Keyhole limpet hemocyanin type 2 (KLH2): detection and immunolocalization of a labile functional unit h

Distribution and characterization of rhogocyte cell types in the mantle tissue of Haliotis laevigata

Molluscan rhogocytes are known to be the only cells able to synthesize hemocyanin that is one of the largest respiratory proteins in nature. However, investigation of rhogocyte cells in vitro is limited due to difficulty in isolating and establishing marine cell culture. The aim of this study was to investigate the nature and distribution of rhogocyte cells of Haliotis laevigata in the mantle tissue with respect to the expression of the two known isoforms of hemocyanin. Rhogocyte cells were identified using immunofluorescence-fluorescence in situ hybridization (IF-FISH) that involved simultaneous staining of localized hemocyanin by a polyclonal antibody while the mRNA was hybridized with FISH probes. The distribution of rhogocyte cells was demonstrated using flow cytometry, followed by cell sorting with fluorescence-activated cell sorter (FACS) and confocal microscope imaging for further characterization. Our results suggested that the mantle tissue is dominated by two distinct populations of rhogocyte cells that synthesize hemocyanin type 1. Observation with confocal microscopy of both populations revealed hemocyanin localization in the periphery of the cell membrane. Cell population with higher antibody signal had irregular and elongated cell morphology with punctate mRNA probe signals. The second population with lower antibody signal had ovoid morphology and wide distribution of mRNA probe signals. We suggest that these populations represent two distinct phases of hemocyanin biosynthesis of a single isoform, which is closely related to Haliotis tuberculata type 1 hemocyanin (HtH1). The knowledge acquired in this study enhances the understanding of the biology of rhogocyte cells and biosynthesis of hemocyanin.

Cryo-EM structure of a molluscan hemocyanin suggests its allosteric mechanism

Hemocyanins are responsible for transporting O2 in the arthropod and molluscan hemolymph. Haliotis diversicolor molluscan hemocyanin isoform 1 (HdH1) is an 8 MDa oligomer. Each subunit is made up of eight functional units (FUs). Each FU contains two Cu ions, which can reversibly bind an oxygen molecule. Here, we report a 4.5 A° cryo-EM structure of HdH1. The structure clearly shows ten asymmetric units arranged with D5 symmetry. Each asymmetric unit contains two structurally distinct but chemically identical subunits. The map is sufficiently resolved to trace the entire subunit Ca backbone and to visualize densities corresponding to some large side chains, Cu ion pairs, and interaction networks of adjacent subunits. A FU topology path intertwining between the two subunits of the asymmetric unit is unambiguously determined. Our observations suggest a structural mechanism for the stability of the entire hemocyanin didecamer and 20 ‘‘communication clusters’’ across asymmetric units responsible for its allosteric property upon oxygen binding.

Characterization of the glutamate-specific endopeptidase from Bacillus licheniformis expressed in Escherichia coli

Glutamate-specific endopeptidase from Bacillus licheniformis (GSE-BL) is widely used in peptide recovery and synthesis because of its unique substrate specificity. However, the mechanism underlying its specificity is still not thoroughly understood. In this study, the roles of the prosegment and key amino acids involved in the proteolytic activity of GSE-BL were investigated. Loss of the GSE-BL prosegment severely restricted enzymatic activity toward Z-Phe-Leu-Glu-pNA. A homologous model of GSE-BL revealed that it contains the catalytic triad "His47, Asp96 and Ser 167", which was further confirmed by site-directed mutagenesis. In vitro mutagenesis further indicated that Val2, Arg89 and His190 are essential for enzymatic activity toward Z-Phe-Leu-Glu-pNA. Moreover, the catalytic efficiency of Phe57Ala GSE-BL toward Z-Phe-Leu-Glu-pNA was 50% higher than that of the native mature GSE-BL. This is the first study to fully elucidate the key amino acids for proteolytic activity of GSE-BL. Mature GSE-BL could be obtained through self-cleavage alone when Lys at -1 position was replaced by Glu, providing a new strategy for the preparation of mature GSE-BL. This study yielded some valuable insights into the substrate specificity of glutamate-specific endopeptidase, establishing a foundation for broadening the applications of GSE-BL.

Evolution of molluscan hemocyanin structures

Hemocyanin transports oxygen in the hemolymph of many molluscs and arthropods and is therefore a central physiological factor in these animals. Molluscan hemocyanin molecules are oligomers composed of many protein subunits that in turn encompass subsets of distinct functional units. The structure and evolution of molluscan hemocyanin have been studied for decades, but it required the recent progress in DNA sequencing, X-ray crystallography and 3D electron microscopy to produce a detailed view of their structure and evolution. The basic quaternary structure is a cylindrical decamer 35nm in diameter, consisting of wall and collar (typically at one end of the cylinder). Depending on the animal species, decamers, didecamers and multidecamers occur in the hemolymph. Whereas the wall architecture of the decamer seems to be invariant, four different types of collar have been identified in different molluscan taxa. Correspondingly, there exist four subunit types that differ in their collar functional units and range from 350 to 550kDa. Thus, molluscan hemocyanin subunits are among the largest polypeptides in nature. In this report, recent 3D reconstructions are used to explain and visualize the different functional units, subunits and quaternary structures of molluscan hemocyanins. Moreover, on the basis of DNA analyses and structural considerations, their possible evolution is traced. This article is part of a Special Issue entitled: Oxygen Binding and Sensing Proteins.

Enhanced structural stability of Concholepas hemocyanin increases its immunogenicity and maintains its non-specific immunostimulatory effects

Hemocyanins, which boost the immune system of mammals, have been used as carrier-adjuvants to promote Ab production against haptens and peptides, as immunostimulants during therapy for bladder carcinoma and as a component in therapeutic vaccines for cancer. These biomedical applications have led to growing interest in obtaining hemocyanins with high immunogenicity. Here, we study the immunological properties of a modified oxidized Concholepas concholepas hemocyanin (Ox-CCH) obtained by the oxidation of its carbohydrates using sodium periodate. We assessed the internalization of Ox-CCH into DCs and its immunogenicity and antitumor effects. Transmission electron microscopy showed no changes in Ox-CCH quaternary structure with respect to native CCH, although proteolytic treatment followed by SDS-PAGE analysis demonstrated that Schiff bases were formed. Interestingly, DCs internalized Ox-CCH faster than CCH, mainly through macropinocytosis. During this process, Ox-CCH remained inside endosome-like structures for a longer period. Mouse immunization experiments demonstrated that Ox-CCH is more immunogenic and a better carrier than CCH. Moreover, Ox-CCH showed a significant antitumor effect in the B16F10 melanoma model similar to that produced by CCH, inducing IFN-γ secretion. Together, these data demonstrate that the aldehydes formed by the periodate oxidation of sugar moieties stabilizes the CCH structure, increasing its adjuvant/immunostimulatory carrier effects.

Keyhole limpet hemocyanin: 9-A CryoEM structure and molecular model of the KLH1 didecamer reveal the interfaces and intricate topology of the 160 functional units

Hemocyanins are blue copper-containing respiratory proteins in the hemolymph of many arthropods and molluscs. Molluscan hemocyanins are decamers, didecamers, or multidecamers of a 340- to 400-kDa polypeptide subunit containing seven or eight globular functional units (FUs; FU-a to FU-h), each with an oxygen-binding site. The decamers are short 35-nm hollow cylinders, with their lumen narrowed by a collar complex. Our recently published 9-A cryo-electron microscopy/crystal structure hybrid model of a 3.4-MDa cephalopod hemocyanin decamer [Nautilus pompilius hemocyanin (NpH)] revealed the pathway of the seven-FU subunit (340 kDa), 15 types of inter-FU interface, and an asymmetric collar consisting of five "arcs" (FU-g pairs). We now present a comparable hybrid model of an 8-MDa gastropod hemocyanin didecamer assembled from two asymmetric decamers [isoform keyhole limpet hemocyanin (KLH) 1 of the established immunogen KLH]. Compared to NpH, the KLH1 subunit (400 kDa) is C-terminally elongated by FU-h, which is further extended by a unique tail domain. We have found that the wall-and-arc structure of the KLH1 decamer is very similar to that of NpH. We have traced the subunit pathway and how it continues from KLH1-g to KLH1-h to form an annulus of five "slabs" (FU-h pairs) at one cylinder edge. The 15 types of inter-FU interface detected in NpH are also present in KLH1. Moreover, we have identified one arc/slab interface, two slab/slab interfaces, five slab/wall interfaces, and four decamer/decamer interfaces. The 27 interfaces are described on the basis of two subunit conformers, yielding an asymmetric homodimer. Six protrusions from the cryo-electron microscopy structure per subunit are associated with putative attachment sites for N-linked glycans, indicating a total of 120 sugar trees in KLH1. Also, putative binding sites for divalent cations have been detected. In conclusion, the present 9-A data on KLH1 confirm and substantially broaden our recent analysis of the smaller cephalopod hemocyanin and essentially solve the gastropod hemocyanin structure.

Comparative 11A structure of two molluscan hemocyanins from 3D cryo-electron microscopy

Hemocyanins are giant extracellular proteins that transport oxygen in the hemolymph of many molluscs. Molluscan hemocyanins are cylindrical decamers or didecamers of a 350-400 kDa subunit that contains seven or eight different covalently linked globular functional units (FUs), arranged in a linear manner. Each FU carries a single copper active site and reversibly binds one dioxygen molecule. As a consequence, the decamer can carry up to 70 or 80 O(2) molecules. Although complete sequence information is now available from several molluscan hemocyanins, many details of the quaternary structure are still unclear, including the topology of the 10 subunits within the decamer. Here we show 3D reconstructions from cryo-electron micrographs of the hemocyanin decamer of Nautilus pompilius (Cephalopoda) and Haliotis tuberculata (Gastropoda) at a resolution of 11A (FSC(1/2-bit) criterion). The wall structure of both hemocyanins is very similar and shows, as in previous reconstructions, three tiers with 20 functional units each that encircle the cylinder wall, and the 10 oblique minor and major wall grooves. However, the six types of wall FUs of the polypeptide subunit, termed a-b-c-d-e-f, are now for the first time individually discernable by their specific orientation, shape, and connections. Also, the internal collar complex of the decamers shows superior resolution which, in this case, reveals striking differences between the two hemocyanins. The five arcs (FU-g pairs) of the central collar (in both hemocyanins) and the five slabs (FU-h pairs) of the peripheral collar (only present in Haliotis hemocyanin), as well as their connections to the wall and to each other are now more clearly defined. The arc is attached to the wall through a feature termed the anchor, a previously undescribed structural element of the hemocyanin wall.

Hemocyanin of the Molluscan Concholepas concholepas Exhibits an Unusual Heterodecameric Array of Subunits

We describe here the structure of the hemocyanin from the Chilean gastropod Concholepas concholepas (CCH), emphasizing some attributes that make it interesting among molluscan hemocyanins. CCH exhibits a predominant didecameric structure as revealed by electron microscopy and a size of 8 MDa by gel filtration, and, in contrast with other mollusc hemocyanins, its stabilization does not require additional Ca(2+) and/or Mg(2+) in the medium. Polyacrylamide gel electrophoresis studies, analyses by a MonoQ FPLC column, and Western blots with specific monoclonal antibodies showed that CCH is made by two subunits noncovalently linked, named CCH-A and CCH-B, with molecular masses of 405 and 350 kDa, respectively. Interestingly, one of the subunits undergoes changes within the macromolecule; we demonstrated that CCH-A has an autocleavage site that under reducing conditions is cleaved to yield two polypeptides, CCH-A1 (300 kDa) and CCH-A2 (108 kDa), whereas CCH-B remains unchanged. The CCH-A nick occurs at 4 degrees C, increases at 37 degrees C, and is not inhibited by the addition of protease inhibitors and/or divalent cations. Since the CCH structure is a heterodimer, we investigated whether subunits would be either intermingled, forming heterodecamers, or assembled as two homogeneous decamers. Light scattering and electron microscope studies of the in vitro reassociation of purified CCH subunits demonstrated that the sole addition of Mg(2+) is needed for its reassembly into the native decameric molecule; no homodecamer reorganization was found with either CCH-A or CCH-B subunits alone. Our evidence showed that C. concholepas hemocyanin is an unusual example of heterodecameric organization.

Oligomeric stability of Rapana venosa hemocyanin (RvH) and its structural subunits

The two structural subunits RvH1 and RvH2 were separated after overnight dialysis of Rapana venosa Hc against 130 mM Gly/NaOH buffer, pH 9.6, on an ion exchange column Hiload 26/10 Sepharose Q using a fast performance liquid chromatography (FPLC) system. The reassociation characteristics of these two RvH isoforms and the native molecule were studied in buffers with different pH values and concentrations of Ca(2+) and Mg(2+). Reassociation of mixed RvH subunits was performed over a period of several days using a stabilizing buffer (SB) of pH 7.0 containing different concentrations of Ca(2+) and Mg(2+) ions. After 2 days of dialysis, an RvH subunit mixture of didecamers and multidecamers was observed in the presence of 100 mM CaCl(2) and MgCl(2), though RvH1 and RvH2 are biochemically and immunologically different and have also different dissociation properties. The reassociation, performed at pH 9.6 with 2 mM CaCl(2) and MgCl(2) at 4 degrees C over a period of one to several weeks, led to the formation of decameric oligomers, while didecamers formed predominantly in the SB at pH 7.0. Higher concentrations of calcium and magnesium ions led to a more rapid reassociation of RvH1 resulting in long stable multidecamers and helical tubules, which were stable and slowly dissociated into shorter multidecamers and decamers at higher pH values. The reassociation of the RvH2 structural subunit in the same buffers processed slowly and yielded didecamers, shorter tubule polymers and long multidecamers which are less stable at higher pH values. The stability of RvH isoforms under varying ionic conditions is compared with the stability of keyhole limpet (KLH, Megathura crenulata) hemocyanin (KLH) and Haliotis tuberculata hemocyanin (HtH) isoforms. The process of dissociation and reassociation is connected with changes of the fluorescence intensity at 600 nm, which can be explained by differences in opalescence of the solutions of these two isoforms. The solutions of longer tubule polymers and multidecamers of RvH1 show a higher opalescence compared to the solutions of shorter helical tubules and multidecamers of RvH2.

A new metallothionein gene from the giant keyhole limpet Megathura crenulata

Metallothioneins (MTs) are small soluble proteins ubiquitously expressed in animals and plants. Different isoforms are present in deuterostomes and protostomes. They do not differ greatly in primary structure, but are clearly distinguishable. Here, I present the gene and the complete cDNA of a novel MT from the mollusk Megathura crenulata. This protein is closely related to the Cu-inducible MTs of the vineyard snail Helix pomatia, but has also some minor sequence features typical of Cd-inducible isoforms of H. pomatia and other molluscs. Overall, the deduced primary structure is similar to the known molluscan MTs, but in addition possesses an insertion of 5 amino acids not found in any other molluscan MTs, protostomic or deuterostomic MTs. In addition, a pentapeptide insertion, characteristic of mammalian MT-3 is present but it lacks the functional tetrapeptide CPCP within the beta-region of those MT-3 proteins that are known to suppress neuronal growth processes. The M. crenulata MT is a novel form of MT in comparison to all other known MTs. Possible functional aspects for this new MT are discussed.

Monoclonal antibodies to molluskan hemocyanin from Concholepas concholepas demonstrate common and specific epitopes among subunits

We studied the reactivity of mouse monoclonal antibodies (MAbs) against the hemocyanin from the Chilean marine gastropod Concholepas concholepas (CCH). This protein has been successfully used as a carrier to produce antibodies to haptens and peptides. All MAbs (13) belonging to IgG subclass exhibit dissociation constants (K(d)) from 1 x 10(-7) M to 1 x 10(-9) M. MAbs were characterized by enzyme-linked immunosorbant assay (ELISA) using CCH treated with different procedures, including dissociation into CCH-A and CCH-B subunits, Western blot, enzymatic digestion, chemical deglycosylation, and thermal denaturation. MAbs were classified into three categories, according to subunit specificity by ELISA. The epitope distribution shows that CCH subunits display common epitopes (group I, 5 MAbs, 1H5, 2A8, 3A5, 3B3, and 3E3), as well as specific epitopes for CCH-A subunits (group II, 3 MAbs, 1B8, 4D8, and 8E5) and for CCH-B subunits (group III, 5 MAbs, 1A4, 1E4, 2H10, 3B7, and 7B4). The results can be summarized as follows: (1). six antibodies react with thermal denatured CCH, suggesting that they recognize linear epitopes, whereas seven recognize conformational epitopes; (2). oxidation of carbohydrate moieties does not affect the binding of the MAbs; (3). enzymatic digestion of CCH decreases the reactivity of all antibodies irrespective of the protease used (elastase or trypsin); (4). bringing together the above data, in addition to epitopic complementarity analysis, we identified 12 different epitopes on the CCH molecule recognized by these MAbs. The anti-CCH MAbs presented here can be useful tools to understand the subunit organization of the CCH and its complex structure, which can explain its immunogenic and immunostimulating properties in mammals.

Topology of the 10 subunits within the decamer of KLH, the hemocyanin of the marine gastropod Megathura crenulata

Immunoelectron microscopy has been performed using negatively stained immune complexes of keyhole limpet hemocyanin isoform 1 (KLH1) decamers and a functional unit-specific monoclonal antibody anti-KLH1-c1. The antibody links hemocyanin molecules at both the collar and the collarless edge of the decamer, indicating a peripheral localization of functional units c. In isoform 2 (KLH2) the positions of functional units c have been identified with the peanut agglutinin (PNA), which has previously been shown to exclusively bind to KLH2-c. Ferritin linked to PNA was used to visualize labeled molecules electron microscopically. The pattern of labeling also indicates a peripheral localization of the c functional units. The data presented in this paper support only one of two possible models for the subunit orientation within the hemocyanin decamer.

Isolation and spectroscopic characterization of the structural subunits of keyhole limpet hemocyanin

Keyhole limpet hemocyanin is a respiratory glycoprotein of high molecular weight from the gastropod mollusc Megathura crenulata. Two subunits, KLH1 and KLH2, were isolated using ion exchange chromatography and their physical properties are compared with the parent molecule. The various proteins are characterized by fluorescence spectroscopy, combined with fluorescence quenching studies, using acrylamide, cesium chloride and potassium iodide as tryptophan quenchers. The conformational stability of the native aggregate and its isolated structural subunits are also studied by circular dichroism and fluorescence spectroscopy as a function of temperature, as well as in the presence of guanidinium hydrochloride and urea. The associated subunits in the hemocyanin aggregates increase considerably the melting temperature to 67 degrees C and the free energy of stabilization in water, DeltaG(H(2)O)(D), towards guanidinium hydrochloride is higher for the decamer as compared to the isolated subunits; this difference can be accounted for by the stabilizing effects of intra-subunit interactions exerted within the oligomer. The copper-dioxygen complex at the active site additionally stabilizes the molecule, and removing of the copper ions increases the tryptophan emission and the quantum yield of the fluorescence.

Haliotis tuberculata hemocyanin (HtH): analysis of oligomeric stability of HtH1 and HtH2, and comparison with keyhole limpet hemocyanin KLH1 and KLH2

The multimeric/higher oligomeric states of the two isoforms of Haliotis tuberculata hemocyanin (HtH1 and HtH2) have been assessed by transmission electron microscopy (TEM) of negatively stained specimens, for comparison with previously published structural data from keyhole limpet hemocyanin (KLH1 and KLH2) [see Harris, J.R., Gebauer, W., Guderian, F.U., Markl, J., 1997a. Keyhole limpet hemocyanin (KLH), I: Reassociation from Immucothel followed by separation of KLH1 and KLH2. Micron, 28, 31-41; Harris, J.R., Gebauer, W., Söhngen, S.M., Nermut, M.V., Markl, J., 1997b. Keyhole limpet hemocyanin (KLH). II: Characteristic reassociation properties of purified KLH1 and KLH2. Micron, 28, 43-56; Harris, J.R., Gebauer, W., Adrian, M., Markl, J., 1998. Keyhole limpet hemocyanin (KLH): Slow in vitro reassociation of KLH1 and KLH2 from Immucothel. Micron, 29, 329-339]. In purified samples of both HtH isoforms, the hollow cylindrical ca 8MDa didecamer predominates together with a small number of decamers, but tri- and longer multidecamers are detectable only in the HtH2. The stability of the two HtH isoforms under varying ionic conditions have been monitored, thereby enabling conditions for the production of stable decamers to be established. The ability of these decamers to reform multimers in the presence of 10 and 100mM concentrations of calcium and magnesium ions in Tris-HCl buffer (pH 7.4), and also of individual HtH1 and HtH2 subunits (produced by pH 9.6 dissociation in glycine-NaOH buffer), to reassociate in the presence of calcium and magnesium ions, has been assessed. For the HtH1 decamers, the predominant multimeric product is the didecamer at 10 and 100mM calcium and magnesium concentrations, whereas for the HtH2 decamers, large numbers of multidecamers are produced, with the reaction proceeding more completely at the higher calcium and magnesium concentration. With the HtH1 subunit, reassociation in the presence of 10 and 100mM calcium and magnesium ions yielded an almost 100% conversion into didecamers, whereas the HtH2 subunit produced a mixture containing large numbers of short multidecamers and relatively few didecamers, together with a considerable number of smaller diameter helical/tubular polymers. The association properties of the HtH1 and HtH2 decamers, and the subunit reassociation, firmly indicate the integrity and structural competency of the protein under the experimental conditions used. Data on the association of KLH2 decamers is also presented, which together with previously published data on the association KLH1 decamers and the reassociation of KLH1 and KLH2 subunits, enables a detailed comparison of the two hemocyanin isoforms from both molluscan species to be made. Biochemical manipulation of the oligomer states and the subunit reassociation of molluscan hemocyanins can usefully be assessed by the study of negatively stained TEM specimens.

Structure of a molluscan hemocyanin didecamer (HtH1 from Haliotis tuberculata) at 12 A resolution by cryoelectron microscopy

A 12 A resolution three-dimensional density map of the Haliotis tuberculata hemocyanin type 1 (HtH1) didecamer has been obtained by cryoelectron microscopy of unstained molecules and angular reconstitution. The dyad symmetry of the 8 MDa D5 HtH1 didecamer, formed by the pairing of two asymmetric 4 MDa ring-like C5 decamers, is emphasised. The major and minor surface helical grooves of the didecamer are well defined, in agreement with earlier data on molluscan hemocyanins. The location of the obliquely orientated repeating unit, a subunit dimer, within the decamer has been defined. Following interactive extraction of this dimer, several new structural features of the dimer and of the subunit have now emerged with improved detail. The subunit dimer possesses pseudo 2-fold symmetry, resulting from the steric arrangement of the wall domains/functional units (FUs-abcdef) of the two subunits. The arc and collar FUs (g and h) depart from this inherent 2-fold symmetry and are thereby responsible for the asymmetry of the C5 decamer, with the internalised collar/arc complex at one edge of the decamer. The FU heterodimers forming the wall morphological units have a hollow centre, and thus create a series of repeating channels that extend within the wall through all three tiers of the decamer. The connections between the wall and the arc are defined with improved clarity, and evidence is provided to indicate that the arc and collar FU pairs have a homodimeric composition (gg and hh, respectively). Two possibilities for the subunit path within the subunit dimer are presented, which correlate with the available structural, immunolabelling and protease cleavage data from HtH1 and other molluscan hemocyanins.

The sequence of a gastropod hemocyanin (HtH1 from Haliotis tuberculata)

The eight functional units (FUs), a-h, of the hemocyanin isoform HtH1 from Haliotis tuberculata (Prosobranchia, Archaeogastropoda) have been sequenced via cDNA, which provides the first complete primary structure of a gastropod hemocyanin subunit. With 3404 amino acids (392 kDa) it is the largest polypeptide sequence ever obtained for a respiratory protein. The cDNA comprises 10,758 base pairs and includes the coding regions for a short signal peptide, the eight different functional units, a 3'-untranslated region of 478 base pairs, and a poly(A) tail. The predicted protein contains 13 potential sites for N-linked carbohydrates (one for HtH1-a, none for HtH1-c, and two each for the other six functional units). Multiple sequence alignments show that the fragment HtH1-abcdefg is structurally equivalent to the seven-FU subunit from Octopus hemocyanin, which is fundamental to our understanding of the quaternary structures of both hemocyanins. Using the fossil record of the gastropod-cephalopod split to calibrate a molecular clock, the origin of the molluscan hemocyanin from a single-FU protein was calculated as 753 +/- 68 million years ago. This fits recent paleontological evidence for the existence of rather large mollusc-like species in the late Precambrian.