Identification of two novel zinc finger modules and nuclear localization signal in rat spermatidal protein TP2 by site-directed mutagenesis

Ursonic acid attenuates spermatogenesis in oligozoospermia mice through inhibiting ferroptosis

Ursonic acid (UNA) is a natural pentacyclic triterpene found in some medicinal plants and foods. The reproductive protective effect of UNA was evaluated in a mouse model of oligozoospermia induced by busulfan (BUS) at 30 mg/kg b.w.. The mice were initially divided into groups with UNA concentrations of 10, 30, 50, 100 mg/kg. Subsequently, based on sperm parameters, the optimal concentration of 50 mg/kg was identified. As a control, an additional group was supplemented with ursolic acid at a concentration of 50 mg/kg. The results indicated that BUS caused the loss of spermatogenic cells in testis, the decrease of sperm in epididymis, the disorder of testicular cytoskeleton, the decrease of serum sex hormones such as testosterone which induced an increase in feedback of androgen receptor and other testosterone-related proteins, the increase of malondialdehyde and reactive oxygen species levels and the increase of ferroptosis in testis while UNA successfully reversed these injuries. High-throughput sequencing revealed that UNA administration significantly upregulated the expression of genes associated with spermatogenesis, such as Tnp1, Tnp2, Prm1, among others. These proteins are crucial in the histone to protamine transition during sperm chromatin remodeling. Network pharmacology analysis reveals a close association between UNA and proteins related to the transformation of histones to protamine. Molecular docking studies reveal that UNA can interact with the ferroptosis-inhibiting gene SLC7A11, thereby modulating ferroptosis. Taken together, UNA alleviated BUS-induced oligozoospermia by regulating hormone secretion, mitigating oxidative stress and promoting recovery of spermatogenesis by inhibiting the ferroptosis.

Citrylglutamate synthase deficient male mice are subfertile with impaired histone and transition protein 2 removal in late spermatids

Chromatin remodelling in spermatids is an essential step in spermiogenesis and involves the exchange of most histones by protamines, which drives chromatin condensation in late spermatids. The gene Rimklb encodes a citrylglutamate synthase highly expressed in testes of vertebrates and the increase of its reaction product, β-citrylglutamate, correlates in time with the appearance of spermatids. Here we show that deficiency in a functional Rimklb gene leads to male subfertility, which could be partially rescued by in vitro fertilization. Rimklb-deficient mice are impaired in a late step of spermiogenesis and produce spermatozoa with abnormally shaped heads and nuclei. Sperm chromatin in Rimklb-deficient mice was less condensed and showed impaired histone to protamine exchange and retained transition protein 2. These observations suggest that citrylglutamate synthase, probably via its reaction product β-citrylglutamate, is essential for efficient chromatin remodelling during spermiogenesis and may be a possible candidate gene for male subfertility or infertility in humans.

Paternal epigenetics: Mammalian sperm provide much more than DNA at fertilization

The spermatozoon is a highly differentiated cell with unique characteristics: it is mobile, thanks to its flagellum, and is very compact. The sperm cytoplasm is extremely reduced, containing no ribosomes, and therefore does not allow translation, and its nucleus contains very closed chromatin, preventing transcription. This DNA compaction is linked to the loss of nucleosomes and the replacement of histones by protamines. Based on these characteristics, sperm was considered to simply deliver paternal DNA to the oocyte. However, some parts of the sperm DNA remain organized in a nucleosomal format, and bear epigenetic information. In addition, the nucleus and the cytoplasm contain a multitude of RNAs of different types, including non-coding RNAs (ncRNAs) which also carry epigenetic information. For a long time, these RNAs were considered residues of spermatogenesis. After briefly describing the mechanisms of compaction of sperm DNA, we focus this review on the origin and function of the different ncRNAs. We present studies demonstrating the importance of these RNAs in embryonic development and transgenerational adaptation to stress. We also look at other epigenetic marks, such as DNA methylation or post-translational modifications of histones, and show that they are sensitive to environmental stress and transmissible to offspring. The post-fertilization role of certain sperm-borne proteins is also discussed.

Fluoride-induced unrestored arrest during haploid period of spermatogenesis via the regulation of DDX25 in rats

The effect of fluoride as an ongoing topic has attracted much attentions due to the decline in overall human fertility worldwide. However, whether fluorine causes a temporary stimulus or permanent damage to the male reproductive system, as well as the mechanism of fluoride influencing spermatogenesis remained unclear. 48 adult male rats were randomly divided into four groups (twelve each). Control group received the distilled water, while the other three groups were treated with 25, 50, 100 mg/L NaF via drinking water for 8 weeks. Six rats from each group were selected randomly to detect the levels of various indices related to spermatogenesis. The remaining rats were given only distilled water and left for recovery of a period of 2 weeks. Results showed that the levels of serum CK, ALP, CHE, BUN, UA, and Cr, testis morphology and the ultrastructure of sperm acrosome and chromatoid body (CB) were significantly changed by fluoride. Interestingly, the elongated spermatid counts, spermatids elongation ratio, and mRNA expressions of Prm1/2 and MIWI, TDRD1, TDRD 6, TDRD7, PABP, and Hsp72 related to CB decreased markedly in fluoride treatment groups compared to the control. Furthermore, the expression levels of DDX25 and associated regulatory proteins like CRM1, HMG2, H4, TP2, and PGK2 were down-regulated by fluoride. After 2-weeks withdrawal period, out of the 19 altered spermatogenesis indicators, 15 indicators in 100 mg/L group and 3 indicators in 50 mg/L group still exhibited a significant change, while none showed change in 25 mg/L group. These results proved that the reversibility of fluoride toxicity is dose-dependent on the male reproductive system. Meanwhile, fluoride caused unrestored arrest during the haploid period of spermatogenesis, where reduced DDX25 and associated regulatory proteins play a crucial role in this process, which could provide the underlying insights to the toxic mechanism of fluoride induced male reproductive toxicity.

Self-Healing Polymeric Hydrogel Formed by Metal-Ligand Coordination Assembly: Design, Fabrication, and Biomedical Applications

Self-healing hydrogels based on metal-ligand coordination chemistry provide new and exciting properties that improve injectability, rheological behaviors, and even biological functionalities. The inherent reversibility of coordination bonds improves on the covalent cross-linking employed previously, allowing for the preparation of completely self-healing hydrogels. In this article, recent advances in the development of this class of hydrogels are summarized and their applications in biology and medicine are discussed. Various chelating ligands such as bisphosphonate, catechol, histidine, thiolate, carboxylate, pyridines (including bipyridine and terpyridine), and iminodiacetate conjugated onto polymeric backbones, as well as the chelated metal ions and metal ions containing inorganic particles, which are used to form dynamic networks, are highlighted. This article provides general ideas and methods for the design of self-healing hydrogel biomaterials based on coordination chemistry.

Mapping of Post-translational Modifications of Transition Proteins, TP1 and TP2, and Identification of Protein Arginine Methyltransferase 4 and Lysine Methyltransferase 7 as Methyltransferase for TP2

In a unique global chromatin remodeling process during mammalian spermiogenesis, 90% of the nucleosomal histones are replaced by testis-specific transition proteins, TP1, TP2, and TP4. These proteins are further substituted by sperm-specific protamines, P1 and P2, to form a highly condensed sperm chromatin. In spermatozoa, a small proportion of chromatin, which ranges from 1 to 10% in mammals, retains the nucleosomal architecture and is implicated to play a role in transgenerational inheritance. However, there is still no mechanistic understanding of the interaction of chromatin machinery with histones and transition proteins, which facilitate this selective histone replacement from chromatin. Here, we report the identification of 16 and 19 novel post-translational modifications on rat endogenous transition proteins, TP1 and TP2, respectively, by mass spectrometry. By in vitro assays and mutational analysis, we demonstrate that protein arginine methyltransferase PRMT4 (CARM1) methylates TP2 at Arg(71), Arg(75), and Arg(92) residues, and lysine methyltransferase KMT7 (Set9) methylates TP2 at Lys(88) and Lys(91) residues. Further studies with modification-specific antibodies that recognize TP2K88me1 and TP2R92me1 modifications showed that they appear in elongating to condensing spermatids and predominantly associated with the chromatin-bound TP2. This work establishes the repertoire of post-translational modifications that occur on TP1 and TP2, which may play a significant role in various chromatin-templated events during spermiogenesis and in the establishment of the sperm epigenome.

Association of TNP2 gene polymorphisms of the bta-miR-154 target site with the semen quality traits of Chinese Holstein bulls

Transition protein 2 (TNP2) participates in removing nucleohistones and the initial condensation of spermatid nucleus during spermiogenesis. This study investigated the relationship between the variants of the bovine TNP2 gene and the semen quality traits of Chinese Holstein bulls. We detected three single nucleotide polymorphisms (SNPs) of the TNP2 gene in 392 Chinese Holstein bulls, namely, g.269 G>A (exon 1), g.480 C>T (intron 1), and g.1536 C>T (3′-UTR). Association analysis showed that the semen quality traits of the Chinese Holstein bulls was significantly affected by the three SNPs. The bulls with the haplotypic combinations H6H4, H6H6, and H6H8 had higher initial semen motility than those with the H7H8 and H8H4 haplotypic combinations (P<0.05). SNPs in the microRNA (miRNA) binding region of the TNP2 gene 3′-UTR may have contributed to the phenotypic differences. The phenotypic differences are caused by the altered expression of the miRNAs and their targets. Bioinformatics analysis predicted that the g.1536 C>T site in the TNP2 3′-UTR is located in the bta-miR-154 binding region. The quantitative real-time polymerase chain reaction results showed that the TNP2 mRNA relative expression in bulls with the CT and CC genotypes was significantly higher than those with the TT genotype (P<0.05) in the g.1536 C>T site. The luciferase assay also indicated that bta-miR-154 directly targets TNP2 in a murine Leydig cell tumor cell line. The SNP g.1536 C>T in the TNP2 3′-UTR, which altered the binding of TNP2 with bta-miR-154, was found to be associated with the semen quality traits of Chinese Holstein bulls.

Acetylation of transition protein 2 (TP2) by KAT3B (p300) alters its DNA condensation property and interaction with putative histone chaperone NPM3

The hallmark of mammalian spermiogenesis is the dramatic chromatin remodeling process wherein the nucleosomal histones are replaced by the transition proteins TP1, TP2, and TP4. Subsequently these transition proteins are replaced by the protamines P1 and P2. Hyperacetylation of histone H4 is linked to their replacement by transition proteins. Here we report that TP2 is acetylated in vivo as detected by anti-acetylated lysine antibody and mass spectrometric analysis. Further, recombinant TP2 is acetylated in vitro by acetyltransferase KAT3B (p300) more efficiently than by KAT2B (PCAF). In vivo p300 was demonstrated to acetylate TP2. p300 acetylates TP2 in its C-terminal domain, which is highly basic in nature and possesses chromatin-condensing properties. Mass spectrometric analysis showed that p300 acetylates four lysine residues in the C-terminal domain of TP2. Acetylation of TP2 by p300 leads to significant reduction in its DNA condensation property as studied by circular dichroism and atomic force microscopy analysis. TP2 also interacts with a putative histone chaperone, NPM3, wherein expression is elevated in haploid spermatids. Interestingly, acetylation of TP2 impedes its interaction with NPM3. Thus, acetylation of TP2 adds a new dimension to its role in the dynamic reorganization of chromatin during mammalian spermiogenesis.

Spatiotemporal organization of AT- and GC-rich DNA and their association with transition proteins TP1 and TP2 in rat condensing spermatids

Transition protein 1 (TP1) and TP2 replace histones during midspermiogenesis (stages 12-15) and are finally replaced by protamines. TPs play a predominant role in DNA condensation and chromatin remodeling during mammalian spermiogenesis. TP2 is a zinc metalloprotein with two novel zinc finger modules that condenses DNA in vitro in a GC-preference manner. TP2 also localizes to the nucleolus in transfected HeLa and Cos-7 cells, suggesting a GC-rich preference, even in vivo. We have now studied the localization pattern of TP2 in the rat spermatid nucleus. Colocalization studies using GC-selective DNA-binding dyes chromomycin A3 and 7-amino actinomycin D and an AT-selective dye, 4',6-diamidino-2-phenylindole, indicate that TP2 is preferentially localized to GC-rich sequences. Interestingly, as spermatids mature, TP2 and GC-rich DNA moves toward the nuclear periphery, and in the late stages of spermatid maturation, TP2 is predominantly localized at the nuclear periphery. Another interesting observation is the mutually exclusive localization of GC- and AT-rich DNA in the elongating and elongated spermatids. A combined immunofluorescence experiment with anti-TP2 and anti-TP1 antibodies revealed several foci of overlapping localization, indicating that TP1 and TP2 may have concerted functional roles during chromatin remodeling in mammalian spermiogenesis.

Quantitative Assessment of Transition Proteins 1, 2 Spermatid-Specific Linker Histone H1-Like Protein Transcripts in Spermatozoa from Normozoospermic and Asthenozoospermic Men

Spermatid-specific linker histone H1-like protein (HILS1), transition proteins 1 and 2 (TNP1 and TNP2), and protamines 1 and 2 (PRM1 and PRM2) contribute to considerable dense packing of spermatid chromatin during spermiogenesis. We evaluated the HILS1, TNP1, and TNP2 transcript levels in spermatozoa isolated from normozoospermic and asthenozoospermic men. Human ejaculates from normozoospermic (n = 70) and asthenozoospermic (n = 100) donors were purified by centrifugation through a discontinuous Percoll density gradient. RNA was isolated from spermatozoa according to the Chomczynski and Sacchi method, treated with DNase I and reverse-transcribed into cDNA. Quantitative analysis of HILS1, TNP1, and TNP2 transcripts was performed by real-time quantitative (RQ-PCR) SYBR green I analysis. We found significantly lower levels of HILS1, TNP1, and TNP2 transcripts in spermatozoa from asthenozoospermic men compared to normozoospermic men. Our observations suggest that a reduction in HILS1, TNP1, and TNP2 transcripts may be associated with asthenozoospermia.

Zinc coordination is required for and regulates transcription activation by Epstein-Barr nuclear antigen 1

Epstein-Barr Nuclear Antigen 1 (EBNA1) is essential for Epstein-Barr virus to immortalize naïve B-cells. Upon binding a cluster of 20 cognate binding-sites termed the family of repeats, EBNA1 transactivates promoters for EBV genes that are required for immortalization. A small domain, termed UR1, that is 25 amino-acids in length, has been identified previously as essential for EBNA1 to activate transcription. In this study, we have elucidated how UR1 contributes to EBNA1's ability to transactivate. We show that zinc is necessary for EBNA1 to activate transcription, and that UR1 coordinates zinc through a pair of essential cysteines contained within it. UR1 dimerizes upon coordinating zinc, indicating that EBNA1 contains a second dimerization interface in its amino-terminus. There is a strong correlation between UR1-mediated dimerization and EBNA1's ability to transactivate cooperatively. Point mutants of EBNA1 that disrupt zinc coordination also prevent self-association, and do not activate transcription cooperatively. Further, we demonstrate that UR1 acts as a molecular sensor that regulates the ability of EBNA1 to activate transcription in response to changes in redox and oxygen partial pressure (pO₂). Mild oxidative stress mimicking such environmental changes decreases EBNA1-dependent transcription in a lymphoblastoid cell-line. Coincident with a reduction in EBNA1-dependent transcription, reductions are observed in EBNA2 and LMP1 protein levels. Although these changes do not affect LCL survival, treated cells accumulate in G0/G1. These findings are discussed in the context of EBV latency in body compartments that differ strikingly in their pO₂ and redox potential.

Epstein-Barr virus (EBV) infects human B-cells and immortalizes them. Immortalization results in diseases that range from infectious mononucleosis to malignancies such as lymphomas. During immortalization, EBV expresses a small number of viral genes that modulate cellular proliferation and differentiation. One of the genes expressed by EBV, Epstein-Barr nuclear antigen 1 (EBNA1), activates the expression of the other viral genes required for immortalization. In this report, we have explored the mechanism by which EBNA1 activates gene expression. We have determined that EBNA1 uses the micronutrient zinc to self-associate, and that self-association is necessary for it to activate gene expression. Further, we have determined that environmental conditions such as oxygen tension and oxidative stress modulate EBNA1's capacity to self-associate, and therefore to activate gene expression. The gene expression profile and proliferative phenotype of EBV-infected cells is known to vary in differing environmental niches in the human body, such as lymph nodes and in peripheral circulation. We interpret our results to postulate that these differences arise as a consequence of varying oxygen tension in these microenvironments on EBNA1's capacity to activate viral gene expression. Our findings can be exploited to devise novel therapeutics against EBV-associated diseases that target EBNA1 through oxidative stress.

Biology of sperm chromatin structure and relationship to male fertility and embryonic survival

Embryonic mortality in mammals is typically thought to result from 'female factor' infertility. There is growing evidence, however, that the status of sperm chromatin (DNA) at the time of fertilisation can also influence embryonic survival. During the final stages of spermatogenesis (spermiogenesis) a number of unique biochemical, morphological and physiological processes take place that are associated with marked changes in the structure of sperm chromatin. In early stages of spermatogenesis, sperm DNA is associated with histone nucleoproteins and structured into classical nucleosome core particles similar to other somatic cells. As spermiogenesis proceeds, the histone nucleoproteins are replaced by transition proteins which are subsequently replaced by protamines. At the completion of spermiogenesis the chromatin of mature sperm has a toroidal structure that is tightly compacted and resistant to denaturation. The compaction is necessary to protect sperm chromatin during transit through the epididymis and female reproductive tract. Disruption to chromatin remodelling during spermiogenesis results in chromatin that is susceptible to denaturation. Inappropriate chromatin structure has been shown in a number of mammalian species to be related to male infertility, and specifically the failure of embryonic development. A range of techniques are available to assess chromatin status in sperm but arguably the most informative is the sperm chromatin structure assay (SCSA). The SCSA is a flow cytometric assay that uses the metachromatic properties of acridine orange to measure the susceptibility of sperm chromatin to acid-induced denaturation. A relationship has been demonstrated, primarily in men, between the SCSA outcome and the probability of continued embryonic development and the establishment of pregnancy after fertilisation. The contribution of sperm chromatin instability to reproductive wastage in both natural mating and assisted reproduction warrants further investigation as it may prove valuable as a means of decreasing the incidence of embryonic mortality. In this regard, it is possible that 'male factor' infertility may emerge as an even more important component in embryonic development.

Involvement of importin-4 in the transport of transition protein 2 into the spermatid nucleus

Mammalian spermiogenesis is characterized by a unique chromatin-remodeling process in which histones are replaced by transition protein 1 (TP1), TP2, and TP4, which are further replaced by protamines. We showed previously that the import of TP2 into the haploid spermatid nucleus requires the components of cytosol and ATP. We have now carried out a detailed analysis to characterize the molecular components underlying the nuclear translocation of TP2. Real-time PCR analysis of the expression of different importins in testicular germ cells revealed that importin-4 and importin-beta3 are significantly up-regulated in tetraploid and haploid germ cells. We carried out physical interaction studies as well as an in vitro nuclear transport assay using recombinant TP2 and the nuclear localization signal of TP2 (TP2(NLS)) fused to glutathione S-transferase in digitonin-permeabilized, haploid, round spermatids and identified importin-4 to be involved in the import of TP2. A three-dimensional model of the importin-4 protein was generated using the crystal structure of importin-beta1 as the template. Molecular docking simulations of TP2(NLS) with the importin-4 structure led to the identification of a TP2(NLS) binding pocket spanning the three helices (helices 21 to 23) of importin-4, which was experimentally confirmed by in vitro interaction and import studies with different deletion mutants of importin-4. In contrast to TP2, TP1 import was accomplished through a passive diffusion process.

Phosphorylation of rat spermatidal protein TP2 by sperm-specific protein kinase A and modulation of its transport into the haploid nucleus

Transition protein 2 (TP2), which is expressed during stages 12-15 of mammalian spermiogenesis, has been shown to undergo phosphorylation immediately after its synthesis. We reported earlier that TP2 is phosphorylated in vitro at threonine 101 and serine 109 by the salt extract of sonication-resistant (elongating and elongated) spermatid nuclei and the protein kinase phosphorylating TP2 was identified to be protein kinase A (PKA). We now report that the cytosol from haploid spermatids but not from premeiotic germ cells is able to phosphorylate recombinant TP2 in vitro at threonine 101 and serine 109. The kinase present in the haploid spermatid cytosol that phosphorylates TP2 has been identified to be the sperm-specific isoform of protein kinase A (Cs-PKA). Reverse transcription-PCR analysis indicated that Cs-PKA was present in the haploid spermatids and absent from premeiotic germ cells. The rat Cs-PKA transcript was amplified and sequenced using the isoform-specific primers. The sequence of rat Cs-PKA at the N terminus differs from mouse and human by one amino acid. Western blot analysis using specific anti-Calpha1 antibodies revealed that Calpha1-PKA is absent in haploid spermatid cytosol. We have also established an in vitro nuclear transport assay for the haploid round spermatids. Using this assay, we have found that the cytoplasmic factors and ATP are absolutely essential for translocation of TP2 into the nucleus. Phosphorylation was found to positively modulate the NLS dependent import of TP2 into the nucleus.

Roles of transition nuclear proteins in spermiogenesis

The transition nuclear proteins (TPs) constitute 90% of the chromatin basic proteins during the steps of spermiogenesis between histone removal and the deposition of the protamines. We first summarize the properties of the two major transition nuclear proteins, TP1 and TP2, and present concepts, based on their time of appearance in vivo and in vitro properties, regarding their roles. Distinct roles for the two TPs in histone displacement, sperm nuclear shaping, chromatin condensation, and maintenance of DNA integrity have been proposed. More definitive information on their roles in spermiogenesis has recently been obtained using mice with null mutations in the Tnp1 or Tnp2 genes for TP1 and TP2, respectively. In these mice, histone displacement and sperm nuclear shaping appear to progress quite normally. Spermatid nuclear condensation occurs, albeit in an abnormal fashion, and the mature sperm of the Tnp -null mutants are not as condensed as wild-type sperm. There is also evidence that sperm from these mutant mice contain an elevated level of DNA strand breaks. The mutant sperm showed several unexpected phenotypes, including a high incidence of configurational defects, such as heads bent back on midpieces, midpieces in hairpin configurations, coils, and clumps, other midpiece defects, reduced levels of proteolytic processing of protamine 2 during maturation, and reduced motility. The two TPs appear partly to compensate for each other as both Tnp1 - and Tnp2 -null mice were able to produce offspring, and appear to have largely overlapping functions as the two mutants had similar phenotypes.

Mutations in PHF6 are associated with Börjeson-Forssman-Lehmann syndrome

Börjeson-Forssman-Lehmann syndrome (BFLS; OMIM 301900) is characterized by moderate to severe mental retardation, epilepsy, hypogonadism, hypometabolism, obesity with marked gynecomastia, swelling of subcutaneous tissue of the face, narrow palpebral fissure and large but not deformed ears. Previously, the gene associated with BFLS was localized to 17 Mb in Xq26-q27 (refs 2-4). We have reduced this interval to roughly 9 Mb containing more than 62 genes. Among these, a novel, widely expressed zinc-finger (plant homeodomain (PHD)-like finger) gene (PHF6) had eight different missense and truncation mutations in seven familial and two sporadic cases of BFLS. Transient transfection studies with PHF6 tagged with green fluorescent protein (GFP) showed diffuse nuclear staining with prominent nucleolar accumulation. Such localization, and the presence of two PHD-like zinc fingers, is suggestive of a role for PHF6 in transcription.

Condensation of DNA by spermatid basic nuclear proteins

Two transition proteins, TP1 and TP2, participate in the repackaging of the spermatid genome early in mammalian spermiogenesis, coincident with the first detectable changes in chromatin condensation. Using an optical trap and a two-channel flow cell to move single DNA molecules into buffer containing protein, we have measured the rates of DNA condensation and decondensation induced by the binding of Syrian hamster transition proteins TP1 and TP2 and protamines P1 and P2. The results show that both transition proteins condense free DNA, with rates similar to those of protamine 1 and 2. DNA molecules condensed with TP1 were significantly less stable than DNA condensed by protamine or by TP2. Experiments conducted with a peptide corresponding to the C-terminal 25 residues of TP2 showed that this domain is responsible for condensing DNA. Experiments conducted with two fragments of TP1 containing arginine and lysine residues demonstrated that DNA binding by TP1 must involve more than these basic sequences. Zinc facilitated the condensation of DNA by P2 but not by TP2. The dissociation rates of TP2 and P2 from DNA were not affected by the addition of zinc.

Involvement of protein kinase A in the phosphorylation of spermatidal protein TP2 and its effect on DNA condensation

Rat spermatidal protein TP2 is rich in serine residues and has several potential sites for phosphorylation by different protein kinases. Recombinant TP2 is phosphorylated upon incubation in vitro with salt extract of testicular sonication resistant nuclei (SRN) (representing elongating and elongated spermatids). The major phosphorylation sites were localized to the C-terminal, V8 protease-derived, fragment (residues 87-114). Phosphorylation experiments with the wild type and different site-specific mutants of TP2 revealed that serine 109 and threonine 101 are the phosphorylation sites. Phosphorylation of the C-terminal fragment of TP2 was also demonstrated in vivo. Phosphorylation was not stimulated by either protein kinase C activators or cGMP but was inhibited by protein kinase A inhibitor (PKI) peptide, showing the involvement of protein kinase A in the phosphorylation of TP2. Phosphorylation of TP2 greatly reduced its DNA condensation property. TP2 when complexed with DNA was not a good substrate for phosphorylation by PKA. Dephosphorylation of the DNA-TP2 complex by calf intestinal alkaline phosphatase restored the DNA condensation property to a level equivalent to that observed with TP2. The physiological significance of the phosphorylation-dephosphorylation cycle is discussed with reference to the two-domain model of TP2.

Basic domains target protein subunits of the RNase MRP complex to the nucleolus independently of complex association

The RNase MRP and RNase P ribonucleoprotein particles both function as endoribonucleases, have a similar RNA component, and share several protein subunits. RNase MRP has been implicated in pre-rRNA processing and mitochondrial DNA replication, whereas RNase P functions in pre-tRNA processing. Both RNase MRP and RNase P accumulate in the nucleolus of eukaryotic cells. In this report we show that for three protein subunits of the RNase MRP complex (hPop1, hPop4, and Rpp38) basic domains are responsible for their nucleolar accumulation and that they are able to accumulate in the nucleolus independently of their association with the RNase MRP and RNase P complexes. We also show that certain mutants of hPop4 accumulate in the Cajal bodies, suggesting that hPop4 traverses through these bodies to the nucleolus. Furthermore, we characterized a deletion mutant of Rpp38 that preferentially associates with the RNase MRP complex, giving a first clue about the difference in protein composition of the human RNase MRP and RNase P complexes. On the basis of all available data on nucleolar localization sequences, we hypothesize that nucleolar accumulation of proteins containing basic domains proceeds by diffusion and retention rather than by an active transport process. The existence of nucleolar localization sequences is discussed.

Targeted disruption of the transition protein 2 gene affects sperm chromatin structure and reduces fertility in mice

During mammalian spermiogenesis, major restructuring of chromatin takes place. In the mouse, the histones are replaced by the transition proteins, TP1 and TP2, which are in turn replaced by the protamines, P1 and P2. To investigate the role of TP2, we generated mice with a targeted deletion of its gene, Tnp2. Spermatogenesis in Tnp2 null mice was almost normal, with testis weights and epididymal sperm counts being unaffected. The only abnormality in testicular histology was a slight increase of sperm retention in stage IX to XI tubules. Epididymal sperm from Tnp2-null mice showed an increase in abnormal tail, but not head, morphology. The mice were fertile but produced small litters. In step 12 to 16 spermatid nuclei from Tnp2-null mice, there was normal displacement of histones, a compensatory translationally regulated increase in TP1 levels, and elevated levels of precursor and partially processed forms of P2. Electron microscopy revealed abnormal focal condensations of chromatin in step 11 to 13 spermatids and progressive chromatin condensation in later spermatids, but condensation was still incomplete in epididymal sperm. Compared to that of the wild type, the sperm chromatin of these mutants was more accessible to intercalating dyes and more susceptible to acid denaturation, which is believed to indicate DNA strand breaks. We conclude that TP2 is not a critical factor for shaping of the sperm nucleus, histone displacement, initiation of chromatin condensation, binding of protamines to DNA, or fertility but that it is necessary for maintaining the normal processing of P2 and, consequently, the completion of chromatin condensation.