Most of the >50,000 different pharmacologically active peptides in Conus venoms belong to a small number of gene superfamilies. In this work, the M-conotoxin superfamily is defined using both biochemical and molecular criteria. Novel excitatory peptides purified from the venoms of the molluscivorous species Conus textile and Conus marmoreus all have a characteristic pattern of Cys residues previously found in the mu-, kappaM-, and psi-conotoxins (CC-C-C-CC). The new peptides are smaller (12-19 amino acids) than the mu-, kappaM-, and psi-conotoxins (22-24 amino acids). One peptide, mr3a, was chemically synthesized in a biologically active form. Analysis of the disulfide bridges of a natural peptide tx3c from C. textile and synthetic peptide mr3a from C. marmoreus showed a novel pattern of disulfide connectivity, different from that previously established for the mu- and psi-conotoxins. Thus, these peptides belong to a new group of structurally and pharmacologically distinct conotoxins that are particularly prominent in the venoms of mollusc-hunting Conus species. Analysis of cDNA clones encoding the novel peptides as well as those encoding mu-, kappaM-, and psi-conotoxins revealed highly conserved amino acid residues in the precursor sequences; this conservation in both amino acid sequence and in the Cys pattern defines a gene superfamily, designated the M-conotoxin superfamily. The peptides characterized can be provisionally assigned to four distinct groups within the M-superfamily based on sequence similarity within and divergence between each group. A notable feature of the superfamily is that two distinct structural frameworks have been generated by changing the disulfide connectivity on an otherwise conserved Cys pattern.

In this report, we document for the first time the occurrence of D-tryptophan in a normally translated polypeptide, contryphan. The peptide, isolated from the venom of the fish-hunting marine snail Conus radiatus, produces the "stiff-tail" syndrome in mice. Characterization of the octapeptide gave the following sequence, The standard amino acids in polypeptides translated from genes are exclusively in the L-configuration. In recent years it has been established that D-amino acids can be post-translationally introduced into such polypeptides (1). Several small peptides have been characterized, which contain a D-amino acid. The first of these was dermorphin, a potent heptapeptide agonist of the -opiate receptor from amphibian skin, discovered by Erspamer and co-workers (2). A number of other peptides from amphibian skin (including the deltorphins and bombinin-H) were also found to have a D-amino acid. The cDNAs encoding these peptides were characterized (3, 4). The results demonstrated unequivocally the presence of mRNA encoding the peptide precursor, indicating that the D-amino acid was post-translationally formed from the corresponding L-isomer.In addition to these vertebrate systems, small peptides with D-amino acids have also been described in invertebrate systems, primarily molluscs. An FMRFamide analog from the bivalve, Mytilus edulis, which contains a D-leucine, has been characterized (5). Likewise, the land snail Achatina fulica has D-amino acid-containing small peptides, achatin-I and fulicin (6, 7). The cDNA encoding the precursor of fulicin was found to contain the usual L-Asn codon at the D-Asn position (8). Recently, the post-translational inversion of an amino acid was demonstrated in vitro for -agatoxin-IVB (also termed -agatoxin-TK), a calcium channel inhibitor from funnel web spider (9). The peptide isomerase that preferentially acts on Ser 46 of the 48-amino acid peptide has been isolated and characterized.The small peptides which appear to be post-translationally modified to convert an L-to a D-amino acid from a variety of phylogenetic systems are shown in Table I. Although there is no homology between vertebrate and invertebrate peptides (and the three molluscan peptides exhibit no sequence similarity), in every case the D-amino acid is found in the second position. This suggests that for small D-amino acid-containing peptides, the proteolytic event that generates the mature peptide and the post-translational enzymatic system that converts an L-to a D-amino acid work in combination to always generate the D-amino acid at position 2.In this report, we describe the purification and characterization of a novel D-amino acid-containing peptide from the venom of Conus radiatus, which causes a "stiff-tail syndrome" in mice. This octapeptide, contryphan, has a D-tryptophan residue. This is the first report of D-tryptophan being formed through posttranslational modification. Furthermore, in contrast to all of the small D-amino acid-containing peptides shown in Table I, contryphan does not have the D-amin...

A new class of Conus peptides, the I-superfamily of conotoxins, has been characterized using biochemical, electrophysiological and molecular genetic methods. Peptides in this superfamily have a novel pattern of eight Cys residues. Five peptides that elicited excitatory symptomatology, r11a, r11b, r11c, r11d and r11e, were purified from Conus radiatus venom; four were tested on amphibian peripheral axons and shown to elicit repetitive action potentials, consistent with being members of the 'lightning-strike cabal' of toxins that effect instant immobilization of fish prey. A parallel analysis of Conus cDNA clones revealed a new class of conotoxin genes that was particularly enriched (with 18 identified paralogues) in a Conus radiatus venom duct library; several C. radiatus clones encoded the excitatory peptides directly characterized from venom. The remarkable diversity of related I-superfamily peptides within a single Conus species is unprecedented. When combined with the excitatory effects observed on peripheral circuitry, this unexpected diversity suggests a corresponding molecular complexity of the targeted signaling components in peripheral axons; the I-conotoxin superfamily should provide a rich lode of pharmacological tools for dissecting and understanding these. Thus, the I-superfamily conotoxins promise to provide a significant new technology platform for dissecting the molecular components of axons.

The 500 different species of venomous cone snails (genus Conus) use small, highly structured peptides (conotoxins) for interacting with prey, predators, and competitors. These peptides are produced by translating mRNA from many genes belonging to only a few gene superfamilies. Each translation product is processed to yield a great diversity of different mature toxin peptides (Ϸ50,000 -100,000), most of which are 12-30 aa in length with two to three disulfide crosslinks. In vitro, forming the biologically relevant disulfide configuration is often problematic, suggesting that in vivo mechanisms for efficiently folding the diversity of conotoxins have been evolved by the cone snails. We demonstrate here that the correct folding of a Conus peptide is facilitated by a posttranslationally modified amino acid, ␥-carboxyglutamate. In addition, we show that multiple isoforms of protein disulfide isomerase are major soluble proteins in Conus venom duct extracts. The results provide evidence for the type of adaptations required before cone snails could systematically explore the specialized biochemical world of ''microproteins'' that other organisms have not been able to systematically access. Almost certainly, additional specialized adaptations for efficient microprotein folding are required.

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Contryphan-R is a disulfide-constrained octapeptide containing a D-tryptophan that was isolated recently from venom of the cone shell Conus radiatus. The polypeptide is present in two forms in solution due to cis-trans isomerization at hydroxyproline 3. The solution structure of the major form of this unusual polypeptide, determined from NMR data, consists of a well-defined fold containing a non-hydrogen-bonded chain reversal from Gly1 to Glu5, which includes a cis-hydroxyproline and a D-Trp, and a type I beta-turn from Glu5 to Cys8. The presence of a putative salt bridge between the Glu5 carboxyl group and the N-terminal ammonium group is investigated by using various solvation models during energy minimization and is compared with the results of a pH titration. A comparison of the structure of contryphan-R with other cyclic peptide structures highlights some of the key structural determinants of these peptides and suggests that the contryphan-R fold could be exploited as a scaffold onto which unrelated protein binding surfaces could be grafted. Comparison with small disulfide-bridged loops in larger proteins shows that contryphan-R is similar to a commonly occurring loop structure found in proteins.

We report a novel post-translational modification involving halogenation of tryptophan in peptides recovered from the venom of carnivorous marine cone snails (Conus). The residue, L-6-bromotryptophan, was identified in the sequence of a heptapeptide, isolated from Conus imperialis, a worm-hunting cone. This peptide does not elicit gross behavioral symptoms when injected centrally or peripherally in mice. L-6-Bromotryptophan was also identified in a 33-amino acid peptide from Conus radiatus; this peptide has been shown to induce a sleep-like state in mice of all ages and is referred to as bromosleeper peptide. The sequences of the two peptides Pca-Cys-Gly-Gln-Ala-Trp*-Cys-NH 2 were determined using a combination of mass spectrometry, amino acid, and chemical sequence analyses, where Pca ‫؍‬ pyroglutamic acid, Hyp ‫؍‬ hydroxyproline, Gla ‫؍‬ ␥-carboxyglutamate, and Trp* ‫؍‬ L-6-bromotryptophan. The precise structure and stereochemistry of the modified residue were determined as L-6-bromotryptophan by synthesis, co-elution, and enzymatic hydrolysis experiments. To our knowledge this is the first documentation of tryptophan residues in peptides/proteins being modified in a eukaryotic system and the first report of halogenation of tryptophan in vivo. SEQUENCE 1 andPolypeptides encoded by genes are primarily made up of the 20 common amino acids that are directly translated using the genetic code. However, many of these amino acids can be further modified post-translationally to yield a set of additional amino acids that contribute to the function of the mature protein. Together the 20 primary amino acids and these "secondary" amino acids which are found in proteins comprise the set of proteinogenous amino acids.These modified amino acids include amino acid conjugates where the side chain is linked to a glycosyl, phosphate, or sulfate group and amino acids such as 5-hydroxylysine, 4-hydroxyproline, and ␥-carboxyglutamate. One notable set of modified amino acids are halogenated derivatives of tyrosine and histidine. Naturally occurring halogenated tyrosine and histidine residues have previously been identified from proteins (1-4). A particularly well documented example of in vivo posttranslational halogenation of an amino acid residue in a protein is afforded by thyroglobulin (5). After iodination of several tyrosine residues, selective cleavages release the iodinated thyroid hormones thyroxine (3,5,3Ј,5Ј-tetraiodothyronine or T 4 ) 1 ; 3,5,3Ј-triiodothyronine (T 3 ), and 3,3Ј-di-iodothyronine. The presence of free T 3 and T 4 has also been shown in protochordates, suggesting a primitive thyroid function exists in tunicates (2).Of the 20 common amino acids, alanine, glycine, isoleucine, leucine, and valine, which lack side chain functional groups, have not been implicated in post-translational modifications (6). In 1907 it was proposed that hydroxytryptophan was a proteinogenous amino acid (7), but this proposal has not been verified (1), and chemical oxidation of tryptophan is likely to be responsible for the observed ...

Post‐translational isomerization of l‐amino acids to d‐amino acids is a subtle modification, not detectable by standard techniques such as Edman sequencing or MS. Accurate predictions require more sequences of modified polypeptides. A 46‐amino‐acid‐long conotoxin, r11a, belonging to the I‐superfamily was previously shown to have a d‐Phe residue at position 44. In this report, we characterize two related peptides, r11b and r11c, with d‐Phe and d‐Leu, respectively, at the homologous position. Electrophysiological tests show that all three peptides induce repetitive activity in frog motor nerve, and epimerization of the single amino acid at the third position from the C‐terminus attenuates the potency of r11a and r11b, but not that of r11c. Furthermore, r11c (but neither r11a nor r11b) also acts on skeletal muscle. We identified more cDNA clones encoding conopeptide precursors with Cys patterns similar to r11a/b/c. Although the predicted mature toxins have the same cysteine patterns, they belong to two different gene superfamilies. A potential correlation between the identity of the gene superfamily to which the I‐conotoxin belongs and the presence or absence of a d‐amino acid in the primary sequence is discussed. The great diversity of I‐conopeptide sequences provides a rare opportunity for defining parameters that may be important for this most stealthy of all post‐translational modifications. Our results indicate that neither the chemical nature of the side chain nor the precise vicinal sequence around the modified residue seem to be critical, but there may be favored loci for isomerization to a d‐amino acid.