The complete amino acid sequence of the major ovine neurophysin (MSEL-neurophysin); comparison with a re-investigated bovine MSEL-neurophysin

Molecular evolution of the polypeptide hormones

Any biological function is at least bimolecular and its evolution therefore is at least dual, with variations in two lines of molecules. The hormone specificity results from a particular fit between the three-dimensional structure of the agent and that of the receptor but, because receptors are not known at the structural level, a discussion on the evolution of the polypeptide hormones is mainly limited to the possible progressive changes of the latter. As for other proteins (enzymes, oxygen carriers etc.) two degrees of complexity can be distinguished according to whether the hormone comprises one or several polypeptide chains. Protein assembly can bring new biological properties, each subunit playing a particular role. In this case, the 'internal' evolution (chain-chain interactions) overlaps the 'external' evolution (hormone-receptor contacts). The 'monomeric' hormones present the following problems: evolution of the prohormone and of the converting enzyme (for insulin), duplication and differentiation of two lines of hormones either by amino acid substitutions (neurohypophysial hormones and neurophysins) or by substitutions and size modifications (corticotropin and lipotropin), duplication and fusion leading to internal homology in the single polypeptide chain (somatotropin, prolactin, placental lactogen). The 'dimeric' hormones lead to several problems: successive duplications giving different subunits, selective associations between subunits, unequal rates of evolution of the subunits, the function of each subunit (lutropin, follitropin, thyrotropin, choriogonadotropin). An attempt is made to integrate the evolution of polypeptide hormones in the frame of the evolution of proteins.

Ostrich MSEL-neurophysin belongs to the class of two-domain "big" neurophysin as indicated by complete amino acid sequence of the neurophysin/copeptin

Mammalian neurohypophyseal hormones, oxytocin and vasopressin, are known to be synthesized as part of two larger precursors containing, respectively, a VLDV-neurophysin and a MSEL-neurophysin together with its associated glycopeptide. Starting from ostrich neurohypophyses, a "big" neurophysin was isolated and chemically characterized. Following sequence determination of the CNBr-derived fragments and of peptides obtained from trypsin and V8-protease digestion of the oxidized protein, this "big" neurophysin was found to contain an MSEL-neurophysin moiety (94 residues) still covalently associated with the COOH-terminal glycopeptide (38 residues, copeptin). This study demonstrates that the ostrich MSEL-neurophysin sequence closely resembles all known MSEL-neurophysin sequences and that, furthermore, it does not contain the single amino acid insertion shown previously in the ostrich VLDV-neurophysin. It is also shown that the stretch of amino acids, linking the MSEL-neurophysin and the copeptin, is clearly different from its mammalian homologues and lacks the Arg residue normally recognized by the cleaving enzyme. This study also demonstrates that the ostrich copeptin is more closely related to the amphibian copeptin sequence than to its mammalian homologue, leading to the hypothesis that two families of copeptin molecules might exist. Thus, the ostrich MSEL-neurophysin-copeptin molecule is the first "big" neurophysin reported in birds and, together with the guinea pig and amphibian homologues, represents the third example of partial or no neurophysin-copeptin cleavage.

Complete amino acid sequence of a VLDV-type neurophysin from ostrich differs markedly from known mammalian neurophysins

The neurohypophyseal hormones vasopressin and oxytocin are known to be synthesized in eutherian mammals as part of larger precursors containing either MSEL- or VLDV-neurophysins. A neurophysin has been isolated from ostrich neurohypophyses and shown by partial amino acid sequence determination to be related to mammalian VLDV-neurophysin. The present report describes the complete amino acid sequence of this ostrich neurophysin containing 93 residues. This amino acid sequence, the first reported in birds, differs in a remarkable manner from its mammalian homolog. Indeed, it contains a large number of substitutions, including one insertion, distributed throughout the polypeptide chain when compared to known VLDV-neurophysins. Whereas many of these substitutions are localized inside the so-called constant region of the neurophysin, the highest variation can be found in the COOH-terminal region.

Isolation and characterization of a neurophysin from ostrich neurohypophyses

A neurophysin has been isolated from ostrich neurohypophyses using acid acetone extraction, salt fractionation and Sephadex G-75 chromatography. The crude neurophysin eluting from the Sephadex G-75 column was subjected to a) reverse-phase HPLC followed by Sephadex G-75 chromatography, b) DEAE-Sephadex A-50 chromatography or c) isoelectric focusing. The different homogeneous ostrich neurophysin fractions so obtained were compared i.t.o. amino acid composition, spectral properties, N-terminal amino acid residues and PAGE. They all revealed a single N-terminal Ala residue and displayed spectral properties (A280/A260 less than 1) which are typical of mammalian neurophysin-like polypeptides. Ultracentrifugation studies on purified ostrich neurophysin over a range of concentrations revealed a reversible concentration dependent association behaviour characterized by the presence of dimeric complexes at higher concentrations. Partial sequencing from the N-terminus revealed the molecule to be VLDV-like. The purified molecule was also submitted to CNBr fragmentation.

mRNA sequence predictions from homologous protein sequences

A model has been developed that permits the prediction of mRNA nucleic acid sequence from the sequences of the translated proteins. The model relies on the information obtained from the comparison of protein sequences in related species to reduce the number of possible codons for those amino acids where mutations are observed. The predictions so obtained have been tested by applying the model to proteins whose mRNA sequences are known. The model's predictions have been found to be 100% accurate if three or more different amino acids are known at a given position and if the protein sequences are restricted to relatively closely related species (within the same class). The use of this model may permit a reduction of the mRNA sequence degeneracy and therefore be helpful in the synthesis of cDNA probes or for the prediction of restriction endonuclease sites. Computer programs have been developed to ease the use of the model.

Identification of human neurophysins: complete amino acid sequences of MSEL- and VLDV-neurophysins

Two human neurophysins have been purified from acetone-desiccated posterior pituitaries by acidic extraction, molecular sieving, and ion-exchange chromatography. The complete amino acid sequence of each protein has been determined by using a sequencer and characterizing two sets of overlapping enzymic peptides. The two neurophysins belong to two structural families previously defined as MSEL- and VLDV-neurophysins according to the nature of the residues in positions 2, 3, 6, and 7. (MSEL-neurophysins contain methionine-2, serine-3, glutamic acid-6, and leucine-7; VLDV-neurophysins contain valine-2, leucine-3, aspartic acid-6, and valine-7.) Human MSEL-neurophysin has only 93 residues instead of 95 usually found in MSEL-neurophysins from other mammalian species, probably because of a deletion of amino acids 91 and 92. Compared with bovine MSEL-neurophysin, nine variations (seven substitutions and two deletions) are observed. Human VLDV-neurophysin has 93 residues, as do the other mammalian VLDV-neurophysins. There are 11 substitutions when the comparison is made with bovine VLDV-neurophysin. Between the two human neurophysins, there are 26 variations. However, the central parts of the proteins (residues 10-70) are nearly identical. Furthermore, in this region identical substitutions are found in positions 29 and 60 of both neurophysins, suggesting either a single exon or some relationship between the two corresponding genes.

Specific cleavage of reduced and S-carboxamidomethylated neurophysin II by the collagenase of Clostridium histolyticum

Purified collagenase of Clostridium histolyticum was shown to cleave reduced and S-carboxamidomethylated bovine neurophysin between Cys-13 and Gly-14. The scission resulted in formation of two separable fragments: a smaller peptide arising from residues 1 through 13, and a larger peptide comprising the remainder of the residues of the protein. By dansylation procedures, the smaller peptide was shown to have amino-terminal alanine as expected from the sequence of neurophysin II, and the larger peptide had amino-terminal glycine as anticipated. These results show that collagenase indeed cleaves bovine neurophysin II in accord with the specificity postulated for that enzyme, i.e., scission between -X-Gly- in a sequence of -Pro-X-Gly-Pro-Y-. This result, obtained with a non-collagenous protein substrate, is further confirmation of the specificity of collagenase as established by its action on collagens and on synthetic oligopeptides.