Structural changes in alpha-2- and ovomacroglobulins studied by gel chromatography and electron microscopy

Deimination Protein Profiles in Alligator mississippiensis Reveal Plasma and Extracellular Vesicle-Specific Signatures Relating to Immunity, Metabolic Function, and Gene Regulation

Alligators are crocodilians and among few species that endured the Cretaceous-Paleogene extinction event. With long life spans, low metabolic rates, unusual immunological characteristics, including strong antibacterial and antiviral ability, and cancer resistance, crocodilians may hold information for molecular pathways underlying such physiological traits. Peptidylarginine deiminases (PADs) are a group of calcium-activated enzymes that cause posttranslational protein deimination/citrullination in a range of target proteins contributing to protein moonlighting functions in health and disease. PADs are phylogenetically conserved and are also a key regulator of extracellular vesicle (EV) release, a critical part of cellular communication. As little is known about PAD-mediated mechanisms in reptile immunology, this study was aimed at profiling EVs and protein deimination in Alligator mississippiensis. Alligator plasma EVs were found to be polydispersed in a 50-400-nm size range. Key immune, metabolic, and gene regulatory proteins were identified to be posttranslationally deiminated in plasma and plasma EVs, with some overlapping hits, while some were unique to either plasma or plasma EVs. In whole plasma, 112 target proteins were identified to be deiminated, while 77 proteins were found as deiminated protein hits in plasma EVs, whereof 31 were specific for EVs only, including proteins specific for gene regulatory functions (e.g., histones). Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analysis revealed KEGG pathways specific to deiminated proteins in whole plasma related to adipocytokine signaling, while KEGG pathways of deiminated proteins specific to EVs included ribosome, biosynthesis of amino acids, and glycolysis/gluconeogenesis pathways as well as core histones. This highlights roles for EV-mediated export of deiminated protein cargo with roles in metabolism and gene regulation, also related to cancer. The identification of posttranslational deimination and EV-mediated communication in alligator plasma revealed here contributes to current understanding of protein moonlighting functions and EV-mediated communication in these ancient reptiles, providing novel insight into their unusual immune systems and physiological traits. In addition, our findings may shed light on pathways underlying cancer resistance, antibacterial and antiviral resistance, with translatable value to human pathologies.

The Kringle-like Domain Facilitates Post-endoplasmic Reticulum Changes to Premelanosome Protein (PMEL) Oligomerization and Disulfide Bond Configuration and Promotes Amyloid Formation

The formation of functional amyloid must be carefully regulated to prevent the accumulation of potentially toxic products. Premelanosome protein (PMEL) forms non-toxic functional amyloid fibrils that assemble into sheets upon which melanins ultimately are deposited within the melanosomes of pigment cells. PMEL is synthesized in the endoplasmic reticulum but forms amyloid only within post-Golgi melanosome precursors; thus, PMEL must traverse the secretory pathway in a non-amyloid form. Here, we identified two pre-amyloid PMEL intermediates that likely regulate the timing of fibril formation. Analyses by non-reducing SDS-PAGE, size exclusion chromatography, and sedimentation velocity revealed two native high Mr disulfide-bonded species that contain Golgi-modified forms of PMEL. These species correspond to disulfide bond-containing dimeric and monomeric PMEL isoforms that contain no other proteins as judged by two-dimensional PAGE of metabolically labeled/immunoprecipitated PMEL and by mass spectrometry of affinity-purified complexes. Metabolic pulse-chase analyses, small molecule inhibitor treatments, and evaluation of site-directed mutants suggest that the PMEL dimer forms around the time of endoplasmic reticulum exit and is resolved by disulfide bond rearrangement into a monomeric form within the late Golgi or a post-Golgi compartment. Mutagenesis of individual cysteine residues within the non-amyloid cysteine-rich Kringle-like domain stabilizes the disulfide-bonded dimer and impairs fibril formation as determined by electron microscopy. Our data show that the Kringle-like domain facilitates the resolution of disulfide-bonded PMEL dimers and promotes PMEL functional amyloid formation, thereby suggesting that PMEL dimers must be resolved to monomers to generate functional amyloid fibrils.

Structural characterization of human general transcription factor TFIIF in solution

Human general transcription factor IIF (TFIIF), a component of the transcription pre-initiation complex (PIC) associated with RNA polymerase II (Pol II), was characterized by size-exclusion chromatography (SEC), electrospray ionization mass spectrometry (ESI-MS), and chemical cross-linking. Recombinant TFIIF, composed of an equimolar ratio of alpha and beta subunits, was bacterially expressed, purified to homogeneity, and found to have a transcription activity similar to a natural one in the human in vitro transcription system. SEC of purified TFIIF, as previously reported, suggested that this protein has a size >200 kDa. In contrast, ESI-MS of the purified sample gave a molecular size of 87 kDa, indicating that TFIIF is an alphabeta heterodimer, which was confirmed by matrix-assisted laser desorption/ionization (MALDI) MS of the cross-linked TFIIF components. Recent electron microscopy (EM) and photo-cross-linking studies showed that the yeast TFIIF homolog containing Tfg1 and Tfg2, corresponding to the human alpha and beta subunits, exists as a heterodimer in the PIC, so the human TFIIF is also likely to exist as a heterodimer even in the PIC. In the yeast PIC, EM and photo-cross-linking studies showed different results for the mutual location of TFIIE and TFIIF along DNA. We have examined the direct interaction between human TFIIF and TFIIE by ESI-MS, SEC, and chemical cross-linking; however, no direct interaction was observed, at least in solution. This is consistent with the previous photo-cross-linking observation that TFIIF and TFIIE flank DNA separately on both sides of the Pol II central cleft in the yeast PIC.

Investigation of molecular size of transcription factor TFIIE in solution

Human general transcription factor IIE (TFIIE), a component of a transcription preinitiation complex associated with RNA polymerase II, was characterized by size-exclusion chromatography, mass spectrometry, analytical ultracentrifugation, and small-angle X-ray scattering (SAXS). Recombinant human TFIIE was purified to homogeneity and shown to contain equimolar amounts of TFIIEalpha (50 kDa) and TFIIEbeta (35 kDa) by SDS-PAGE. In the analysis of size-exclusion chromatography of the purified sample, as already reported, TFIIE was shown to be a 170-kDa alpha(2)beta(2) heterotetramer. However, by using electrospray ionization mass spectrometry the purified sample gave the molecular mass of 84,152 +/- 5, indicating that TFIIE is an alphabeta heterodimer but not a heterotetramer. Analytical ultracentrifugation experiment of TFIIE provided that only a single component with the molecular mass of ca. 80,000 existed in solution, also suggesting an alphabeta heterodimer. In addition, its extraordinarily rod-like molecular shape was confirmed by SAXS. It is likely that the rod-like molecular shape of TFIIE has misled larger molecular size in size-exclusion chromatography, which was calibrated by globular proteins. It is demonstrated that TFIIE exists as a heterodimer under our present conditions in solution, although two molecules of heterodimer might be required for the formation of the preinitiation complex with RNA polymerase II for starting the transcription process.

Identification and partial characterization of alpha 2-macroglobulin from the tuatara (Sphenodon punctatus)

alpha 2-Macroglobulin (alpha 2-M), a large molecular mass proteinase-binding protein, was identified in plasma from tuatara (Sphenodon), a rare reptile endemic to New Zealand. In this genus, alpha 2-M constitutes 11-13% of total plasma protein (approximately 2.2-3.9 mg/mL). Analysis of blood samples collected at approximately monthly intervals from individual tuatara indicated that the plasma level of alpha 2-M remains fairly constant. The subunits of tuatara alpha 2-M have an apparent molecular mass of approximately 160 kDa as determined by SDS-polyacrylamide gel electrophoresis and the intact protein is an oligomer that contains inter-chain disulfide bonds. N-terminal sequence analyses of tuatara alpha 2-M revealed a distinct similarity to alpha-macroglobulins of other vertebrates and that at least two types of alpha 2-M subunits are present in plasma of tuatara.

Small-angle scattering study of alpha 1 inhibitor III from rat blood plasma

The alpha 1 proteinase inhibitor III from rat blood plasma, homologous to the alpha 2-macroglobulin family of proteins, has been studied in solution using small-angle scattering of X-rays and of neutrons: the radius of gyration, Rg, was found to be 4.5 nm, and the largest distance within the molecule, Dmax = 14 nm. When the inhibitor reacts with chymotrypsin or methylamine, the resulting derivatives yield slightly higher Rg-values, 4.7 and 4.85 nm, respectively. The data of the native protein are consistent with a model, the projection of which resembles the letter V and which is formed by the two identical halves of an elliptic cylinder with semi-axes of 2.1 and 5.5 nm and a length of 11 nm. This elliptic cylinder model also explained the scattering from the monomeric complement proteins C3 and C4, as well as that from the monomers of the dimeric and tetrameric alpha 2-macroglobulin family of proteins (Osterberg, R., et al. (1991), Biochemistry 30, 7873-7878). Due to the conformational change occurring when the thiol ester bond is split, the cleft in the V-form seems to be closed; and as a result, the models of the chymotrypsin and methylamine derivatives are more compact than that of the native protein.

The kinetics of conformational changes of alpha 2-macroglobulin determined by time resolved X-ray solution scattering

The rate of gross conformational change of alpha 2-macroglobulin (alpha 2M) during its proteinase trapping was directly determined for the first time using time-resolved X-ray solution scattering. Decrease of radius of gyration was observed under pseudo-first-order conditions with excess proteinases, which exhibited a monophasic time-course. The rate constants were 0.5 +/- 0.1 s-1 and 0.8 +/- 0.2 s-1 for the reaction with chymotrypsin and trypsin, respectively. There was no concentration dependence of the observed rate constants. Therefore, the rate-limiting step of the gross conformational change was not the bimolecular encounter reaction between alpha 2M and proteinases, which requires a new proposal of pre-trapping of proteinases before the gross conformational change.

Direct kinetics of bait region cleavage of alpha-2-macroglobulin by a rapid quenching method

The rate of bait region cleavage of human alpha-2-macroglobulin by chymotrypsin was determined by a rapid quenching method under conditions where the bimolecular encounter between the two reactants was not rate-limiting. alpha 2M was first mixed with a 30 molar excess of chymotrypsin in a sequential stopped-flow apparatus and after programmed time intervals the activity of chymotrypsin was quenched with 1 N HCl. The fraction of uncleaved subunits was quantitated by SDS-PAGE under reducing conditions. The result indicated that the bait region cleavage proceeded following a two-exponential decay curve with respective rate constants of k1 = 40 s-1 and k2 = 2 s-1.

Evidence from sequence analysis that hen egg-white ovomacroglobulin (ovostatin) is devoid of an internal beta-Cys-gamma-Glu thiol ester

53 residues of the internal sequence from the proteinase-binding hen egg-white ovostatin have been determined. The stretch corresponds to residues 945-997 of human alpha 2-macroglobulin. The degree of conservation of residues of the two stretches is approx. 74%. Cys-949, being one constituent of the internal thiol ester site of members of the family of proteins related to alpha 2-macroglobulin, is an Asn-residue in hen egg-white ovostatin, but the other constituent, Gln-952, is preserved. The Cys-to-Asn substitution forms the chemical basis for the lack of thiol esters in hen egg-white ovostatin.

Ultrastructure of alpha 2-macroglobulins

New results concerning the ultrastructure of human alpha 2-macroglobulin (alpha 2M) molecules are presented in connection and comparison with the historical, the current and our own most recent, even unpublished results on the structure and function of alpha 2M and related proteins. The electron microscopic approach uses classical negative staining, combined with the new imaging mode "Electron Energy Loss Spectroscopy", which provides unusual contrast, resolution and readability of the electron micrographs. Immuno- and cryoelectron microscopy, as well as image processing has provided new data necessary to the building of tentative 3D models of the molecule. A model for the native tetrameric alpha 2M is described for the first time, and tries to explain and gather the various observations, sometimes contradictory, taken from different laboratories. A revised version for a model of the methylamine- and proteinase-transformed forms of alpha 2M is also shown. The probable positions of the bait regions and the thiol esters are given on both models. We confirm that alpha 2M is a twin trap capable of inactivating one or two proteinases by partial immobilization. Preliminary results on the production of crystals of alpha 2M-chymotrypsin complexes are also presented. A critical analysis of our models is presented in comparison with others. The technical limitations reached with some techniques and some possible extensions of future research in the field are also presented.

alpha 2-Macroglobulin is cleaved by HIV-1 protease in the bait region but not in the C-terminal inter-domain region

alpha 2-Macroglobulin is cleaved by human immunodeficiency virus-1 protease. The cleavage site is the Phe684-Tyr685 bond in the "bait region", an exposed part of alpha 2-macroglobulin, creating the "F-form". The methylamine derivative of alpha 2-macroglobulin is also cleaved at the same bond. The homologous chicken ovomacroglobulin does not form an F-form structure with the protease, although, F-form generation by other enzymes is known. This is possibly due to the lack of a suitable cleavage sequence in the corresponding region of ovomacroglobulin. In human alpha 2-macroglobulin, the interdomain segment between the main part of the molecule and the receptor-binding C-terminal domain is not cleaved by the HIV protease although typical cleavage sequences occur. In AIDS, therefore, HIV protease from infected cells in unlikely to interfere with receptor-binding of alpha 2-macroglobulin.

X-ray scattering study of hagfish protease inhibitor, a protein structurally related to complement and alpha 2-macroglobulin

A protease inhibitor from hagfish blood plasma, homologous to human alpha 2-macroglobulin, has been studied in solution using small-angle X-ray scattering; the radius of gyration, R, was found to be 7.0 nm, the molecular weight 340,000 +/- 20,000, and the largest distance within the molecule, Dmax, 22 nm. When the inhibitor reacts with chymotrypsin, its 1:1 chymotrypsin complex is found to be more compact than the native molecule, R = 6.1 nm. A very similar conformational change is observed after the protein is reacted with methylamine. The data are consistent with models consisting of two equal elliptic cylinders with the same size as the one used as a model for the complement proteins C3 and C4 [cf. Osterberg et al. (1989) Eur. J. Biochem. 183, 507-511]. In the model for the native protein, these cylinders are arranged in an extended form, and in the one for the methylamine derivative (or chymotrypsin complex), they are closer together so that the projection of their elliptic surfaces forms an angle of about 70 degrees. These models for the hagfish protease inhibitor were expanded to models for the twice as large human alpha 2-macroglobulin using symmetry operations, and the resulting alpha 2-macroglobulin models were found to agree with those emerged from earlier studies involving electron microscopy and X-ray scattering methods.

Inhibitory effects of ovomacroglobulin on bacterial keratitis in rabbits

We studied the inhibitory effects of chicken egg-white ovomacroglobulin (ovoM) on keratitis induced by 56,000-Da protease (56 KP) of Serratia marcescens and by elastase (PE) and alkaline protease (PAP) of Pseudomonas aeruginosa. The effects of ovoM on the serratial and pseudomonal keratitis in rabbits were also elucidated. In one model, four drops of 56 KP, PE, or PAP (1 mg/ml) were applied to wounded corneas of eight eyes. Thereafter, 80 microliters ovoM (10 mg/ml) was dropped into four eyes and 0.01 M phosphate-buffed 0.15 M saline (pH 7.4) into the other eyes as a control. The other in vivo test system involved intrastromal injection of S. marcescens or P. aeruginosa, by which each sample (10(5)-10(7) colony-forming units) mixed with ovoM was injected into one cornea and the other cornea received organisms without ovoM. OvoM completely inhibited the activity of these bacterial proteases in vitro and reduced corneal destruction in experimental keratitis in rabbits. In addition, greatly accelerated wound healing was observed.

Structural studies of human alpha 2-macroglobulin: concordance between projected views obtained by negative-stain and cryoelectron microscopy

Two views of native alpha 2-macroglobulin are revealed by electron microscopy of negatively stained samples; in one view the molecule resembles a padlock and in the other, a pair of lips. Interconversion of the two views upon tilting establishes that these are two different projected views of the same structure. Furthermore, the two views are related by a 45 degrees rotation about their major axis because they interconvert when the specimens are titled +/- 22.5 degrees. Negatively stained molecules on Butvar films present a nearly equal distribution of the two views, whereas in frozen-hydrated samples the molecules almost exclusively are oriented in the lip view. Measurements from both views indicate that the alpha 2-macroglobulin molecule is approximately 200 A long and approximately 140 A wide. Our results suggest that alpha 2-macroglobulin is composed of two protomeric units, each in the shape of a twisted letter S. These units are joined together at their ends to form a complex with point group symmetry 222. The 45 degrees interconversion angle between the lip and padlock views support this arrangement. Average images of unstained and stained lips are quite similar, indicating that the native structure is consistently preserved by the two electron microscopy procedures used in this investigation. This is substantiated by the interconversion between the lip and padlock views that occurs when the molecule is rotated 45 degrees [corrected] about its major twofold axis.

The structure of alpha 2-macroglobulin-methylamine after papain digestion as determined by electron microscopy

alpha 2-Macroglobulin-methylamine (alpha 2M-CH3NH2) was digested with papain at pH 5.0. The major 600 kDa fragment was purified by molecular-exclusion chromatography. In a non-denaturing gel-electrophoresis system, the 600 kDa fragment migrated in a single band at a rate that was comparable with that for the untreated alpha 2M-CH3NH2. The elution volume of the 600 kDa fragment on Superose-6 was slightly increased. In primary cultures of rat hepatocytes, cellular uptake of 125I-alpha 2M-CH3NH2 was not affected by the 600 kDa fragment, confirming the results of other investigators. The 600 kDa fragment was negatively stained with uranyl formate and analysed by transmission electron microscopy. The major structural characteristics of the parent protein (alpha 2M-CH3NH2) remained intact. The most common image included prominent lateral walls and two centrally located regions of stain exclusion termed 'paddle structures'. The distance between the paddle structures was equivalent in alpha 2M-CH3NH2 and the 600 kDa fragment [approximately 13.5 nm (135 A)]. By contrast, the lateral walls in the 600 kDa fragment were decreased in length by approximately 0.37 nm (37 A) (19%). It is proposed that the 600 kDa structure retains the 'hollow cylinder' shape of alpha 2M-CH3NH2. The structure of the cylinder is formed by the lateral walls and four paddle structures (only two are imaged, owing to overlapping). The paddle structures in the 600 kDa fragment are intact and relatively closer to the apices of the molecule, owing to the decrease in lateral wall length. Since the alpha 2M receptor-binding sites are removed by papain digestion, the studies presented here support the location of the receptor-binding sites near the apices of the lateral walls.

Enzyme-replacement therapy: problems and prospects

Several diseases can, at least in theory, be treated by the administration of an enzyme, the deficiency of which is the cause of the disease. Various attempts have been made to correct enzymatic deficiencies responsible for the clinical manifestation of diseases for which prevention cannot be achieved by modification of the diet or by supportive therapy with drugs. Except for treating certain digestive disorders, enzyme-replacement therapy has not yet found a broad application. In this review article a compilation is given of the problems and prospects of enzyme-replacement therapy in diseases caused by the deficiency of an enzyme.

Polymerization of turtle alpha-macroglobulin through newly exposed sulfhydryls reveals the location of ex-thiolester bonds

Green turtle alpha-macroglobulin, which has previously been shown to contain thiolester bonds, formed linear polymers after being treated with proteinases. Biochemical analyses showed that the polymerization proceeded through disulfide-bond formation between monomers. The only sulfhydryl groups available for such polymerization after proteinase treatment were those created as the product of thiolester hydrolysis. Electron micrographs of polymers revealed H-shaped monomeric units aligned lengthwise in linear polymers. The average length per monomeric unit in the polymer estimated from the discrete distribution of polymer lengths was approximately 80% of the average length of free monomers, indicating that monomers overlapped each other within a region of about 4 nm. From such observations we concluded that the newly produced sulfhydryl groups were located on the four arms of the H-shaped molecule. The location of sulfhydryls can be taken as the site of the exposure of thiolesters which were originally sequestered in the hydrophobic interior of the molecule. Since the structure of turtle alpha-macroglobulin is very similar to that of human serum alpha 2-macroglobulin the results predict a similar location of sulfhydryls in human alpha 2-macroglobulin after proteinase treatment. The observed polymerization property is unique to sea turtle alpha-macroglobulin and has not been observed with human alpha 2-macroglobulin or other homologous proteins.

Structure of native alpha 2-macroglobulin and its transformation to the protease bound form

Well-preserved structures of native and alpha-chymotrypsin-bound alpha 2-macroglobulin were obtained by electron microscopy. Computer processing of these images has shown that the native structure has the shape of a padlock 19 nm long. It is proposed that the native alpha 2-macroglobulin consists of the juxtaposition of two protomers with one protomer shaped like a distorted letter "S" and with the other its reverse image, to form a binding site between the two protomers near the bottom of the complex. On cleavage of the subunits with chymotrypsin, the native structure condenses to 16.7 nm and rearranges so that the interaction between the protomers is near the middle. Two images of the alpha 2-macroglobulin-chymotrypsin conjugate were obtained. We suggest that these images represent the end and side view of this complex. Based on the manner in which the native structure is assembled, we propose that the proteolyzed form of alpha 2-macroglobulin is functionally asymmetric in that both protease binding sites reside on the same half of the complex.

The role of histidyl residues in zinc-induced precipitation of alpha 2-macroglobulin-proteinase complexes

When alpha 2-macroglobulin (alpha 2M) is reacted with proteinases including trypsin, plasmin, alpha-thrombin, or with CH3NH2, each resulting alpha 2M derivative is precipitated by Zn2+ in a similar manner. By contrast, unreacted alpha 2M is not precipitated over the same Zn2+ concentration range. Zn2+-induced precipitation of alpha 2M-CH3NH2 or alpha 2M-trypsin is prevented by acylation of the protein employing the histidine-specific reagent diethylpyrocarbonate (DEP). The Zn2+-induced precipitation of alpha 2M-trypsin is prevented by acylation of the preformed alpha 2M-trypsin complex or by the reaction of acylated native alpha 2M with trypsin. Acylation of alpha 2M by treatment with DEP results in the modification of 13.5 histidyl residues per subunit of either native alpha 2M or alpha 2M-CH3NH2. Subsequent treatment with hydroxylamine reverses the modification of 10.5 histidyl residues per subunit in each protein preparation. These results indicate that histidyl residues are involved in the Zn2+-induced precipitation of alpha 2M-proteinase or alpha 2M-CH3NH2 complexes, and that these residues are accessible to extensive protein-metal interactions only after alpha 2M has undergone a major conformational change. These appear to be the same histidyl residues which, upon acylation by DEP, are responsible for recognition of alpha 2M-proteinase complexes by the acyl-low-density-lipoprotein cell surface receptor (S. V. Pizzo, P. A. Roche, S. R. Feldman, and S. L. Gonias (1986) Biochem. J. 238, 217-225).

Endocytotic internalization of alpha-2-macroglobulin: alpha-galactosidase conjugate by cultured fibroblasts derived from Fabry hemizygote

Endocytotic internalization of alpha-galactosidase by cultured fibroblasts derived from a patient with Fabry's disease was achieved via receptor-mediated endocytosis of alpha-2-macroglobulin (alpha-2-M). alpha-galactosidase of coffee beans was conjugated to alpha-2-M when the latter was treated with trypsin. Internalization of the conjugate resulted in an increase of alpha-galactosidase activity in the crude cell extracts. The observed internalization was blocked by the presence of bacitracin, an inhibitor of binding between alpha-2-M and its receptor on the cell surface. When the cells were incubated at 4 degrees C with the conjugate, internalization was also inhibited. The alpha-galactosidase activity in the cells was saturated when the concentration of the conjugate in the medium was 40 micrograms/ml. Since non-conjugated alpha-galactosidase was not effectively internalized, the observed internalization of the conjugate was mediated by recognition of alpha-2-M by its receptor. The effective internalization of alpha-galactosidase described in this paper has a potential use in the enzyme replacement therapy of Fabry's disease.

Proton nuclear magnetic resonance study of human plasma alpha-2-macroglobulin

A proton nuclear magnetic resonance (NMR) study is reported of human alpha-2-macroglobulin (alpha-2-M). It was observed that alpha-2-M, which consists of four identical subunits and has a molecular weight of 720,000, gives several sharp resonances. After cleavage of the "bait" region peptide with trypsin and subsequent removal of the peptide under a high salt condition, most of the sharp resonances disappeared, indicating that the sharp resonances observed in the native alpha-2-M originate from the amino acid residues in the bait region. Resonances due to the aromatic protons of the Tyr residue, which exists in the bait region, have been assigned on the basis of chemical shift. It was observed that the C3- and C5-H proton resonances for the Tyr residue are especially narrow, indicating that the side chain of the Tyr residue in the bait region is in a highly mobile state. Photochemically induced dynamic nuclear polarization experiments clearly show that the Tyr residue is actually exposed to the solvent. It was possible to identify resonances due to several His residues that are exposed to solvent. Other resonances, which probably originate from Arg residues in the bait region, were also observable in the conventional NMR spectra. On the basis of the present NMR data, we conclude that the bait region of the native alpha-2-M is highly flexible and exposed to solvent. On treatment of alpha-2-M with methylamine, no significant change has been detected in the NMR spectra observed in both the conventional and CIDNP mode.(ABSTRACT TRUNCATED AT 250 WORDS)

Open quaternary structure of the hagfish proteinase inhibitor with similar properties to human alpha-2-macroglobulin

A homologous protein to human plasma alpha-2-macroglobulin (alpha-2-M) was purified from the blood plasma of hagfish (Eptatretus buergeri) and its structure and function were studied. The hagfish protein inhibited several proteinases and its inhibitory activity was blocked with methylamine as in the case of human alpha-2-M. The molecular weight and sedimentation coefficient of the hagfish inhibitor were 390,000 +/- 20,000 and 11.0 S, respectively, as determined by sedimentation studies. The frictional ratio calculated from these parameters was 1.75. The Stokes radius estimated from HPLC gel chromatography was 8.8-8.9 nm, which was similar to that of human alpha-2-M despite the fact that the hagfish inhibitor was only one-half as large as human alpha-2-M in molecular weight. The hagfish inhibitor was expected to be more asymmetric and/or more hydrated than the human inhibitor. The electron micrographs of the negatively stained hagfish inhibitor showed that it had an open, rectangular quaternary structure of 15 +/- 1.5 X 19 +/- 2 nm in which two semiglobular units were located at the two shorter sides with a gap of 8 +/- 1 nm in width. Each semiglobular unit had an approximate width of 5 +/- 0.5 nm. The thickness of the unit was estimated to be 3 to 3.5 nm from the result of fixed-angle shadowing experiments. Although the two semiglobular units must be connected by some structure, very little material could be seen between them. Such an open quaternary structure may explain the high frictional ratio and large Stokes radius of this protein. The structural change of the inhibitor after reaction with proteinases or methylamine could be detected by electron microscopy and gel chromatography.

Calculation and experimental verification of the frictional ratio of hagfish proteinase inhibitor

A major structural feature of the alpha-2-macroglobulin-like inhibitor of hagfish described in Osada, Nishigai, and Ikai [(1987) J. Ultrastruct. Mol. Struct. Res. 96, 00-00] was its highly open quaternary structure observed under an electron microscope. We drew a qualitative conclusion that the high frictional ratio obtained from the result of sedimentation study and the large Stokes radius obtained in gel chromatographic experiment were the reflection of such an open quaternary structure. In this paper I present several structural models of hagfish inhibitor based on its electron micrographs and calculate expected frictional ratios for such models according to the method developed by Bloomfield and his co-workers. Their method allows the calculation of frictional coefficient of a body of an arbitrary shape by approximating it with a collection of small spheres. To test the validity of such a method, macroscopic models were built from plastic spheres or cylindrical capsules and their translational frictional coefficients were measured by the free-falling method under experimental conditions where the Reynolds number was between 10(-3) and 10(-4).

Unusual properties of crocodilian ovomacroglobulin shown in its methylamine treatment and sulfhydryl titration

The inhibitory activity of chicken and crocodilian ovomacroglobulins against trypsin was measured before and after their incubation with methylamine. The result for crocodilian ovomacroglobulin showed that methylamine treatment destroyed half of its activity, in unique contrast to human alpha 2-macroglobulin and chicken ovomacroglobulin for which methylamine either destroys the inhibitory activity of the former completely or does not affect that of the latter at all. Free sulfhydryl groups of chicken and crocodilian ovomacroglobulins were titrated with 5,5'-dithiobis(2-nitrobenzoic acid) before and after incubation with trypsin. Prior to the incubation with trypsin the chicken and crocodilian proteins respectively had 0 and 1 titratable sulfhydryl per molecule of Mr 720,000. After treatment with trypsin the crocodilian protein had 3.5-4 titratable sulfhydryls, whereas there were no titratable sulfhydryls in the chicken protein. After denaturation of the crocodilian protein in sodium dodecyl sulfate at 100 degrees C the number of titratable sulfhydryls was 4. Chicken ovomacroglobulin again did not have an appreciable number of titratable sulfhydryls under similar denaturing conditions. Incubation of crocodilian protein with [14C]methylamine showed an incorporation of at least 2 mol of methylamine per molecule. The result indicated the presence of three intramolecular thiol ester bonds in crocodilian ovomacroglobulin with differential stability against external perturbations.