereas H-(4(R)Hyp-Pro-Gly)(10)-OH does not form a triple helix. Compared with H-(Pro-Pro-Gly)(10)-OH, the melting temperature of H-(Pro-4(R)Hyp-Gly)(10)-OH is higher, suggesting that 4(R)Hyp in the Yaa position has a stabilizing effect. The inability of triple helix formation of H-(4(R)Hyp-Pro-Gly)(10)-OH has been explained by a stereoelectronic effect, but the details are unknown. In this study, we synthesized a peptide that contains 4(R)Hyp in both the Xaa and the Yaa positions, that is, Ac-(Gly-4(R)Hyp-4(R)Hyp)(10)-NH(2) and compared it to Ac-(Gly-Pro-4(R)Hyp)(10)-NH(2), and Ac-(Gly-4(R)Hyp-Pro)(10)-NH(2). Ac-(Gly-4(R)Hyp-4(R)Hyp)(10)-NH(2) showed a polyproline II-like circular dichroic spectrum in water. The thermal transition temperatures measured by circular dichroism and differential scanning calorimetry were slightly higher than the values measured for Ac-(Gly-Pro-4(R)Hyp)(10)-NH(2) under the same conditions. For Ac-(Gly-4(R)Hyp-4(R)Hyp)(10)-NH(2), the calorimetric and the van't Hoff transition enthalpy DeltaH were significantly smaller than that of Ac-(Gly-Pro-4(R)Hyp)(10)-NH(2). We postulate that the denatured states of the two peptides are significantly different, with Ac-(Gly-4(R)Hyp-4(R)Hyp)(10)-NH(2) forming a more polyproline II-like structure instead of a random coil. Two-dimensional nuclear Overhauser effect spectroscopy suggests that the triple helical structure of Ac-(Gly-4(R)Hyp-4(R)Hyp)(10)-NH(2) is more flexible than that of Ac-(Gly-Pro-4(R)Hyp)(10)-NH(2). This is confirmed by the kinetics of amide (1)H exchange with solvent deuterium of Ac-(Gly-4(R)Hyp-4(R)Hyp)(10)-NH(2), which is faster than that of Ac-(Gly-Pro-4(R)Hyp)(10)-NH(2). The higher transition temperature of Ac-(Gly-4(R)Hyp-4(R)Hyp)(10)-NH(2), can be explained by the higher trans/cis ratio of the Gly-4(R)Hyp peptide bonds than that of the Gly-Pro bonds, and this ratio compensates for the weaker interchain hydrogen bonds.

Calorimetric Studies on the Tight Binding Metal Interactions of Escherichia coli Manganese Superoxide Dismutase

Escherichia coli apomanganese superoxide dismutase, prepared by removing the native metal ion under denaturing conditions, exhibits thermally triggered metal uptake behavior previously observed for thermophilic and hyperthermophilic superoxide dismutases but over a lower temperature range. Differential scanning calorimetry of aposuperoxide dismutase and metalated superoxide dismutase unfolding transitions has provided quantitative estimates of the metal binding affinities for manganese superoxide dismutase. The binding constant for Mn(II) (K(Mn(II)) = 3.2 x 10(8) m(-1)) is surprisingly low in light of the essentially irreversible metal binding characteristic of this family of proteins and indicates that metal binding and release processes are dominated by kinetic, rather than thermodynamic, constraints. The kinetic stability of the metalloprotein complex can be traced to stabilization by elements of the protein that are independent of the presence or absence of the metal ion reflected in the thermally triggered metalation characteristic of these proteins. Binding constants for Mn(III), Fe(II), and Fe(III) complexes were estimated using quasireversible values for the unfolding enthalpy and DeltaC(p) for apo-Mn superoxide dismutase and the observed T(m) values for unfolding the metalated species in the absence of denaturants. For manganese and iron complexes, an oxidation state-dependent binding affinity reflects the protein perturbation of the metal redox potential.

Prolyl 3-Hydroxylase 1, Enzyme Characterization and Identification of a Novel Family of Enzymes

The collagen prolyl hydroxylases are enzymes that are required for proper collagen biosynthesis, folding, and assembly. They reside within the endoplasmic reticulum and belong to the group of 2-oxoglutarate and iron-dependent dioxygenases. Although prolyl 4-hydroxylase has been characterized as an alpha2beta2 tetramer in which protein disulfide isomerase is the beta subunit with two different alpha subunit isoforms, little is known about the enzyme prolyl 3-hydroxylase (P3H). It was initially characterized and shown to have an enzymatic activity distinct from that of prolyl 4-hydroxylase, but no amino acid sequences or genes were ever reported for the mammalian enzyme. Here we report the characterization of a novel prolyl 3-hydroxylase enzyme isolated from embryonic chicks. The primary structure of the enzyme, which we now call P3H1, demonstrates that P3H1 is a member of a family of prolyl 3-hydroxylases, which share the conserved residues present in the active site of prolyl 4-hydroxylase and lysyl hydroxylase. P3H1 is the chick homologue of mammalian leprecan or growth suppressor 1. Two other P3H family members are the genes previously called MLAT4 and GRCB. In this study we demonstrate prolyl 3-hydroxylase activity of the purified enzyme P3H1 on a full-length procollagen substrate. We also show it to specifically interact with denatured collagen and to exist in a tight complex with other endoplasmic reticulum-resident proteins. Immunohistochemistry with a monoclonal antibody specific for chick P3H1 localizes P3H1 specifically to tissues that express fibrillar collagens, suggesting that other P3H family members may be responsible for modifying basement membrane collagens.

Laser light-scattering evidence for an altered association of beta B1-crystallin deamidated in the connecting peptide

Deamidation is a prevalent modification of crystallin proteins in the vertebrate lens. The effect of specific sites of deamidation on crystallin stability in vivo is not known. Using mass spectrometry, a previously unreported deamidation in beta B1-crystallin was identified at Gln146. Another deamidation was investigated at Asn157. It was determined that whole soluble beta B1 contained 13%-17% deamidation at Gln146 and Asn157. Static and quasi-elastic laser light scattering, circular dichroism, and heat aggregation studies were used to explore the structure and associative properties of recombinantly expressed wild-type (wt) beta B1 and the deamidated beta B1 mutants, Q146E and N157D. Dimer formation occurred for wt beta B1, Q146E, and N157D in a concentration-dependent manner, but only Q146E showed formation of higher ordered oligomers at the concentrations studied. Deamidation at Gln146, but not Asn157, led to an increased tendency of beta B1 to aggregate upon heating. We conclude that deamidation creates unique effects depending upon where the deamidation is introduced in the crystallin structure.

Hydroxylation-induced stabilization of the collagen triple helix. Further characterization of peptides with 4(R)-hydroxyproline in the Xaa position

4(R)-Hydroxyproline in the Yaa position of the -Gly-Xaa-Yaa-repeated sequence of collagen plays a crucial role in the stability of the triple helix. Since the peptide (4(R)-Hyp-Pro-Gly)10 does not form a triple helix, it was generally believed that polypeptides with a -Gly-4(R)-Hyp-Yaa-repeated sequence do not form a triple helix. Recently, we found that acetyl-(Gly-4(R)-Hyp-Thr)10-NH2 forms a triple helix in aqueous solutions. To further study the role of 4(R)-hydroxyproline in the Xaa position, we made a series of acetyl-(Gly-4(R)-Hyp-Yaa)10-NH2 peptides where Yaa was alanine, serine, valine, and allo-threonine. We previously hypothesized that the hydroxyl group of threonine might form a hydrogen bond to the hydroxyl group of 4(R)hydroxyproline. In water, only the threonine- and the valine-containing peptides were triple helical. The remaining peptides did not form a triple helix in water. In 1,2- and in 1,3-propanediol at 4 degrees C, all the soluble peptides were triple helical. From the transition temperature of the triple helices, it was found that among the examined residues, threonine was the most stable residue in the acetyl-(Gly-4(R)-Hyp-Yaa)10-NH2 peptide. The transition temperatures of the valine- and allo-threonine-containing peptides were 10 degrees lower than those of the threonine peptide. Surprisingly, the serine-containing peptide was the least stable. These results indicate that the stability of these peptides depends on the presence of a methyl group as well as the hydroxyl group and that the stereo configuration of the two groups is essential for the stability. In the threonine peptide, we hypothesize that the methyl group shields the interchain hydrogen bond between the glycine and the Xaa residue from water and that the hydroxyl groups of threonine and 4(R)hydroxyproline can form direct or water-mediated hydrogen bonds.

Role of carbohydrate in stabilizing the triple-helix in a model for a deep-sea hydrothermal vent worm collagen

The glycopeptide Ac-(Gly-Pro-Thr(beta-Gal))(10)-NH(2) forms a collagen-like triple-helix. A (1)H NMR structural analysis is reported for the peptides Ac-(Gly-Pro-Thr)(n)-NH(2) and Ac-(Gly-Pro-Thr(beta-Gal))(n)-NH(2), where n = 1, 5, and 10. NMR assignments for the individual peptides are made using one- and two-dimensional TOCSY, ROESY, and NOESY experiments. The NMR and corroborating CD data show that Ac-(Gly-Pro-Thr)(n)-NH(2), n = 1, 5, or 10, as well as Ac-(Gly-Pro-Thr(beta-Gal))(n)-NH(2), n = 1 or 5 peptides are unable to form collagen-like triple-helical structures. Furthermore, the equilibrium ratio of cis to trans isomers of the Pro residues is unaffected by the presence of carbohydrate. For Ac-(Gly-Pro-Thr(beta-Gal))(10)-NH(2), the kinetics of amide (1)H exchange with solvent deuterium indicate a slow rate of exchange for both the Gly and the Thr amide. The data are thus consistent with a model in which the carbohydrate stabilizes the triple helix through an occlusion of water molecules and by hydrogen bonding but not through an influence on the cis to trans isomer ratio.

Collagen triple helix formation can be nucleated at either end

The directional dependence of folding rates for rod-like macromolecules such as parallel alpha-helical coiled-coils, DNA double-helices, and collagen triple helices is largely unexplored. This is mainly due to technical difficulties in measuring rates in different directions. Folding of collagens is nucleated by trimeric non-collagenous domains. These are usually located at the COOH terminus, suggesting that triple helix folding proceeds from the COOH to the NH(2) terminus. Evidence is presented here that effective nucleation is possible at both ends of the collagen-like peptide (Gly-Pro-Pro)(10), using designed proteins in which this peptide is fused either NH(2)- or COOH-terminal to a nucleation domain, either T4-phage foldon or the disulfide knot of type III collagen. The location of the nucleation domain influences triple-helical stability, which might be explained by differences in the linker sequences and the presence or absence of repulsive charges at the carboxyl-terminal end of the triple helix. Triple helical folding rates are found to be independent of the site of nucleation and consistent with cis-trans isomerization being the rate-limiting step.

Deamidation, but not truncation, decreases the urea stability of a lens structural protein, betaB1-crystallin

Crystallins, the major structural proteins in the lens of the eye, are maintained with little turnover throughout the lifetime of the host. With time, lens crystallins undergo post-translational modifications that may play an important role in loss of vision during aging and cataract formation. Specific modifications include deamidation and truncation. Urea-induced denaturation was studied for recombinantly expressed wild-type betaB1 (WT), the deamidated mutant (Q204E), an N-terminally truncated mutant (betaB1(DeltaN41)), and other truncated versions of these proteins generated by calpain II digestion. Tryptophan fluorescence was used to monitor loss of global tertiary structure. Loss of secondary structure was followed by circular dichroism, and electron paramagnetic resonance site-directed spin labeling was used to monitor loss of tertiary structure selectively in the N-terminal domain. Our results indicated that the deamidated mutant was significantly destabilized relative to WT. Q204E showed a two-step denaturation curve with transitions at 4.1 and 7.2 M urea, whereas denaturation of WT occurred in a cooperative single step with a transition midpoint of 5.9 M urea. Unfolding of WT was completely reversible, whereas Q204E failed to fully refold. Prolonged incubation under denaturing conditions led to aggregation, which was also more pronounced for Q204E dimers than for WT. Truncation of 41 residues from the N-terminus or 47 and 5 residues from the N- and C-termini did not affect stability. These studies indicated that a single-site deamidation could significantly diminish the stability of lens betaB1-crystallin, supporting the idea that such modifications may play an important role in age-related cataract formation.

Decreased heat stability and increased chaperone requirement of modified human betaB1-crystallins

PURPOSE

To determine how deamidation and partial loss of the N- and C-terminal extensions alter the heat stability of betaB1-crystallin.

METHODS

Human lens betaB1, a deamidated betaB1, Q204E, and alphaA-crystallins were expressed. Truncated betaB1 was generated by proteolytic removal of part of its terminal extensions. The aggregation and precipitation of these proteins due to heating was monitored by circular dichroism and light scattering. The effect of heat on the stability of both monomers and oligomers was investigated. The flexibility of the extensions in wild type and deamidated betaB1 was assessed by 1H NMR spectroscopy.

RESULTS

With heat, deamidated betaB1 precipitated more readily than wild type betaB1. Similar effects were obtained for either monomers or oligomers. Flexibility of the N-terminal extension in deamidated betaB1 was significantly reduced compared to the wild type protein. Truncation of the extensions further increased the rate of heat-induced precipitation of deamidated betaB1. The presence of the molecular chaperone, alphaA-crystallin, prevented precipitation of modified betaB1s. More alphaA was needed to chaperone the truncated and deamidated betaB1 than deamidated betaB1 or truncated betaB1.

CONCLUSIONS

Deamidation and truncation of betaB1 led to destabilization of the protein and decreased stability to heat. Decreased stability of lens crystallins may contribute to their insolubilization and cataract formation.

Hantavirus nucleocapsid protein coiled-coil domains

The nucleocapsid (N) proteins of hantaviruses such as the Sin Nombre virus (SNV) bind to membranes and viral RNAs, associate with transcription and replication complexes, and oligomerize during the process of virus assembly. N proteins trimerize in vitro and in vivo, and associate via assembly domains at their amino- and carboxyl-terminal ends. Because structure prediction algorithms suggested that N protein residues 3-75 form two coiled-coil motifs separated by an intervening kink or turn sequence, we examined the properties of peptides representing SNV N protein residues 3-35, 43-75, and 3-75. Of the three peptides, N-(3-35) assembled coiled-coil oligomers only at high concentration and low temperature. In contrast, N-(43-75) efficiently trimerized at low concentration, implying that it carries a coiled-coil trigger sequence. Interestingly, while the longer peptide, N-(3-75), assembled dimers and/or trimers at high concentration, at low concentration it appeared to adopt an intramolecular helix-turn-helix conformation. These results suggest that N protein oligomerization involves the bundling of intramolecular antiparallel coils or a conformational switch from intra- to intermolecular coiled-coils.

Nucleation and propagation of the collagen triple helix in single-chain and trimerized peptides: transition from third to first order kinetics

The kinetics of triple helix formation from single non-crosslinked peptide chains were studied for the collagen models (ProProGly)10 and (ProHypGly)10 in a broad concentration range and compared with those in nucleated trimers. At very low peptide concentrations the reaction order is 3 but decreases at higher concentrations. For (ProProGly)10 the third order rate constant is 800 M(-2) x s(-1) at 7 degrees C, which corresponds to a very long half time of 15 hours at 60 microM chain concentration. For (ProHypGly)10 the rate constant is about 1000-fold higher, which is consistent with the stabilizing effect of 4-hydroxyproline in collagens. The concentration dependence of the reaction order is explained by a nucleation mechanism in which a very unstable dimer is in fast equilibrium with the monomeric chains and addition of the third chain occurs in a rate-limiting step. At high concentrations nucleation is faster than propagation of helix formation and propagation becomes rate-limiting. To test this hypothesis an artificial nucleus was introduced by fusion of (ProProGly)10 with the trimeric foldon domain of T4 phage or the crosslinking domain of collagen III GlyProProGlyProCysCysGlyGlyGly. These domains were recombinantly attached to the C terminus of (GlyProPro)10 and link the three chains in a similar way to the C-terminal propeptide domain in collagen III. This results in a local intrinsic chain concentration of about 1 M. A first order reaction is observed for the folding of the triple helix in (GlyProPro)10foldon with a half time of 8.3 minutes, which approximately matches the rate of folding from single chains at 1 M peptide concentration. A high activation energy of 54 kJ/mol is found for this reaction, whereas the temperature dependence of the nucleation step is close to zero, confirming earlier findings on natural collagens that cis-trans isomerization of peptide bonds is the rate-limiting step in propagation.

Hfq: a bacterial Sm-like protein that mediates RNA-RNA interaction

The bacterial Hfq protein modulates the stability or the translation of mRNAs and has recently been shown to interact with small regulatory RNAs in E. coli. Here we show that Hfq belongs to the large family of Sm and Sm-like proteins: it contains a conserved sequence motif, known as the Sm1 motif, forms a doughnut-shaped structure, and has RNA binding specificity very similar to the Sm proteins. Moreover, we provide evidence that Hfq strongly cooperates in intermolecular base pairing between the antisense regulator Spot 42 RNA and its target RNA. We speculate that Sm proteins in general cooperate in bimolecular RNA-RNA interaction and that protein-mediated complex formation permits small RNAs to interact with a broad range of target RNAs.

The basic helix-loop-helix domain of the aryl hydrocarbon receptor nuclear transporter (ARNT) can oligomerize and bind E-box DNA specifically

The aryl hydrocarbon receptor nuclear transporter (ARNT) is a basic helix-loop-helix (bHLH) protein that contains a Per-Arnt-Sim (PAS) domain. ARNT heterodimerizes in vivo with other bHLH PAS proteins to regulate a number of cellular activities, but a physiological role for ARNT homodimers has not yet been established. Moreover, no rigorous studies have been done to characterize the biochemical properties of the bHLH domain of ARNT that would address this issue. To begin this characterization, we chemically synthesized a 56-residue peptide encompassing the bHLH domain of ARNT (residues 90-145). In the absence of DNA, the ARNT-bHLH peptide can form homodimers in lower ionic strength, as evidenced by dynamic light scattering analysis, and can bind E-box DNA (CACGTG) with high specificity and affinity, as determined by fluorescence anisotropy. Dimers and tetramers of ARNT-bHLH are observed bound to DNA in equilibrium sedimentation and dynamic light scattering experiments. The homodimeric peptide also undergoes a coil-to-helix transition upon E-box DNA binding. Peptide oligomerization and DNA affinity are strongly influenced by ionic strength. These biochemical and biophysical studies on the ARNT-bHLH reveal its inherent ability to form homodimers at concentrations supporting a physiological function and underscore the significant biochemical differences among the bHLH superfamily.

Selective intracellular retention of extracellular matrix proteins and chaperones associated with pseudoachondroplasia

Mutations in the cartilage oligomeric matrix protein (COMP) gene result in pseudoachondroplasia (PSACH), which is a chondrodysplasia characterized by early-onset osteoarthritis and short stature. COMP is a secreted pentameric glycoprotein that belongs to the thrombospondin family of proteins. We have identified a novel missense mutation which substitutes a glycine for an aspartic acid residue in the thrombospondin (TSP) type 3 calcium-binding domain of COMP in a patient diagnosed with PSACH. Immunohistochemistry and immunoelectron microscopy both show abnormal retention of COMP within characteristically enlarged rER inclusions of PSACH chondrocytes, as well as retention of fibromodulin, decorin and types IX, XI and XII collagen. Aggrecan and types II and VI collagen were not retained intracellularly within the same cells. In addition to selective extracellular matrix components, the chaperones HSP47, protein disulfide isomerase (PDI) and calnexin were localized at elevated levels within the rER vesicles of PSACH chondrocytes, suggesting that they may play a role in the cellular retention of mutant COMP molecules. Whether the aberrant rER inclusions in PSACH chondrocytes are a direct consequence of chaperone-mediated retention of mutant COMP or are otherwise due to selective intracellular protein interactions, which may in turn lead to aggregation within the rER, is unclear. However, our data demonstrate that retention of mutant COMP molecules results in the selective retention of ECM molecules and molecular chaperones, indicating the existence of distinct secretory pathways or ER-sorting mechanisms for matrix molecules, a process mediated by their association with various molecular chaperones.

HSP47 binds cooperatively to triple helical type I collagen but has little effect on the thermal stability or rate of refolding

HSP47, a collagen-specific molecular chaperone, interacts with unfolded and folded procollagens. Binding of chicken HSP47 to native bovine type I collagen was studied by fluorescence quenching and cooperative binding with a collagen concentration at half saturation (K(half)) of 1.4 x 10(-7) m, and a Hill coefficient of 4.3 was observed. Similar results are observed for the binding of mouse HSP47 recombinantly expressed in Escherichia coli. Chicken HSP47 binds equally well to native type II and type III procollagen without the carboxyl-terminal propeptide (pN type III collagen), but binding to triple helical collagen-like peptides is much weaker. Weak binding occurred to both hydroxylated and nonhydroxylated collagen-like peptides, and a significant chain length dependence was observed. Binding of HSP47 to native type I collagen had no effect on the thermal stability of the triple helix. Refolding of type I collagen in the presence of HSP47 showed minor changes, but these are probably not biologically significant. Binding of HSP47 to bovine pN type III collagen has only minor effects on the thermal stability of the triple helix and does not influence the refolding kinetics of the triple helix.

Design, engineering, and production of human recombinant t cell receptor ligands derived from human leukocyte antigen DR2

Major histocompatibility complex (MHC) class II molecules are membrane-anchored heterodimers on the surface of antigen-presenting cells that bind the T cell receptor, initiating a cascade of interactions that results in antigen-specific activation of clonal populations of T cells. Susceptibility to multiple sclerosis is associated with certain MHC class II haplotypes, including human leukocyte antigen (HLA) DR2. Two DRB chains, DRB5*0101 and DRB1*1501, are co-expressed in the HLA-DR2 haplotype, resulting in the formation of two functional cell surface heterodimers, HLA-DR2a (DRA*0101, DRB5*0101) and HLA-DR2b (DRA*0101, DRB1*1501). Both isotypes can present an immunodominant peptide of myelin basic protein (MBP-(84-102)) to MBP-specific T cells from multiple sclerosis patients. We have previously demonstrated that the peptide binding/T cell recognition domains of rat MHC class II (alpha1 and beta1 domains) could be expressed as a single exon for structural and functional characterization; Burrows, G. G., Chang, J. W., Bächinger, H.-P., Bourdette, D. N., Wegmann, K. W., Offner, H., and Vandenbark A. A. (1999) Protein Eng. 12, 771-778; Burrows, G. G., Adlard, K. L., Bebo, B. F., Jr., Chang, J. W., Tenditnyy, K., Vandenbark, A. A., and Offner, H. (2000) J. Immunol. 164, 6366-6371). Single-chain human recombinant T cell receptor ligands (RTLs) of approximately 200 amino acid residues derived from HLA-DR2b were designed using the same principles and have been produced in Escherichia coli with and without amino-terminal extensions containing antigenic peptides. Structural characterization using circular dichroism predicted that these molecules retained the antiparallel beta-sheet platform and antiparallel alpha-helices observed in the native HLA-DR2 heterodimer. The proteins exhibited a cooperative two-state thermal unfolding transition, and DR2-derived RTLs with a covalently linked MBP peptide (MBP-(85-99)) showed increased stability to thermal unfolding relative to the empty DR2-derived RTLs. These novel molecules represent a new class of small soluble ligands for modulating the behavior of T cells and provide a platform technology for developing potent and selective human diagnostic and therapeutic agents for treatment of autoimmune disease.

Mutations within a furin consensus sequence block proteolytic release of ectodysplasin-A and cause X-linked hypohidrotic ectodermal dysplasia

X-linked hypohidrotic ectodermal dysplasia (XLHED) is a heritable disorder of the ED-1 gene disrupting the morphogenesis of ectodermal structures. The ED-1 gene product, ectodysplasin-A (EDA), is a tumor necrosis factor (TNF) family member and is synthesized as a membrane-anchored precursor protein with the TNF core motif located in the C-terminal domain. The stalk region of EDA contains the sequence -Arg-Val-Arg-Arg156-Asn-Lys-Arg159-, representing overlapping consensus cleavage sites (Arg-X-Lys/Arg-Arg( downward arrow)) for the proprotein convertase furin. Missense mutations in four of the five basic residues within this sequence account for approximately 20% of all known XLHED cases, with mutations occurring most frequently at Arg156, which is shared by the two consensus furin sites. These analyses suggest that cleavage at the furin site(s) in the stalk region is required for the EDA-mediated cell-to-cell signaling that regulates the morphogenesis of ectodermal appendages. Here we show that the 50-kDa EDA parent molecule is cleaved at -Arg156Asn-Lys-Arg(159 downward arrow)- to release the soluble C-terminal fragment containing the TNF core domain. This cleavage appears to be catalyzed by furin, as release of the TNF domain was blocked either by expression of the furin inhibitor alpha1-PDX or by expression of EDA in furin-deficient LoVo cells. These results demonstrate that mutation of a functional furin cleavage site in a developmental signaling molecule is a basis for human disease (XLHED) and raise the possibility that furin cleavage may regulate the ability of EDA to act as a juxtacrine or paracrine factor.

Stabilization of short collagen-like triple helices by protein engineering

Recombinant expression of collagens and fragments of collagens is often difficult, as their biosynthesis requires specific post-translational enzymes, in particular prolyl 4-hydroxylase. Although the use of hydroxyproline-deficient variants offers one possibility to overcome this difficulty, these proteins usually differ markedly in stability when compared with the hydroxyproline-containing analogs. Here, we report a method to stabilize collagen-like peptides by fusing them to the N terminus of the bacteriophage T4 fibritin foldon domain. The isolated foldon domain and the chimeric protein (GlyProPro)(10)foldon were expressed in a soluble form in Escherichia coli. The recombinant proteins and the synthetic (ProProGly)(10) peptide were characterized by circular dichroism (CD) spectroscopy, differential scanning calorimetry, and analytical ultracentrifugation. We show that the foldon domain, which comprises only 27 amino acid residues, forms an obligatory trimer with a high degree of thermal stability. The CD thermal unfolding profiles recorded from foldon are monophasic and completely reversible upon cooling. Similar Van't Hoff and calorimertic enthalpy values of trimer formation indicated a cooperative all-or-none transition. As reported previously, (ProProGly)(10) peptides form collagen triple helices of only moderate stability. When fused to the foldon domain, however, triple helix formation of (GlyProPro)(10) is concentration independent, and the midpoint temperature of the triple helix unfolding is significantly increased. The stabilizing function of the trimeric foldon domain is explained by the close vicinity of its N termini, which induce a high local concentration in the range of 1 M for the C termini of the collagen-like-peptide. Collagen-foldon fusion proteins should be potentially useful to study receptor-collagen interactions.

Structure of the gating domain of a Ca2+-activated K+ channel complexed with Ca2+/calmodulin

Small-conductance Ca2+-activated K+ channels (SK channels) are independent of voltage and gated solely by intracellular Ca2+. These membrane channels are heteromeric complexes that comprise pore-forming alpha-subunits and the Ca2+-binding protein calmodulin (CaM). CaM binds to the SK channel through the CaM-binding domain (CaMBD), which is located in an intracellular region of the alpha-subunit immediately carboxy-terminal to the pore. Channel opening is triggered when Ca2+ binds the EF hands in the N-lobe of CaM. Here we report the 1.60 A crystal structure of the SK channel CaMBD/Ca2+/CaM complex. The CaMBD forms an elongated dimer with a CaM molecule bound at each end; each CaM wraps around three alpha-helices, two from one CaMBD subunit and one from the other. As only the CaM N-lobe has bound Ca2+, the structure provides a view of both calcium-dependent and -independent CaM/protein interactions. Together with biochemical data, the structure suggests a possible gating mechanism for the SK channel.

Glycosylation/Hydroxylation-induced stabilization of the collagen triple helix. 4-trans-hydroxyproline in the Xaa position can stabilize the triple helix

We have shown recently that glycosylation of threonine in the peptide Ac-(Gly-Pro-Thr)(10)-NH(2) with beta-d-galactose induces the formation of a collagen triple helix, whereas the nonglycosylated peptide does not. In this report, we present evidence that a collagen triple helix can also be formed in the Ac-(Gly-Pro-Thr)(10)-NH(2) peptide, if the proline (Pro) in the Xaa position is replaced with 4-trans-hydroxyproline (Hyp). Furthermore, replacement of Pro with Hyp in the sequence Ac-(Gly-Pro-Thr(beta-d-Gal))(10)-NH(2) increases the T(m) of the triple helix by 15.7 degrees C. It is generally believed that Hyp in the Xaa position destabilizes the triple helix because (Pro-Pro-Gly)(10) and (Pro-Hyp-Gly)(10) form stable triple helices but the peptide (Hyp-Pro-Gly)(10) does not. Our data suggest that the destabilizing effect of Hyp relative to Pro in the Xaa position is only true in the case of (Hyp-Pro-Gly)(10). Increasing concentrations of galactose in the solvent stabilize the triple helix of Ac-(Gly-Hyp-Thr)(10)-NH(2) but to a much lesser extent than that achieved by covalently linked galactose. The data explain some of the forces governing the stability of the annelid/vestimentiferan cuticle collagens.

Cooperative equilibrium transitions coupled with a slow annealing step explain the sharpness and hysteresis of collagen folding

Heat and guanidinium-induced denaturation curves of collagen III and its fragments were fitted by theoretical models to explain the extreme sharpness and the hysteresis between unfolding and refolding. It was shown that a recently proposed kinetic model for collagen denaturation does not account for the observed steepness, with physically reasonable values of activation energy and frequency factors in the Arrhenius equation. The extreme slope, which amounts to 0.38 per centigrade for collagen III at the midpoint of its transition, can only be explained by a highly cooperative equilibrium model. The refolding curve is shifted to lower temperatures by 6 degrees C for collagen III and reversible unfolding matching the initial profile of the native protein is observed only after long-time annealing. A simple formalism is proposed by which experimental denaturation and refolding curves are quantitatively described. The transition proceeds via many cooperative steps with slightly different equilibrium constants for unfolding and refolding. Hysteresis and annealing are caused by very slow steps, which are probably connected with a rearrangement of misfolded regions. These slow steps disappear with decreasing size of collagen fragments and hysteresis is not found for collagen model peptides.

Sweet is stable: glycosylation stabilizes collagen

For most collagens, the melting temperature (T(m)) of the triple-helical structure of collagen correlates with the total content of proline (Pro) and 4-trans-hydroxyproline (Hyp) in the Xaa and Yaa positions of the -Gly-Xaa-Yaa- triplet repeat. The cuticle collagen of the deep-sea hydrothermal vent worm Riftia pachyptila, despite a very low content of Pro and Hyp, has a relatively high thermal stability. Rather than Hyp occupying the Yaa position, as is normally found in mammalian collagens, this position is occupied by threonine (Thr) which is O-glycosylated. We compare the triple-helix forming propensities in water of two model peptides, Ac-(Gly-Pro-Thr)(10)-NH(2) and Ac-(Gly-Pro-Thr(Galbeta))(10)-NH(2), and show that a collagen triple-helix structure is only achieved after glycosylation of Thr. Thus, we show for the first time that glycosylation is required for the formation of a stable tertiary structure and that this modification represents an alternative way of stabilizing the collagen triple-helix that is independent of the presence of Hyp.

Mutations in calcium-binding epidermal growth factor modules render fibrillin-1 susceptible to proteolysis. A potential disease-causing mechanism in Marfan syndrome

Most extracellular proteins consist of various modules with distinct functions. Mutations in one common type, the calcium-binding epidermal growth factor-like module (cbEGF), can lead to a variety of genetic disorders. Here, we describe as a model system structural and functional consequences of two typical mutations in cbEGF modules of fibrillin-1 (N548I, E1073K), resulting in the Marfan syndrome. Large (80-120 kDa) wild-type and mutated polypeptides were recombinantly expressed in mammalian cells. Both mutations did not alter synthesis and secretion of the polypeptides into the culture medium. Electron microscopy after rotary shadowing and comparison of circular dichroism spectra exhibited minor structural differences between the wild-type and mutated forms. The mutated polypeptides were significantly more susceptible to proteolytic degradation by a variety of proteases as compared with their wild-type counterparts. Most of the sensitive cleavage sites were mapped close to the mutations, indicating local structural changes within the mutated cbEGF modules. Other cleavage sites, however, were observed at distances beyond the domain containing the mutation, suggesting longer range structural effects within tandemly repeated cbEGF modules. We suggest that proteolytic degradation of mutated fibrillin-1 may play an important role in the pathogenesis of Marfan syndrome and related disorders.

Structural organization of distinct domains within the non-collagenous N-terminal region of collagen type XI

Collagen XI is a heterotrimeric molecule found predominantly in heterotypic cartilage fibrils, where it is involved in the regulation of fibrillogenesis. This function is thought to involve the complex N-terminal domain. The goal of this current study was to examine its structural organization to further elucidate the regulatory mechanism. The amino-propeptide (alpha1-Npp) alone or with isoforms of the variable region were recombinantly expressed and purified by affinity and molecular sieve chromatography. Cys-1-Cys-4 and Cys-2-Cys-3 disulfide bonds were detected by liquid chromatography-tandem mass spectrometry. This pattern is identical to the homologous alpha2-Npp, indicating that the recombinant proteins were folded correctly. Anomalous elution on molecular sieve chromatography suggested that the variable region was extended, which was confirmed using rotary shadowing; the alpha1-Npp formed a globular "head" and the variable region an extended "tail." Circular dichroism spectra analysis determined that the alpha1-Npp comprised 33% beta-sheet, whereas the variable region largely comprised non-periodic structure. Taken together, these results imply that the alpha1-Npp cannot be accommodated within the core of the fibril and that the variable region and/or minor helix facilitates its exclusion to the fibril surface. This provides further support for regulation of fibril diameter by steric hindrance or by interactions with other matrix components that affect fibrillogenesis.