Disulfide bond reduction: A powerful, chemical probe for the study of structure-function relationships in the hemocyanins

Hemocyanins of Muricidae: New 'Insights' Unravel an Additional Highly Hydrophilic 800 kDa Mass Within the Molecule

Hemocyanins are giant oxygen transport proteins that freely float within the hemolymph of most molluscs. The basic quaternary structure of molluscan hemocyanins is a cylindrical decamer with a diameter of 35 nm which is built of 400 kDa subunits. Previously published results, however, showed that one out of two hemocyanin subunits of Rapana venosa encompasses two polypeptides, one 300 kDa and one 100 kDa polypeptide which aggregate to typical 4 MDa and 8 MDa hemocyanin (di-)decamer molecules. It was shown that the polypeptides are bound most probably by one or more cysteine disulfide bridges but it remained open if these polypeptides were coded by one or two genes. Our here presented results clearly showed that both polypeptides are coded by one gene only and that this phenomenon can also be found in the gastropod Nucella lapillus. Thus, it can be defined as clade-specific for Muricidae, a group of the very diverse Caenogastropoda. In addition, we discovered a further deviation of this hemocyanin subunit within both species, namely a region of 340 mainly hydrophilic amino acids (especially histidines and aspartic acids) which have not been identified in any other molluscan hemocyanin, yet. Our results indicate that, within the quaternary structure, these additional amino acids most probably protrude within the inner part of didecamer cylinders, forming a large extra mass of up to 800 kDa. They presumably influence the structure of the protein and may affect the functionality. Thus, these findings reveal further insights into the evolution and structures of gastropod hemocyanins.

The cDNA sequence of three hemocyanin subunits from the garden snail Helix lucorum

Hemocyanins are blue copper containing respiratory proteins residing in the hemolymph of many molluscs and arthropods. They can have different molecular masses and quaternary structures. Moreover, several molluscan hemocyanins are isolated with one, two or three isoforms occurring as decameric, didecameric, multidecameric or tubule aggregates. We could recently isolate three different hemocyanin isopolypeptides from the hemolymph of the garden snail Helix lucorum (HlH). These three structural subunits were named α(D)-HlH, α(N)-HlH and β-HlH. We have cloned and sequenced their cDNA which is the first result ever reported for three isoforms of a molluscan hemocyanin. Whereas the complete gene sequence of α(D)-HlH and β-HlH was obtained, including the 5' and 3' UTR, 180bp of the 5' end and around 900bp at the 3' end are missing for the third subunit. The subunits α(D)-HlH and β-HlH comprise a signal sequence of 19 amino acids plus a polypeptide of 3409 and 3414 amino acids, respectively. We could determine 3031 residues of the α(N)-HLH subunit. Sequence comparison with other molluscan hemocyanins shows that α(D)-HlH is more related to Aplysia californicum hemocyanin than to each of its own isopolypeptides. The structural subunits comprise 8 different functional units (FUs: a, b, c, d, e, f, g, h) and each functional unit possesses a highly conserved copper-A and copper-B site for reversible oxygen binding. Potential N-glycosylation sites are present in all three structural subunits. We confirmed that all three different isoforms are effectively produced and secreted in the hemolymph of H. lucorum by analyzing a tryptic digest of the purified native hemocyanin by MALDI-TOF and LC-FTICR mass spectrometry.

Identification of glycosylated sites in Rapana hemocyanin by mass spectrometry and gene sequence, and their antiviral effect

Molluscan hemocyanins (Hcs) have recently received particular interest due to their significant immunostimulatory properties. This is mainly related to their high carbohydrate content and specific monosaccharide composition. We have now analyzed the oligosaccharides and the carbohydrate linkage sites of the Rapana venosa hemocyanin (RvH) using different approaches. We analyzed a number of glycopeptides by LC/ESI-MS/MS and identified the sugar chains and peptide sequences of 12 glycopeptides. Additionally, the potential carbohydrate linkage sites of 2 functional units, RvH-b and RvH-c, were determined by gene sequence analysis. Only RvH-c shows a potential N-glycosylation site. During this study, we discovered a highly conserved linker-intron, separating the coding exons of RVH-b and RvH-c. Following reports on antiviral properties from arthropod hemocyanin, we conducted a preliminary study of the antiviral activity of RvH and the functional units RvH-b and RvH-c. We show that the glycosylated FU RvH-c has antiviral properties against the respiratory syncytial virus (RSV), whereas native RvH and the nonglycosylated FU RvH-b have not. This is the first report of the fact that also molluscan hemocyanin functional units possess antiviral activity.

The first complete cDNA sequence of the hemocyanin from a bivalve, the protobranch Nucula nucleus

By cDNA sequencing we have achieved the first, and complete, hemocyanin sequence of a bivalve (Nucula nucleus). This extracellular oxygen-binding protein consists of two immunologically distinguishable isoforms, here termed NnH1 and NnH2. They share a mean sequence identity of 61%, both contain a linear arrangement of eight paralogous, ca.50-kDa functional units (FUs a-h), and in both isoforms the C-terminal FU-h possesses an extension of ca. 100 amino acids. The cDNA of NnH1 comprises 11,090 bp, subdivided into a 5'utr of 75 bp, a 3'utr of 791 bp, and an open reading frame for a signal peptide of 19 amino acids plus a polypeptide of 3389 amino acids (Mr = 385 kDa). The cDNA of NnH2 comprises 10,849 bp, subdivided into a 5'utr of 47 bp, a 3'utr of 647 bp, and an open reading frame for a signal peptide of 16 amino acids plus a polypeptide of 3369 amino acids (Mr = 387 kDa). In contrast to other molluscan hemocyanins, which are highly glycosylated, the bivalve hemocyanin sequence exhibits only four potential N-glycosylation sites, and within both isoforms a peculiar indel is present, surrounding the highly conserved copper-binding site CuA. Phylogenetic analyses of NnH1 and NnH2, compared to the known hemocyanin sequences of gastropods and cephalopods, reveal a statistically sound closer relationship between gastropod and protobranch hemocyanin than to cephalopod hemocyanin. Assuming a molecular clock, the last common ancestor of protobranch and gastropods lived 494 million +/- 50 million years ago, in conformity with fossil records from the late Cambrian.

The Hemocyanin from a Living Fossil, the Cephalopod Nautilus pompilius: Protein Structure, Gene Organization, and Evolution

By electron microscopic and immunobiochemical analyses we have confirmed earlier evidence that Nautilus pompilius hemocyanin (NpH) is a ring-like decamer (M(r) = approximately 3.5 million), assembled from 10 identical copies of an approximately 350-kDa polypeptide. This subunit in turn is substructured into seven sequential covalently linked functional units of approximately 50 kDa each (FUs a-g). We have cloned and sequenced the cDNA encoding the complete polypeptide; it comprises 9198 bp and is subdivided into a 5' UTR of 58 bp, a 3' UTR of 365 bp, and an open reading frame for a signal peptide of 21 amino acids plus a polypeptide of 2903 amino acids (M(r) = 335,881). According to sequence alignments, the seven FUs of Nautilus hemocyanin directly correspond to the seven FU types of the previously sequenced hemocyanin "OdH" from the cephalopod Octopus dofleini. Thirteen potential N-glycosylation sites are distributed among the seven Nautilus hemocyanin FUs; the structural consequences of putatively attached glycans are discussed on the basis of the published X-ray structure for an Octopus dofleini and a Rapana thomasiana FU. Moreover, the complete gene structure of Nautilus hemocyanin was analyzed; it resembles that of Octopus hemocyanin with respect to linker introns but shows two internal introns that differ in position from the three internal introns of the Octopus hemocyanin gene. Multiple sequence alignments allowed calculation of a rather robust phylogenetic tree and a statistically firm molecular clock. This reveals that the last common ancestor of Nautilus and Octopus lived 415 +/- 24 million years ago, in close agreement with fossil records from the early Devonian.

Subunit sequences of the 4 x 6-mer hemocyanin from the golden orb-web spider, Nephila inaurata

The transport of oxygen in the hemolymph of many arthropod and mollusc species is mediated by large copper-proteins that are referred to as hemocyanins. Arthropod hemocyanins are composed of hexamers and oligomers of hexamers. Arachnid hemocyanins usually form 4 x 6-mers consisting of seven distinct subunit types (termed a-g), although in some spider taxa deviations from this standard scheme have been observed. Applying immunological and electrophoretic methods, six distinct hemocyanin subunits were identified in the red-legged golden orb-web spider Nephila inaurata madagascariensis (Araneae: Tetragnathidae). The complete cDNA sequences of six subunits were obtained that corresponded to a-, b-, d-, e-, f- and g-type subunits. No evidence for a c-type subunit was found in this species. The inclusion of the N. inaurata hemocyanins in a multiple alignment of the arthropod hemocyanins and the application of the Bayesian method of phylogenetic inference allow, for the first time, a solid reconstruction of the intramolecular evolution of the chelicerate hemocyanin subunits. The branch leading to subunit a diverged first, followed by the common branch of the dimer-forming b and c subunits, while subunits d and f, as well as subunits e and g form common branches. Assuming a clock-like evolution of the chelicerate hemocyanins, a timescale for the evolution of the Chelicerata was obtained that agrees with the fossil record.

Complete hemocyanin subunit sequences of the hunting spider Cupiennius salei: recent hemocyanin remodeling in entelegyne spiders

Hemocyanins are large copper-containing respiratory proteins found in many arthropod species. Scorpions and orthognath spiders possess a highly conserved 4 x 6-mer hemocyanin that consists of at least seven distinct subunit types (termed a to g). However, many "modern" entelegyne spiders such as Cupiennius salei differ from the standard arachnid scheme and have 2 x 6-mer hemocyanins. Here we report the complete primary structure of the 2 x 6-mer hemocyanin of C. salei as deduced from cDNA sequencing, gel electrophoresis, and matrix-assisted laser desorption spectroscopy. Six distinct subunit types (1 through 6) and three additional allelic sequences were identified. Each 1 x 6-mer half-molecule most likely is composed of subunits 1-6, with subunit 1 linking the two hexamers via a disulfide bridge located in a C-terminal extension. The C. salei hemocyanin subunits all belong to the arachnid g-type, whereas the other six types (a-f) have been lost in evolution. The reconstruction of a complex hemocyanin from a single g-type subunit, which commenced about 190 million years ago and was completed about 90 million years ago, might be explained by physiological and behavioral changes that occurred during the evolution of the entelegyne spiders.