TSG-6 transfers proteins between glycosaminoglycans via a Ser28-mediated covalent catalytic mechanism

Studies of the interaction between Bikunin proteins, tumor necrosis factor-stimulated gene-6 protein (TSG-6), and glycosaminoglycans have revealed a unique catalytic activity where TSG-6/heavy chain 2 transfer heavy chain subunits between glycosaminoglycan chains. The activity is mediated by TSG-6/heavy chain 2 and involves a transient SDS stable interaction between TSG-6 and the heavy chain to be transferred. The focus of this study was to characterize the molecular structure of this cross-link to gain further insight into the catalytic mechanism. The result showed that the C-terminal Asp residue of the heavy chains forms an ester bond to Ser(28) beta-carbon of TSG-6 suggesting that this residue plays a role during catalysis.     

The TSG-6 and I alpha I interaction promotes a transesterification cleaving the protein-glycosaminoglycan-protein (PGP) cross-link

During co-incubation of human inter-alpha-inhibitor (IalphaI) and human tumor necrosis factor-stimulated gene 6 protein (TSG-6) SDS-stable interactions are formed between the two proteins. We have analyzed the products of this reaction and characterized the mechanism of complex formation. Following the incubation seven new bands not previously identified were apparent in SDS-PAGE. Three of these bands did not contain TSG-6, including heavy chain (HC)1.bikunin, HC2.bikunin, and free bikunin. In addition high molecular weight complexes composed of the same components as I alpha I, including HC1, HC2, and bikunin, were formed. The formation of these complexes was prevented by the addition of hyaluronan. The cross-links stabilizing these complexes displaying properties similar to the protein-glycosaminoglycan-protein (PGP) cross-link. The TSG-6-containing SDS-stable complexes were composed of HC1.TSG-6 or HC2.TSG-6 exclusively. Both glycosylated and non-glycosylated TSG-6 participated in the complex formation. The HC.TSG-6 cross-links were different from the PGP cross-link and were determined to be ester bonds between the alpha-carbonyl of the C-terminal Asp of the heavy chain and most likely a hydroxyl group containing the TSG-6 residue. The mechanism involved cleaving the PGP cross-link of I alpha I during a transesterification reaction. A TSG-6 hydroxyl group reacts with the ester bond between the alpha-carbonyl of the C-terminal Asp residues of HC1 or HC2 and carbon-6 of an internal N-acetylgalactosamine of the chondroitin-4-sulfate chain. An intermediate is formed resulting in a partitioning of the reaction between HC(1 or 2).TSG-6 complexes and transfer of HC(1 or 2) to the chondroitin via competing pathways.     

Enhanced secretory ability for the human factor VIII light chain produced in baculovirus-infected insect cells

The light and truncated heavy chains of human factor VIII, expressed separately in baculovirus-infected insect cells, exhibited different secretory behaviour when compared with each other and with a biologically active fusion molecule of the truncated heavy and light chains.The light chain was very efficiently secreted into culture medium, as judged by high extracellular protein levels and the absence of evidence for light chain retention within cells.Alternatively, proteins containing the heavy chain sequence were poorly secreted and appeared to be sequestered within cells, suggesting that regions within the heavy chain are responsible for the low levels of secreted protein which have generally been observed for recombinant factor VIII.     

Evidence for a two-step mechanism involved in the formation of covalent HC x TSG-6 complexes

IalphaI and TSG-6 interact to form a covalent bond between the C-terminal Asp alpha-carbon of an IalphaI heavy chain (HC) and an unknown component of TSG-6. This event disrupts the protein-glycosaminoglycan-protein (PGP) cross-link and dissociates IalphaI. In simple terms the interaction involves 5 components: (i) the IalphaI HCs, (ii) bikunin, (iii) chondroitin sulfate chain, (iv) TSG-6, and (v) divalent cations. To understand the molecular mechanism of complex formation, the effect of these were separately examined. The data show that although the mature covalent cross-link between the HCs and TSG-6 only involves the C-terminal Asp residue, the native fold of both IalphaI and TSG-6 was essential for the reaction to occur. Similarly, complex formation was prevented if the chondroitin sulfate chain was cleaved, releasing bikunin but maintaining the HC1 and HC2 PGP cross-links. In contrast, releasing the majority of the bikunin protein moiety by limited proteolysis did not prevent complex formation. An analysis of the divalent-cation requirements revealed two distinct interactions between IalphaI and TSG-6: (i) a noncovalent manganese, magnesium, or calcium-independent interaction between TSG-6 and the chondroitin sulfate chain (Kd 180 nM) and (ii) a covalent manganese, magnesium, or calcium-dependent interaction generating HC1 x TSG-6, HC2 x TSG-6, and high molecular weight (HMW) IalphaI. Significantly, both free TSG-6 and HC x TSG-6 complexes were able to bind the chondroitin sulfate chain suggesting that the sites on TSG-6 were distinct. On the basis of these findings, we propose a two-step reaction mechanism involving two putative binding sites. Initially, a cation-independent interaction between TSG-6 and the chondroitin sulfate chain is formed at site 1. Subsequently, a cation-dependent transesterification occurs, generating the covalent HC x TSG-6 cross-link at another site, site 2.     

Glycosaminoglycans promote fibril formation by amyloidogenic immunoglobulin light chains through a transient interaction

Amyloid formation occurs when a precursor protein misfolds and aggregates, forming a fibril nucleus that serves as a template for fibril growth. Glycosaminoglycans are highly charged polymers known to associate with tissue amyloid deposits that have been shown to accelerate amyloidogenesis in vitro. We studied two immunoglobulin light chain variable domains from light chain amyloidosis patients with 90% sequence identity, analyzing their fibril formation kinetics and binding properties with different glycosaminoglycan molecules. We find that the less amyloidogenic of the proteins shows a weak dependence on glycosaminoglycan size and charge, while the more amyloidogenic protein responds only minimally to changes in the glycosaminoglycan. These glycosaminoglycan effects on fibril formation do not depend on a stable interaction between the two species but still show characteristic traits of an interaction-dependent mechanism. We propose that transient, predominantly electrostatic interactions between glycosaminoglycans and the precursor proteins mediate the acceleration of fibril formation in vitro.     

Effect of skeletal muscle myosin light chain 2 on the Ca2+-sensitive interaction of myosin and heavy meromyosin with regulated actin

A low-speed centrifugation assay has been used to examine the binding of myosin filaments to F-action and to regulated actin in the presence of MgATP. While the cross-linking of F-actin by myosin was Ca2+ insensitive, much less regulated actin was cross-linked by myosin in the absence of Ca2+ than in its presence. Removal of the 19000-dalton, phosphorylatable light chain from myosin resulted in the loss of this Ca2+ sensitivity. Readdition of this light chain partially restored the Ca2+-sensitive cross-linking of regulated actin by myosin. Urea gel electrophoresis has been used to distinguish that fraction of heavy meromyosin which contains intact phosphorylatable light chain from that which contains a 17000-dalton fragment of this light chain. In the absence of Ca2+, heavy meromyosin which contained digested light chain bound to regulated actin in MgATP about 10-fold more tightly than did heavy meromyosin which contained intact light chain. The regulated actin-activated ATPases of heavy meromyosin also showed that cleavage of this light chain causes a substantial increase in the affinity of heavy meromyosin for regulated actin in the absence of Ca2+. Thus, the binding of both myosin and heavy meromyosin to regulated actin is Ca2+ sensitive, and this sensitivity is dependent on the phosphorylatable light chain.     

Native nonmuscle myosin II stability and light chain binding in Drosophila melanogaster

Native nonmuscle myosin IIs play essential roles in cellular and developmental processes throughout phylogeny. Individual motor molecules consist of a heterohexameric complex of three polypeptides which, when properly assembled, are capable of force generation. Here, we more completely characterize the properties, relationships and associations that each subunit has with one another in Drosophila melanogaster. All three native nonmuscle myosin II polypeptide subunits are expressed in close to constant stoichiometry to each other throughout development. We find that the stability of two subunits, the heavy chain and the regulatory light chain, depend on one another whereas the stability of the third subunit, the essential light chain, does not depend on either the heavy chain or regulatory light chain. We demonstrate that heavy chain aggregates, which form when regulatory light chain is lacking, associate with the essential light chain in vivo-thus showing that regulatory light chain association is required for heavy chain solubility. By immunodepletion we find that the majority of both light chains are associated with the nonmuscle myosin II heavy chain but pools of free light chain and/or light chain bound to other proteins are present. We identify four myosins (myosin II, myosin V, myosin VI and myosin VIIA) and a microtubule-associated protein (asp/Abnormal spindle) as binding partners for the essential light chain (but not the regulatory light chain) through mass spectrometry and co-precipitation. Using an in silico approach we identify six previously uncharacterized genes that contain IQ-motifs and may be essential light chain binding partners.     

Characterization of matteuccin, the 2.2S storage protein of the ostrich fern. Evolutionary relationship to angiosperm seed storage proteins

The 2.2S spore storage protein (matteuccin) of the ostrich fern, Matteuccia struthiopteris, has been isolated and characterized. It is a small basic protein consisting of two disulfide-linked polypeptides with approximate molecular masses of 3.0 kDa and 8.0 kDa. At least four different isoforms exist where two of the forms differ from the other by having a slightly smaller heavy chain. Amino acid analysis reveals that the 2.2S protein is rich in arginine. Almost complete amino acid sequence information was obtained for the light chain and a partial sequence for the heavy chain. Amino acid sequence comparison reveals that this protein shows a high similarity to seed storage proteins in different angiosperm species in spite of the fact that the common ancestor of ferns and angiosperms lived more than 300 million years ago.     

Structure-function studies on Acanthamoeba myosins IA, IB, and II

Myosins IA and IB are globular proteins with only a single, short (for myosins) heavy chain (140,000 and 125,000 daltons for IA and IB, respectively) and are unable to form bipolar filaments. The amino acid sequence of IB heavy chain shows 55% similarity to muscle myosins in the N-terminal 670 residues, which contain the active sites, and a unique 500-residue C-terminus highly enriched in proline, glycine, and alanine. The C-terminal region contains a second actin-binding site which allows myosins IA and IB to cross-link actin filaments and support contractile activity. Myosins IA and IB are regulated solely by phosphorylation of one serine on the heavy chain positioned between the catalytic site and the actin-binding site that activates ATPase. Myosin II is a more conventional myosin in composition (two heavy chains and two pairs of light chains), heavy chain sequence (globular head 45% identical to muscle myosins and a coiled-coil helical tail), and structure (bipolar filaments). The tail of myosin II is much shorter than that of other conventional myosins, and it contains a 25 amino acid sequence in which helical structure is predicted to be weak or absent. The position of this sequence corresponds to the position of a bend in the monomer. Myosin II heavy chains also have a 29-residue nonhelical tailpiece which contains three regulatory, phosphorylatable serines. Phosphorylation at the tip of the tail regulates ATPase activity in the globular head apparently through an effect on filament structure.     

The central helix of calmodulin functions as a flexible tether

Using site-directed mutagenesis we have created an altered calmodulin in which Gln-3 and Thr-146 have both been replaced by cysteines. We have reacted this protein with the bifunctional reagent, bismaleimidohexane, forming an intramolecular cross-link between the two cysteines. In the crystal structure of native calmodulin alpha-carbons at positions 3 and 146 are 37 A apart. In the bismaleimidohexane cross-linked protein these atoms can be no more than 19 A apart, and model building studies indicate that there is probably a bend in the central helix of calmodulin. A second modified calmodulin was generated by cleaving the central helix of the cross-linked protein at Lys-77 with trypsin. In this molecule, the two lobes of calmodulin are joined solely by the bismaleimidohexane cross-link, which bridges Cys-3 and Cys-146. Vm and Kact values for activation of myosin light chain kinase activity by the cross-linked and cross-linked/trypsinized proteins are not significantly different from those for the control protein. This result indicates that one role for the central helix may be to serve as a flexible tether between the calmodulin lobes. This is consistent with a model calmodulin-enzyme complex in which the central helix is bent, and the two lobes exert a concerted effect. A detailed model of this type has been proposed for the calmodulin-myosin light chain kinase complex (Persechini, A. and Kretsinger, R.H. (1988) J. Cardiovasc. Pharmacol., in press).     

The heavy chain of conventional kinesin interacts with the SNARE proteins SNAP25 and SNAP23

Recent studies on the conventional motor protein kinesin have identified a putative cargo-binding domain (residues 827-906) within the heavy chain. To identify possible cargo proteins which bind to this kinesin domain, we employed a yeast two-hybrid assay. A human brain cDNA library was screened, using as bait residues 814-963 of human ubiquitous kinesin heavy chain. This screen initially identified synaptosome-associated protein of 25 kDa (SNAP25) as a kinesin-binding protein. Subsequently, synaptosome-associated protein of 23 kDa (SNAP23), the nonneuronal homologue of SNAP25, was also confirmed to interact with kinesin. The sites of interaction, determined from in vivo and in vitro assays, are the N-terminus of SNAP25 (residues 1-84) and the cargo-binding domain of kinesin heavy chain (residues 814-907). Both regions are composed almost entirely of heptad repeats, suggesting the interaction between heavy chain and SNAP25 is that of a coiled-coil. The observation that SNAP23 also binds to residues 814-907 of heavy chain would indicate that the minimal kinesin-binding domain of SNAP23 and SNAP25 is most likely residues 45-84 (SNAP25 numbering), a heptad-repeat region in both proteins. The major binding site for kinesin light chain in kinesin heavy chain was mapped to residues 789-813 at the C-terminal end of the heavy chain stalk domain. Weak binding of light chain was also detected at the N-terminus of the heavy chain tail domain (residues 814-854). In support of separate binding sites on heavy chain for light chain and SNAPs, a complex of heavy and light chains was observed to interact with SNAP25 and SNAP23.     

Nucleotide-induced movements in the myosin head near the converter region

Structural changes in subfragment 1 of skeletal muscle myosin were investigated by cross-linking trypsin-cleaved S1 with 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide. In the absence of nucleotide the alkali light chains are cross-linked to the 27 kDa heavy chain fragment; the presence of MgATP reduces the efficiency of this reaction. On the other hand, MgATP promotes the cross-link formation between the N-terminal 27 kDa and C-terminal 20 kDa fragments of the heavy chain. The chemical cleavage of the cross-linked heavy chains fragments with N-chlorosuccinimide and hydroxylamine indicates that the cross-links are formed between the regions spanning residues 131-204 and 699-809. These results indicate that the two regions of the heavy chain that are relatively distant in nucleotide-free skeletal S1 [Rayment et al. (1993) Science 261, 50-58] can potentially interact upon addition of nucleotide.     

Multiple nuclear factors interact with the immunoglobulin enhancer sequences

To characterize proteins that bind to the immunoglobulin (Ig) heavy chain and the kappa light chain enhancers, an electrophoretic mobility shift assay with end-labeled DNA fragments was used. Three binding proteins have been found. One is NF-A, a factor found in all tested cell types that binds to the octamer sequence found upstream of all Ig variable region gene segments and to the same octamer in the heavy chain enhancer. The second, also ubiquitous, protein binds to a sequence in both the heavy chain and the kappa enhancers that was previously shown to be protected from methylation in vivo. Other closely related sites do not compete for this binding, implying a restriction enzyme-like binding specificity. The third protein binds to a sequence in the kappa enhancer (and to an identical sequence in the SV40 enhancer) and is restricted in its occurrence to B cells.     

Interchain disulphide-bond formation in the assembly of immunoglobulin G. Heavy-chain dimer as an intermediate

1. The role of disulphide-bond formation in the assembly of G_2a myeloma protein 5563 was studied by pulse-labelling ascitic plasma cells of tumour-line 5563 for 2–8min. with radioactive amino acids, and analysing the intracellular proteins. Myeloma-protein determinants were first purified by ion-exchange chromatography under conditions that do not dissociate non-covalently linked sub-units of immunoglobulin G. The pulse-labelled material was then analysed by electrophoresis on polyacrylamide gels in sodium dodecyl sulphate–phosphate–urea buffer, which dissociates non-covalently linked sub-units; after gel electrophoresis, radioactive protein bands were located by radioautography, and characterized immunologically after elution. 2. Two heavy-chain intermediates were detected: (i) heavy-chain dimer; (ii) the dimer with one light chain attached. Free light chains had previously been shown to be intermediates in assembly. No evidence for the presence of half-molecules (one light chain attached to one heavy chain) was obtained. The formation of the disulphide bond between the heavy chains thus appears to precede the light-chain–heavy-chain linkage in immunoglobulin G assembly.     

Characterization of primary amino acid sequence of human complement control protein factor I from an analysis of cDNA clones.

A cDNA clone of the mRNA coding for the human complement system control protein Factor I has been isolated. The predicted amino acid sequence obtained from the DNA sequence demonstrates a protein consisting of a heavy chain (Mr 35,400) linked to a light chain (Mr 27,600), both of which contain three sites for N-linked glycosylation. The light chain has clear homology with other serine proteinases, most notably in the region of the catalytically active and structurally important amino acids and shares some of the features characteristic of the plasminogen activators. The heavy chain has a clear 'mosaic' nature typical of the plasma serine proteinases; in particular it contains class A and class B LDL (low-density lipoprotein) receptor repeats with conserved cysteine residues similar to those found in other complement proteins.     

Multi-subunit proteins on the surface of filamentous phage: methodologies for displaying antibody (Fab) heavy and light chains.

The display of proteins on the surface of phage offers a powerful means of selecting for rare genes encoding proteins with binding activities. Recently we found that antibody heavy and light chain variable (V) domains fused as a single polypeptide chain to a minor coat protein of filamentous phage fd, could be enriched by successive rounds of phage growth and panning with antigen. This allows the selection of antigen-binding domains directly from diverse libraries of V-genes. Now we show that heterodimeric Fab fragments can be assembled on the surface of the phage by linking one chain to the phage coat protein, and secreting the other into the bacterial periplasm. Furthermore by introducing an amber mutation between the antibody chain and the coat protein, we can either display the antibody on phage using supE strains of bacteria, or produce soluble Fab fragment using non-suppressor strains. The use of Fab fragments may offer advantages over single chain Fv fragments for construction of combinatorial libraries.     

Developmental consequences of the lack of myosin heavy chain in Dictyostelium discoideum

Two different Dictyostelium discoideum cell lines that lack myosin heavy chain protein (MHC A) have been previously described. One cell line (mhcA) was created by antisense RNA inactivation of the endogenous mRNA and the other (HMM) by insertional mutagenesis of the endogenous myosin gene. The two cell lines show similar developmental defects; they are delayed in aggregation and become arrested at the mound stage. However, when cells that lack myosin heavy chain are mixed with wild-type cells, some of the mutant cells are capable of completing development to form mature spores. The pattern of expression of a number of developmentally regulated genes has been examined in both mutant cell lines. Although morphogenesis becomes aberrant before aggregation is completed, all of the markers that we have examined are expressed normally. These include genes expressed prior to aggregation as well as prespore genes expressed later in development. It appears that the signals necessary for cell-type differentiation are generated in the aborted structures formed by cells lacking MHC A. The mhcA cells have negligible amounts of MHC A protein while the HMM cells express normal amounts of a fragment of the myosin heavy chain protein similar to heavy meromyosin (HMM). The expression of myosin light chain was examined in these two cell lines. HMM cells accumulate normal amounts of the 18,000-D light chain, while the amount of light chain in mhcA cells is dramatically reduced. It is likely that the light chains assemble normally with the HMM fragment in HMM cells, while in cells lacking myosin heavy chain (mhcA) the light chains are unstable.     

The N-terminal lobes of both regulatory light chains interact with the tail domain in the 10 S-inhibited conformation of smooth muscle myosin

In the presence of ATP, unphosphorylated smooth muscle myosin can form a catalytically inactive monomer that sediments at 10 Svedbergs (10 S). The tail of 10 S bends into thirds and interacts with the regulatory domain. ADP-P(i) is "trapped" at the active site, and consequently the ATPase activity is extremely low. We are interested in the structural basis for maintenance of this off state. Our prior photocross-linking work with 10 S showed that tail residues 1554-1583 are proximal to position 108 in the C-terminal lobe of one of the two regulatory light chains ( Olney, J. J., Sellers, J. R., and Cremo, C. R. (1996) J. Biol. Chem. 271, 20375-20384 ). These data suggested that the tail interacts with only one of the two regulatory light chains. Here we present data, using a photocross-linker on position 59 on the N-terminal lobe of the regulatory light chain (RLC), demonstrating that both regulatory light chains of a single molecule can cross-link to the light meromyosin portion of the tail. Mass spectrometric data show four specific cross-linked regions spanning residues 1428-1571 in the light meromyosin portion of the tail, consistent with cross-linking two RLC to one light meromyosin. In addition, we find that position 59 can cross-link internally to residues 42-45 within the same RLC subunit. The internal cross-link only forms in 10 S and not in unphosphorylated heavy meromyosin (lacking the light meromyosin), suggesting a structural rearrangement within the RLC attributed to the interaction of the tail with the head.     

Expression of antibodies using single‐open reading frame vector design and polyprotein processing from mammalian cells

We describe a novel polyprotein precursor‐based approach to express antibodies from mammalian cells. Rather than expressing heavy and light chain proteins from separate expression units, the antibody heavy and light chains are contained in one single‐open reading frame (sORF) separated by an intein gene fused in frame. Inside mammalian cells this ORF is transcribed into a single mRNA, and translated into one polypeptide. The antibody heavy and light chains are separated posttranslationally, assembled into the functional antibody molecule, and secreted into culture medium. It is demonstrated that Pol I intein from P. horikoshii mediates protein splicing and cleavage reactions in mammalian cells, in the context of antibody heavy and light chain amino acid sequences. To allow the separation of antibody heavy chain, light chain, and the intein, we investigated a number of intein mutations designed to inhibit intein‐mediated splicing but preserve cleavage reactions. We have also designed constructs in which the signal peptide downstream from intein has altered hydrophobicity. The use of some of these mutant constructs resulted in more efficient antibody secretion, highlighting areas that can be further explored in improving such an expression system. An antibody secreted using one of the sORF constructs was characterized. This antibody has correct N‐terminal sequences for both of its heavy and light chains, correct heavy and light chain MW as well as intact MW as measured by mass spectrometry. Its affinity to antigen, as measured by surface plasmon resonance (SPR), is indistinguishable from that of the same antibody produced using conventional method. © 2009 American Institute of Chemical Engineers Biotechnol. Prog., 2009