Protamines of reptiles

Protamines and the sperm nuclear basic proteins Pandora's Box of insects

Insects are the largest group of animals when it comes to the number and diversity of species. Yet, with the exception of Drosophila, no information is currently available on the primary structure of their sperm nuclear basic proteins (SNBPs). This paper represents the first attempt in this regard and provides information about six species of Neoptera: Poecillimon thessalicus, Graptosaltria nigrofuscata, Apis mellifera, Nasonia vitripennis, Parachauliodes continentalis, and Tribolium castaneum. The SNBPs of these species were characterized by acetic acid urea gel electrophoresis (AU-PAGE) and high-performance liquid chromatography fractionated. Protein sequencing was obtained using a combination of mass spectrometry sequencing, Edman N-terminal degradation sequencing and genome mining. While the SNBPs of several of these species exhibit a canonical arginine-rich protamine nature, a few of them exhibit a protamine-like composition. They appear to be the products of extensive cleavage processing from a precursor protein which are sometimes further processed by other post-translational modifications that are likely involved in the chromatin transitions observed during spermiogenesis in these organisms.

Sperm nuclear protamines: A checkpoint to control sperm chromatin quality

Protamines are nuclear proteins which are specifically expressed in haploid male germ cells. Their replacement of histones and binding to DNA is followed by chromatin hypercondensation that protects DNA from negative influences by environmental factors. Mammalian sperm contain two types of protamines: PRM1 and PRM2. While the proportion of the two protamines is highly variable between different species, abnormal ratios within a species are known to be associated with male subfertility. Therefore, it is more than likely that correct protamine expression represents a kind of chromatin checkpoint during sperm development rendering protamines as suitable biomarkers for the estimation of sperm quality. This review presents an overview of our current knowledge on protamines comparing gene and protein structures between different mammalian species with particular consideration given to man, mouse and stallion. At last, recent insights into the possible role of inherited sperm histones for early embryo development are provided.

Sperm nuclear basic proteins of tunicates and the origin of protamines

Sperm nuclear basic proteins (SNBPs) are the chromosomal proteins that are found associated with DNA in sperm nuclei at the end of spermiogenesis. These highly specialized proteins can be classified into three major types: histone type (H-type), protamine-like type (PL-type), and protamine type (P-type). A hypothesis from early studies on the characterization of SNBPs proposed a mechanism for the vertical evolution of these proteins that involved an H1 → PL → P transition. However, the processes and mechanisms involved in such a transition were not understood. In particular, it was not clear how a molecular transition from a lysine-rich protein precursor (H1 histone) to the arginine-rich protamines might have taken place. In deuterostomes, the presence of SNBPs of the H-type in echinoderms and of protamines in the higher phylogenetic groups of vertebrates had long been known. The initial work on the characterization of tunicate SNBPs attempted to define the types and range of SNBPs that characterize this phylogenetically intermediate group. It was found that tunicate SNBPs belong to the PL-type. In this work we discuss how the study of SNBPs in the tunicates has been key to providing support to the H1 → PL → P transition. Most significantly, it was in tunicates that a potential molecular mechanism to explain the lysine-to-arginine transition was first reported.

Spermiogenic nuclear protein transitions and chromatin condensation. Proposal for an ancestral model of nuclear spermiogenesis

We have chosen three species (Sparus aurata, Dicentrarchus labrax, and Monodonta turbinata) that represent different transition patterns in the composition and structure of spermiogenic nuclei. The transition patterns of these species are representative of spermiogenesis in a large number of animal species. We analyze: (a) nuclear protein exchange; (b) chromatin condensation pattern; and (c) histone acetylation during spermiogenic development. In the simplest spermiogenesis histones and nucleosomes remain in mature sperm. Chromatin of spermatids is organized into 20 nm granules, simultaneous with a nuclear volume reduction. The granules coalesce in the final stage of spermiogenesis. Granular chromatin is correlated with acetylation of histones H3 and H4, whereas final coalescence is associated with histone deacetylation. We also studied two other spermiogenesis where a basic protein substitutes histones. Each species has a very different substituting protein. One has a typical protamine of 34 amino acids; the other has a sperm nuclear basic proteins (SNBP) of 106 amino acids. In both, the structural transitions and histone acetylation pattern are similar: in early spermiogenesis chromatin is organized into 20 nm granules, and histones are significantly acetylated, while the nuclear volume decreases. Subsequently, acetylated histones are displaced by the protamine or SNBP. Histone substitution causes chromatin remodelling and additional reduction in nuclear volume. We analyze these three cases together with earlier works and propose that the formation of 20 nm granules containing acetylated H3 and H4 accomplishes the minimum functional requirement to be considered the most evolutionarily ancestral chromatin conformation preceding condensation in animal spermiogenesis.

A unique vertebrate histone H1-related protamine-like protein results in an unusual sperm chromatin organization

Protamine-like proteins constitute a group of sperm nuclear basic proteins that have been shown to be related to somatic linker histones (histone H1 family). Like protamines, they usually replace the chromatin somatic histone complement during spermiogenesis; hence their name. Several of these proteins have been characterized to date in invertebrate organisms, but information about their occurrence and characterization in vertebrates is still lacking. In this sense, the genus Mullus is unique, as it is the only known vertebrate that has its sperm chromatin organized by virtually only protamine-like proteins. We show that the sperm chromatin of this organism is organized by two type I protamine-like proteins (PL-I), and we characterize the major protamine-like component of the fish Mullus surmuletus (striped red mullet). The native chromatin structure resulting from the association of these proteins with DNA was studied by micrococcal nuclease digestion as well as electron microscopy and X-ray diffraction. It is shown that the PL-I proteins organize chromatin in parallel DNA bundles of different thickness in a quite distinct arrangement that is reminiscent of the chromatin organization of those organisms that contain protamines (but not histones) in their sperm.

Sperm nuclear basic proteins of two closely related species of Scorpaeniform fish (Sebastes maliger, Sebastolobus sp.) with different sexual reproduction and the evolution of fish protamines

In this paper, we present a review of sperm nuclear basic proteins (SNBPs) in teleost fish. The distribution of the three basic groups of SNBPs [histone (H)-type, protamine-like (PL)-type and protamine (P)-type], their evolution and possible relation to the mode of fertilization are described. In this regard, we have characterized the SNBPs from two closely related species of Scorpaeniform fish: internally fertilizing Sebastes maliger and externally fertilizing Sebastolobus sp., both in the family Scorpaenidae. Despite the different reproductive behavior of these two closely related rockfish species, in both instances the SNBP consists of protamines. However, there is a significant increase in the arginine content of the protamine in the internally fertilizing rockfish. The relevance of this observation is discussed within the context of the P-type SNBP in teleosts. The rapid evolution of teleost protamines, including those in rockfish, has also allowed us to obtain a molecular phylogeny for this group of bony fish that is almost indistinguishable from that currently available from the use of conventional anatomical/paleontological markers.

Protamines in the internally fertilizing neobatrachian frogEleutherodactylus coqui

The internally fertilizing primitive frog Ascaphus truei (family Ascaphidae) from the Pacific Northwest is the only frog with an intromittent organ. The more advanced neobatrachian frog Eleutherodactylus coqui (family Leptodactylidae) from Puerto Rico has secondarily acquired internal fertilization but mates by cloacal apposition. Nonetheless, both frogs have introsperm with an elongated head containing highly condensed chromatin. Characterization of sperm nuclear basic proteins (SNBPs) in E. coqui by acid-urea polyacrylamide gel electrophoresis indicates that, as in A. truei, testes from a single animal contain several protamines. Amino acid analysis indicates a composition for the most rapidly moving protamine of each species as follows: in E. coqui, ARG (35.6 mol %) + LYS (3.8 mol %) + HIS (7.6 mol %) = 47 mol % total basic residues and in A. truei, ARG (42.1 mol %) + LYS (11.1 mol %) = 53.2 mol % total basic residues. Transmission electron microscopy shows that E. coqui introsperm, like those in A. truei, are elongate with highly condensed chromatin. However, E. coqui introsperm lacks an axial perforatorium that extends into an endonuclear canal. These morphological features are plesiomorphic (primitive) and shared by A. truei with urodeles and basal amniotes (Jamieson et al. (1993) Herpetologica 49:52-65). In E. coqui introsperm, the nucleoprotein complex has a cross-sectional axis of 420 + 20 angstroms and shows a knobby chromatin structural organization in TEM. The presence of arginine-enriched protamines in both a basal anuran like the ascaphid A. truei and a more advanced neobatrachian like the leptodactylid E. coqui supports the hypothesis that internal fertilization acts as a constraint on the range of SNBP diversity in animals.

A walk though vertebrate and invertebrate protamines

An updated comparative analysis of protamines and their corresponding genes is presented, including representative organisms from each of the vertebrate classes and one invertebrate (squid, Loligo opalescens). Special emphasis is placed on the implications for sperm chromatin organization and the evolutionary significance. The review is based on some of the most recent publications in the field and builds upon previously published reviews on this topic.

Chromatin condensation, cysteine-rich protamine, and establishment of disulphide interprotamine bonds during spermiogenesis of Eledone cirrhosa (Cephalopoda)

During spermiogenesis in Eledone cirrhosa a single protamine substitutes for histones in nuclei of developing spermatids. This protein displays a peculiar primary structure. It contains 22.6 mol% cysteine residues (19 cysteines in 84 residues). This makes it the most cysteine-rich protamine known. The proportion of basic residues is relatively low (arginine 36.9 mol%, lysine 19.0 mol%). The protamine of E. cirrhosa condenses spermiogenic chromatin in a pattern which comprises fibres with a progressively larger diameter and lamellae that finally undergo definitive coalescence. We have also performed a study that estimates the number of interprotamine disulphide bonds formed during the process of spermiogenic chromatin condensation by means of sequential disappearance of MMNA (monomaleimido-nanogold) labelling. During the first step of spermiogenesis, protamines are found spread over very slightly condensed chromatin with their cysteines in a reactive state (protamine-cys-SH). From this stage the interprotamine disulphide bonds are established in a progressive way. First they are formed inside the chromatin fibres. Subsequently, they participate in the mechanism of fibre coalescence and finally, in the last step of spermiogenesis, the remaining free reactive -SH groups of cysteine form disulphide bonds, thus promoting a definitive stabilization of the nucleoprotein complex in the ripe sperm nucleus.

Characterization and evolutionary relevance of the sperm nuclear basic proteins from stickleback fish

We have characterized the sperm nuclear basic proteins (SNBPs) of the sticklebacks in the suborder Gasterosteoidei. The complete amino acid sequence of the protamines from Aulorhynchus flavidus, Pungitius pungitius, Gasterosteus aculeatus, (anadromous) and G. wheatlandi, as well as the sequences of the protamines of several species pairs of freshwater G. aculeatus, have been determined. Analysis of the primary structure of these proteins has shown that: a) despite the relatively low amino acid complexity and small molecular mass of these basic proteins, they are very good molecular markers at the generic level. The bootstrap parsimony analysis using their sequences provides a phylogenetic relationship for the old anadromous species of Gasterosteoidei which is identical to that obtained from morphological and behavioral analysis; b) the comparison of the sequences also suggests that protamines from the suborder Gasterosteoidei have most likely evolved from a common gene in the early Acanthopterygii by an extension of the carboxy terminal portion of the molecule; c) protamines are not good markers for recent postglacial freshwater isolates of G. aculeatus. However, in the unique case of Enos Lake (British Columbia), we have been able to detect an additional minor protamine component in the benthic forms of G. aculeatus that is not present in the limnetic forms. Thus, this new protamine must have appeared during the past 12,000 years concomitantly with the speciation of benthics and limnetics in this lake.

Cysteine-containing histone H1-like (PL-I) proteins of sperm

We have determined the presence of cysteine in the protein PL-I from the sperm of the surf clam Spisula solidissima. The existence of cysteine in this histone H1-related protein is responsible for its previously described aggregation behavior. The location of this residue, within the trypsin-resistant domain of the protein, has been established. We have also shown that cysteine is ubiquitously present in the PL-I proteins from the sperm of other bivalve mollusks but is absent from other PL of smaller molecular mass (PL-II, PL-III, PL-IV). We have also found cysteine to be present in the PL-I from a tunicate (Chelysoma productum) but absent in a PL-I from a fish (Mullus barbatus). The possible significance of the unusual occurrence of cysteine in these histone-H1-related proteins is discussed.

Reconstitution of native-like nucleosome core particles from reversed-phase-HPLC-fractionated histones

We have reconstituted nucleosome core particles from reversed-phase-HPLC-purified chicken erythrocyte core histones and 145 bp random-sequence DNA fragments. Characterization of the resulting nucleoprotein complexes by sedimentation velocity, CD and DNase I footprinting showed that they are structurally indistinguishable from native nucleosome core particles. Furthermore, we have shown that the ability to reproduce these native-like structural features in these reconstituted nucleosome core particles is basically independent of the biological source or the method used (i.e. salt versus acid) for the extraction of histones before their HPLC fractionation. The usefulness and relevance of this approach for the reconstitution of native-like chromatin structures from histone types (histone variants/post-translationally modified histones), which are usually available only in relatively small amounts, is discussed.

The sperm nuclear basic proteins (SNBPs) of the sponge Neofibularia nolitangere: implications for the molecular evolution of SNBPs

We have isolated and characterized for the first time, the SNBPs from an organism (Neofibularia nolitangere) of the phylum Porifera (Sponges). We have shown that these proteins consist of histones which, as expected, exhibit an amino acid composition very similar to that of other eukaryotic histones. The finding of histones in the sperm of these primitive organisms provides support to the notion that histones (SNBPs of the histone, H, type) were the proteins present at the onset of SNBP evolution. In contrast, a discrete number of alternative SNBP types (protamine-like, PL; protamine, P, types) seem to have appeared later on in the course of evolution and are found in both protostomes and deuterostomes, most likely as a result of processes of parallel evolution.