Nuclear basic proteins in spermiogenesis.

In animal species, spermiogenesis, the late stage of spermatogenesis, is characterized by a dramatic remodelling of chromatin which involves morphological changes and various modifications in the nature of the nuclear basic proteins. According to the evolution of species, three situations can be observed: a) persistence of somatic histones or appearance of sperm-specific histones; b) direct replacement of histones by generally smaller and more basic proteins called protamines; and c) occurrence of a double nuclear basic protein transition: histones are not directly replaced by protamines but by intermediate basic proteins which are themselves replaced by one or several protamines. However, in some species, two kinds of intermediate basic proteins can be distinguished in spermatid nuclei: transition proteins and protamine precursors. Whereas transition proteins are not structurally related either to histones or to protamines, protamine precursors are further processed at the end of spermiogenesis to give rise to the mature protamine. The molecular characteristics of the protamines as well as number of protamine types present in the spermatozoon vary from species to species. In some cases, protamine-encoding genes, although present, are not expressed to a significant level. The diversity and the precise function of intermediate basic proteins remain open to discussion. Some of them are the precursors of protamines but the mechanism, sequential or not, as well as the enzyme(s) involved in the proteolytic processing, remain to be discovered.

Molecular cloning and expression of a metallothionein mRNA in Xenopus laevis.

Metallothioneins (MT) are expressed during early development in species of several groups. To understand MT function in developmental processes, we studied the MT system of Xenopus laevis, a model vertebrate species in experimental embryology. We first purified and sequenced the liver MT from copper-treated animals. This 62-amino-acid protein shares the main structural properties of known vertebrate MT, and is more closely related to avian than to fish or mammalian MT. Using this sequence, we designed oligonucleotide primers to amplify and isolate a MT clone from a XL2 cell line cDNA library. This 752-bp cDNA encodes a putative 62-amino-acid-long protein that is 100% identical with the sequenced MT. Zinc, cadmium, and copper ions are very efficient inducers of MT mRNA accumulation in Xenopus liver and cell lines.

Nuclear protein transitions in cuttle-fish spermiogenesis: immunocytochemical localization of a protein specific for the spermatid stage.

The changes in basic nuclear proteins throughout cuttle-fish spermiogenesis were investigated both by immunocytochemical procedures and by isolation of late spermatid nuclei (by virtue of their resistance to sonication). Antibodies were raised in rabbits to a protein, named protein T, isolated from testis chromatin. The anti-protein T immune serum was found to recognize protein T and not histones from the testis. Immunoperoxidase staining of sections or of smears of testis with anti-protein T antibodies showed that protein T appears in the nuclei of round spermatids, is abundant in elongating spermatid nuclei, but cannot be detected in elongated spermatids. Nuclei from these elongated spermatids were isolated by sonication treatment of testis cells. A protein, named protein Sp, with the characteristic mobility of a protamine, was isolated from elongated spermatid nuclei. This protein has the same mobility as the protamine present in mature spermatozoa. Taken together, the results indicate that in cuttle-fish, nuclear protein transitions involve the replacement of histones by a spermatid-specific protein (protein T), which is replaced at the end of elongation of the nucleus by a protamine (protein Sp). Thus, spermiogenesis of the cuttle-fish (and perhaps of other cephalopods), shows two basic nuclear protein transitions, which are similar to the transitions observed in higher vertebrates such as mammals.

Isolation and characterization of a small putative zinc finger protein from cuttlefish epididymal sperm cells.

At the end of spermiogenesis, sperm chromatin stabilization is ensured by protamine dephosphorylation and, in mammals, by the formation during epididymal transit, of intra- and inter-molecular disulfide bridges between protamines. In cuttlefish, the nuclear protein transition histones-->spermatid-specific protein T-->protamine Sp is very similar to that occurring in mammals during spermiogenesis. However, in cuttlefish, the protamine Sp is devoid of cysteine residues. The protein complement of cuttlefish epididymal sperm nuclei has been investigated. A minor basic protein, called protein E, has been isolated. Its primary structure was established from sequence analysis and mass spectrometry data of the protein and its fragments. Protein E contains a motif -Cys-Xaa2-Cys-Xaa23-His-Cys-Xaa2-Cys- which is likely to adopt a zinc finger conformation. Reduced protein E does fix zinc whereas alkylation of cysteine residues abolishes this ability. The sequence of protein E does not correspond to that of any known protein, but presents some similarities with a part of ZFY protein, a putative human transcription factor specifically expressed in germinal cells and which could be involved in spermatogenesis.

Primary structure of a myohemerythrin-like cadmium-binding protein, isolated from a terrestrial annelid oligochaete.

Two isoforms of a cadmium-binding protein (Cd-BP 14a and Cd-BP 14b) were isolated from the terrestrial oligochaete annelid, Allolobophora caliginosa. The complete amino acid sequence of the major isoform Cd-BP 14a (molecular mass: 13441 Da; 119 residues) and the amino-terminal sequence (57 residues) of Cd-BP 14b were determined. The sequence of Cd-BP 14a is highly similar to that of myohemerythrins present in marine invertebrates. Furthermore, as myohemerythrins, Cd-BP 14a and Cd-BP 14b bind two atoms of iron and their ultraviolet/visible spectra are typical of non-heme iron-binding proteins. Three substitutions were found in the amino-terminal half of the proteins at positions 19, 21 and 41. The substitutions at positions 19 and 21 are conservative, whereas that at position 41 consists of the replacement of an aspartate residue in isoform a by a lysine residue in isoform b. To our knowledge, it is the first report of a protein belonging to the hemerythrin family in a terrestrial invertebrate.

The amino-acid sequence of histone H2A from cuttlefish Sepia officinalis.

The amino acid sequence of cuttlefish testis histone H2A (124 residues) was established from structural data obtained by automated sequencing of large peptides generated by the cleavage of the protein with V8 staphylococcal protease or by limited chymotryptic hydrolysis. Compared to the calf thymus homologous histone, cuttlefish H2A shows 14 substitutions (most of them conservative) and 5 deletions. Extensive evolutionary changes were mainly observed in the basic amino-terminal and carboxy-terminal regions of the molecule, which are the primary DNA-binding sites. Few punctual changes are observed in the central region (residues 18-118), which interacts strongly with histone H2B to form the dimer H2A-H2B.

Cuttlefish spermatid-specific protein T. Molecular characterization of two variants T1 and T2, putative precursors of sperm protamine variants Sp1 and Sp2.

In cuttlefish, as in selachians and mammals, spermiogenesis is characterized by the double nuclear protein transition histones----intermediate protein (protein T)----protamine (protein Sp). The cuttlefish protein T, which consists of two structural variants phosphorylated at different degrees, is the first invertebrate spermatid-specific protein to be fully characterized and sequenced. The primary structures of these two variants were established from sequence analysis and mass spectrometric data of the proteins and their fragments. T1 and T2 are two highly related proteins of 78 and 77 residues, respectively, which differ only by four conservative substitutions, two inversions Ser in equilibrium with Arg, and the deletion of 1 residue of arginine in variant T2. The asymmetrical distribution of the hydrophobic and basic residues determines two well defined domains: an amino-terminal domain (residues 1-21) devoid of arginine and aromatic residues and containing all the aliphatic hydrophobic residues and a highly basic carboxyl-terminal domain (residues 22-77 or 78) that contains 77% of arginine, all the tyrosine residues, and most of the phosphorylated serine residues present in the protein. The complete structural identity of the basic carboxyl-terminal domain of spermatidal proteins T1 and T2 with the protamine variants Sp1 and Sp2 isolated from cuttlefish spermatozoa strongly suggests that T1 and T2 could be precursors of Sp1 and Sp2, respectively.

Cuttlefish sperm protamines. 2. Mass spectrometry of protamines and related peptides.

The sequence of very basic proteins such as protamines (more than 50% arginines) and related peptides has been determined using mass spectrometry in conjunction with Edman degradation. The capabilities of three mass spectrometric (MS) techniques [fast-atom-bombardment (FAB), 252Cf plasma desorption (252CFPD) and electrospray (ES)] have been evaluated on stallion protamine 1, cuttlefish protamine, and the corresponding cleavage peptides. In contrast to FAB-MS and 252Cf PD-MS, ES-MS made possible an easy determination of the molecular mass of the intact protamines (approximately 8 kDa). With ES-MS about 0.2 nmol was sufficient to yield a mass measurement with an accuracy of 0.05%. On peptides smaller than 3500 Da, both FAB-MS and 252Cf PD-MS allowed mass measurements with an accuracy of 0.1%. 252Cf PD-MS appeared more sensitive than FAB-MS by about a factor of 10. FAB-MS is nevertheless particularly interesting since in most cases it produced spectra with intense A-type fragmentation ions which provided reliable primary structure information.

Cuttlefish sperm protamines. 1. Amino acid sequences of two distinct variants.

The amino acid sequences of two cuttlefish protamine variants Sp1 and Sp2 have been established from automated sequence analysis and mass spectrometry data. Sp1 (57 residues) and Sp2 (56 residues) have molecular masses of 8410 and 8253 Da, respectively. They are almost identical proteins which differ only by one residue of arginine and the position of two of the serine residues (14 and 37 in Sp1; 13 and 35 in Sp2). With an arginine content of about 77%, cuttlefish protamine is one of the most basic proteins which have ever been characterized and the first typical protamine sequenced in invertebrates. It is closely similar to sperm basic proteins identified in squids but strongly differs from the protamine-like components isolated from the sperm of bivalve molluscs.

Squid spermiogenesis: molecular characterization of testis-specific pro-protamines.

Cuttlefish spermiogenesis is characterized by a two-step nuclear protein transition: histones-->spermatid-specific protein (protein T)-->sperm protamine (protein Sp). A similar situation can be observed in another Cephalopod species, the squid Loligo pealeii. The protein T from Loligo consists of two structural variants, T1 and T2 (molecular masses: 10788 and 10791 Da respectively), phosphorylated to different degrees (2-6 phosphate groups). The primary structures of these two variants and of the protamine variant Sp2 were established from sequence analysis and mass spectrometric data of the proteins and their fragments. T1 and T2 are closely related proteins of 79 residues. The complete structural identity of the C-terminal domain (residues 22-79) of protein T2 with the sperm protamine Sp2 (molecular mass 8562 Da, 58 residues) strongly suggests that the testis-specific protein T2 is indeed the precursor of the protamine. The transition between the precursor protein T and protein Sp results from a hydrolytic cleavage similar to that found in many proteins that are synthesized as precursors. The processing mechanism involves the specific cleavage of a Gly-Arg bond in the sequence Met/Leu18-Lys-Gly-Gly-Arg-Arg23. Furthermore, the study provides molecular evidence on the taxonomic relationship between Loligo and Sepia.

DNA-interacting proteins in the spermiogenesis of the mollusc Murex brandaris.

Sperm chromatin of Murex brandaris (a neogastropod mollusc) undergoes a series of structural transitions during spermiogenesis. The DNA-interacting proteins responsible for these changes as well as the mature protamines present in the ripe sperm nucleus have been characterized. The results reveal that spermiogenic nuclear proteins are protamine precursors that are subjected to a substantial number of small N-terminal deletions that gradually modify their overall charge. The composition of mature protamines is remarkably simple in turn, promoting an efficient and extremely tight packaging of DNA. The pattern of spermiogenic chromatin condensation in M. brandaris clearly departs from that corresponding to vertebrate chromatin.