Expression of uncharacterized male germ cell-specific genes and discovery of novel sperm-tail proteins in mice

The identification and characterization of germ cell-specific genes are essential if we hope to comprehensively understand the mechanisms of spermatogenesis and fertilization. Here, we searched the mouse UniGene databases and identified 13 novel genes as being putatively testis-specific or -predominant. Our in silico and in vitro analyses revealed that the expressions of these genes are testis- and germ cell-specific, and that they are regulated in a stage-specific manner during spermatogenesis. We generated antibodies against the proteins encoded by seven of the genes to facilitate their characterization in male germ cells. Immunoblotting and immunofluorescence analyses revealed that one of these proteins was expressed only in testicular germ cells, three were expressed in both testicular germ cells and testicular sperm, and the remaining three were expressed in sperm of the testicular stages and in mature sperm from the epididymis. Further analysis of the latter three proteins showed that they were all associated with cytoskeletal structures in the sperm flagellum. Among them, MORN5, which is predicted to contain three MORN motifs, is conserved between mouse and human sperm. In conclusion, we herein identify 13 authentic genes with male germ cell-specific expression, and provide comprehensive information about these genes and their encoded products. Our finding will facilitate future investigations into the functional roles of these novel genes in spermatogenesis and sperm functions.


Introduction
During spermatogenesis, germ cells are processed from primordial germ cells (PGCs) to mature sperm [1,2]. The tightly regulated nature of this process, which occurs in the seminiferous tubules of testes, indicates that a highly organized network of genes is expressed in male germ cells during their development [3]. An intrinsic program determines which genes are utilized and when they are expressed in germ cells, and an interactive regulation between germ cells and somatic cells is necessary for germ cell proliferation and progression. Extrinsic influences, including steroid and peptide hormones, control this interactive regulation. However, the intrinsic genetic program is central to the development and fertilization of male germ cells. Many unique genes and variant transcripts are expressed during spermatogenesis, and the identification and characterization of male germ cell-specific genes are crucial to our understanding of the mechanisms of sperm development [4,5]. Analysis of the proteins encoded by such genes has revealed that they are developmentally regulated and involved in diverse functions during spermatogenesis and fertilization [6][7][8]. For example, Mm.290718/Zfp541 was found to encode a protein that forms a complex involved in chromatin remodeling during late spermatogenesis [7]. The protein encoded by Mm.87328/Shsp1 localizes to the head region of mature sperm, particularly the surface of the acrosomal region [6]. Finally, three novel male germ cell-specific genes (Mm.386907/Sfap1, Mm.157049/Sfap2, and Mm.442063/Als2r12) were found to encode proteins associated with cytoskeletal structures in the sperm flagellum [6,8].
Here, as part of an ongoing study of germ cell-specific genes, we identified 13 testis-specific genes using sequence information from the mouse testis UniGene database, and analyzed their characteristics at the gene and transcript levels. Furthermore, we obtained original findings on the developmental expression patterns and localizations of seven of the encoded proteins. In particular, we found that three of the novel proteins were present in mature sperm and associated with cytoskeletal structures of the sperm tail. Therefore, our study represents the first transcript-and protein-level characterization of 13 novel testis-specific genes that may play roles in spermatogenesis and fertilization.

Ethics statements
The biospecimens used in this study were provided by the Pusan National University Hospital, which is a member of the National Biobank of Korea supported by the Ministry of Health, Welfare and Family Affairs. All samples from the National Biobank of Korea were obtained with informed consent under institutional review board-approved protocols. The study of human sperm was also ratified through the Ethics Committee of Gwangju Institute of Science and Technology (GIST) and Chonnam National University (permit number: 20140818-BR-14-01-02). All participants signed an informed consent form permitting use of their semen remnants in this study.

UniGene database analysis
Mouse UniGene profile data (Build #193) was obtained from the NCBI UniGene database. The profile data contained expressed sequence tag (EST) expression profile of each gene entry in 47 different tissues. Transcripts per million (TPM, indicating the normalized expression levels) and testis specificity were calculated as described [9]. Genes with unassigned functions were regarded as 'unknown' according to Gene References into Function (GeneRIF) in the NCBI Gene database. We discarded previously characterized testis-specific genes by PubMed search.

Reverse-transcription (RT) PCR
RT-PCR experiments were performed using total RNA from eight tissues (testis, brain, heart, kidney, liver, lung, muscle, and spleen) of male mice. RT-PCR was also performed using RNA from germ cell-lacking testes of W/W v mutant mice (SLC) [10], to determine whether the genes were expressed in somatic cells of the testis. Total RNA was extracted using the TRIzol reagent (Molecular Research Center) according to the manufacturer's protocol, and cDNA was synthesized by random hexamer and oligo(dT) primers using the Omniscript reverse transcriptase (Qiagen). The utilized gene-specific primers are listed in S1 Table. PCR was performed for 30 cycles of 94˚C for 30 s, 55˚C for 30 s, and 72˚C for 1 min. Glyceraldehyde-3-phosphate dehydrogenase (Gapdh) was amplified as a control (forward, 5'-TGA AGG TCG  GAG TCA ACG GAT TTG GT-3' and reverse, 5'-CAT GTG GGC CAT GAG GTC CAC CAC-3'). Specific expression at different stages of spermatogenesis was established using total RNA obtained from the testes of prepubertal and adult male mice (ages 8,10,12,14,16,20, and 30 days after birth). All animal investigations were carried out according to the guidelines of the Animal Care and Use of Gwangju Institute of Science and Technology. The protocol was approved by the Animal Care and Use Committee of Gwangju Institute of Science and Technology (Permit number: GIST 2011-13).

Antibodies
For production of polyclonal antibodies, glutathione S-transferase (GST)-fusion proteins containing the specific antigenic regions of seven candidate proteins were expressed in Escherichia coli BL21 and affinity purified with glutathione Sepharose 4B (GE Healthcare). The recombinant proteins were used as antigens for producing rabbit polyclonal antisera. The antibodies were affinity purified using the appropriate proteins and an AminoLink immobilization kit (Pierce). The following commercially available antibodies were also used: a mouse monoclonal antibody against ADAM2 (1/1000, MAB19292) from Millipore; an antibody against α-tubulin (1/1000, T6199) from Sigma-Aldrich; and an antibody against GAPDH (1/1000, MCA4739) from Bio-Rad. As secondary antibodies for Western blot analysis, we used horseradish peroxidase (HRP)-conjugated anti-rabbit or anti-mouse IgG (Jackson ImmunoResearch).

Preparation of protein samples
Mouse tissues were lysed in a nonionic detergent lysis buffer (1% NP-40) containing 1% protease inhibitor cocktail (GenDEPOT) for 1 h on ice. Debris was removed by centrifugation at 13,000 g for 15 min at 4˚C. The supernatant fractions from the lysate were mixed with 2x SDS sample buffer and boiled for 5 min. Testicular cells and sperm were prepared as previously described [11]. Briefly, cells were isolated by suspension in 52% isotonic Percoll (GE Healthcare) followed by centrifugation for 10 min (27,000 g, 4˚C), and then resuspended in Mg 2 + -HEPES buffer. Sperm from the cauda epididymis and vas deferens were released into PBS. The sperm suspension was centrifuged twice at 800 g for 3 min to remove contaminants, and the pelleted sperm were resuspended in 2x SDS sample buffer and boiled for 10 min. For sperm fractionation, sperm were collected and sonicated. An equal volume of 1.8 M sucrose was added and the suspension was layered over a discontinuous sucrose gradient containing equal volumes of 2.05 M and 2.2 M sucrose solutions. The sample was centrifuged at 100,000 g at 4˚C for 16 h, and the sperm heads and tails were collected from the pellets at the bottom and the middle of the 2.05 M sucrose layer, respectively. To test the solubility of sperm tail proteins, sperm were lysed in lysis buffer (10 mM Tris-Cl, pH 8.0, 150 mM KCl, 5 mM MgCl2, 0.5 mM EDTA) containing 2, 3, 4 or 6 M urea, or in 1% NP-40 or Triton X-100. Lysis was performed on ice for 2 h, except for experiments that used the lysis buffer containing 6 M urea, which involved incubation at room temperature. Soluble and insoluble fractions were separated by centrifugation at 10,000 g for 15 min, and each faction was subjected to Western blot analysis. milk in TBS-T (50 mM Tris-HCl, pH 7.5, 150 mM NaCl [TBS] containing 0.1% Tween-20) and incubated with primary antibodies at room temperature for 1 h. After three 10-min washes with TBS-T, the membranes were incubated with HRP-conjugated secondary antibodies for 1 h at room temperature. Following three washes with TBS-T, immunoreactive proteins were detected using a chemiluminescence kit (Thermo Scientific).

Immunofluorescence
Freshly dissected testes were fixed in 4% paraformaldehyde, embedded in paraffin and sectioned at 5 μm. The sections were deparaffinized in xylene and an ethanol series. Antigen retrieval was performed by boiling the sections for 20 min in 10 mM tribasic sodium citrate, pH 6. The samples were washed in phosphate-buffered saline (PBS) and then immersed in 0.4% Triton X-100 for 10 min before being blocked with a buffer (3% bovine serum albumin in PBS) for 30 min. The sections were incubated overnight at 4˚C with the primary antibodies (5 μgÁmL -1 ), and then placed in 1/1000 dilutions of Rhodamine Red™-X goat anti-rabbit IgG (Molecular Probes) for 30 min at room temperature. DNA was stained with DAPI, and the sections were mounted and visualized by fluorescence microscopy (DMLB, Leica).

Identification of testis-specific novel genes
To select for genes that are exclusively transcribed in testis, we calculated testis specificity using information from the mouse UniGene database (Build #193) [9], which includes the EST expression profile of a given gene in particular tissues of mice at a specific age and/or health status, as expressed in TPM. We selected genes on the basis of the following criteria: (i) genes with testis specificity >50% and (ii) genes with unassigned function. We collected the information available for the selected genes, such as the presence of coding sequences, known homologies, and cell lines with known mutations. Using this strategy, we identified 13 novel genes as being putatively testis-specific or -predominant. The UniGene ID numbers and names of these genes are Mm.276332/MORN repeat containing 5 (Morn5); Mm.23509/phosphatidylethanol amine binding protein 4 (Pebp4); Mm.56430/coiled-coil glutamate rich protein 1 (Ccer1); Mm.73222/testis expressed 33 (Tex33); Mm.131623/transmembrane and coiled-coil domains 5

Expression patterns of the 13 novel genes
To determine whether the genes selected from the mouse UniGene library represented authentic genes with testis-specific expression, we performed various expression analyses. RT-PCR analysis showed that all of the genes were expressed at the expected sizes exclusively in mouse testis, which was consistent with our in silico prediction (Fig 1). When we examined their expressions in the germ cell-lacking testis of W/W v (c-kit) mutant mice [10], we found little or no expression of the selected genes (Fig 1), further validating that their expression is germ cellspecific.
In the first round of spermatogenesis in a prepubertal mouse, spermatogonial stem cells increasingly proliferate and differentiate through the sequence of spermatogonia, spermatocytes, and spermatids (Fig 2) [12]. We hypothesized that if a given gene is transcribed in germ cells during spermatogenesis, the transcripts will appear in the testis at an exact post-partum time point corresponding to a specific stage of spermatogenesis. Indeed, our RT-PCR results showed that all of the selected genes were expressed at least after day 12, indicating that their expressions are developmentally regulated. With the exception of Mm.159422/4930505A04Rik, the examined genes were all first expressed between day 12 and day 20, which corresponds to the spermatocyte stage. Mm.159422/4930505A04Rik was expressed in germ cells after meiosis (Fig 2). Collectively, the results of our in vitro analyses indicate that the in silico-selected genes show testis-and germ cell-specific expression, and are developmentally regulated during spermatogenesis.  Germ cell-specific genes and sperm-tail proteins In silico analysis of genomic, transcript, and protein characteristics To characterize the genomic, transcript, and protein natures of the selected genes, we collected the information available from various databases. Fig 3 shows the exon organizations and chromosomal locations of the genes, the predicted transcript sizes, and the numbers of amino acids, specific domains, and gene ontology (GO) of the predicted proteins. The exon numbers of the genes ranged from 1 to 12 exons, and the genes were found to be widely distributed across the mouse chromosomes. To extend our findings, we searched the human genome database and found that human orthologs for 12 of the 13 selected mouse genes are present in genomic regions that show conserved synteny between mice and humans. Ten of these human genes were predicted to be expressed predominantly in testis; MORN5 and PEBP4 were not. The protein-coding region of each gene was defined by selecting the longest amino acid sequence before a stop codon. All of the translated gene products were predicted to contain various domains and motifs, and could be annotated with GO codes.  and used to generate polyclonal sera (S1 Fig), and affinity purification of antibodies was performed against GST and GST fusion proteins. To verify the specificity of the antibodies for the novel proteins and confirm whether the proteins were expressed in testis, we performed Western blot analysis with lysates of liver and testis. All of the generated antibodies recognized appropriately sized bands in the testis, but not in the liver, and the bands disappeared when the GST-fusion proteins were added along with the primary antibodies (S2 Fig). These results support the specificity of the antibodies and the authenticity of the selected proteins.

Generation of antibodies and expression patterns of novel proteins
To determine whether the selected proteins are expressed exclusively in testis, we performed Western blot analysis with protein extracts from eight different tissues. Consistent with our RT-PCR results, all of the proteins were expressed specifically in the testis (Fig 4A  and S3 and S4 Figs). Further immunoblot analyses performed with mouse testes obtained at different times after birth revealed that Mm.276332/MORN5 and Mm.258841/FAM71E1 were expressed in spermatocytes (day 21); Mm.271255/1700013F07Rik was weakly expressed in spermatocytes during the first spermatogenesis; and Mm.73222/TEX33, Mm.131623/TMCO5, Mm.272519/1700013G24Rik, and Mm.159422/4930505A04Rik were expressed in round spermatids (day 28) (Fig 4B and S5 Fig).
To investigate the expression pattern of the selected proteins in germ cells during sperm development, we performed immunoblotting with lysates of testicular cells (TC), testicular sperm (TS), and mature sperm (S) from the cauda epididymis and vas deferens (Fig 5A and  S6 Fig). We found that Mm.73222/TEX33 was present only in testicular cells; Mm.258841/ FAM71E1, Mm.131623/TMCO5, and Mm.272519/1700013G24Rik existed in testicular cells and sperm but not in mature sperm; and Mm.276332/MORN5, Mm.271255/1700013F07Rik, and Mm.159422/4930505A04Rik were present in all stages. None of these proteins exhibited any change in molecular weight during spermatogenesis. These results indicate that the seven analyzed proteins exhibited differential regulation during male germ cell development.
To confirm and further investigate the developmental expression and localization of the selected proteins, we performed immunofluorescence analysis in paraffin sections of adult testis. Antibodies to Mm.276332/MORN5, Mm.258841/FAM71E1, and Mm.159422/4930505A04Rik did not display immunoreactivity (S8 Fig). In contrast, we observed immunofluorescence signals in analyses with antibodies to the other proteins (Fig 6A). Mm.73222/TEX33 was found to exist in the cytoplasm of round spermatids. Mm.131623/TMCO5 was identified exclusively in the tail region of elongated spermatids. Mm.272519/1700013G24Rik showed a dot pattern in the cytoplasm of round spermatids, but was mainly present in the tail of elongated spermatids. Mm.271255/1700013F07Rik was expressed in the cytoplasm of spermatocytes and round spermatids, and then observed in the tail of elongated spermatids (Fig 6A). These data confirm and extend the Western blot results (Figs 4 and 5).

Localization and characterization of novel proteins in mature sperm
Because three of the novel proteins (Mm.276332/MORN5, Mm.271255/1700013F07Rik, and Mm.159422/4930505A04Rik) were found to exist in mature sperm, we hypothesized that they might be related to sperm function and fertilization. To elucidate the distribution of these proteins in mature sperm, we isolated sperm from the epididymis and vas deferens, separated them into head and tail fractions, and subjected the protein lysates of these fractions to Western blot analysis. ADAM2 and α-tubulin were used as positive controls for the head and tail fractions, respectively. All three proteins were found exclusively in the tail fraction (Fig 5B and  S7 Fig). In addition, immunofluorescence analysis revealed that Mm.271255/1700013F07Rik was localized to the neck and midpiece of sperm tail (Fig 6B). Outer dense fibers (ODFs) and the fibrous sheath (FS) are specialized cytoskeletal structures of the mammalian sperm tail. The proteins that comprise these structures show similar solubilities in various detergents [13]. To examine the possible association of Mm.276332/MORN5, Mm.271255/1700013F07Rik, and Mm.159422/4930505A04Rik with these structures, we examined their solubilities in nonionic detergents and 2-6 M urea. None of the tested proteins was solubilized by 1% NP-40 or 1% Triton X-100 (Fig 7A and S9 Fig). Mm.276332/MORN5 was solubilized only in 6 M urea, Mm.271255/1700013F07Rik and Mm.159422/4930505A04Rik resisted the lower concentrations but showed a gradual increase in solubility as the concentration of urea increased (Fig 7B and S9 Fig). These results indicate that MORN5, Mm.271255, and Mm.159422 are likely to be components of or associated with cytoskeletal structures of the sperm flagellum, such as ODFs and the FS.

Expression of MORN5 in human sperm
As the three sperm tail proteins (Mm.276332/MORN5, Mm.271255/1700013F07Rik, and Mm.159422/4930505A04Rik) identified from the mouse database showed 68-79% sequence identity to their human homologs, we hypothesized that our generated antibodies might recognize the human homologs. To examine this possibility, we performed Western blot analysis with protein extracts from mouse and human sperm. As shown in Fig 8A, the anti-MORN5 antibody recognized an 18-kDa band in human sperm, which was 2-kDa smaller than that recognized in mouse sperm. The antigenic region of MORN5 exhibits an 85% sequence identity between mouse and human ( Fig 8B). These findings suggest that MORN5 might be functionally conserved in mouse and human sperm. Our Western blot analyses using the antibodies against Mm.271255/1700013F07Rik and Mm.159422/4930505A04Rik did not recognize any band with the expected size of the proteins in human sperm (S10 Fig), suggesting that these mouse antibodies did not cross-react with their human orthologs.

Discussion
In our previous studies, we identified a number of novel male germ cell-specific genes from the McCarrey Eddy spermatocyte (Lib. 6787) and spermatid (Lib.6786) UniGene libraries [4,5]. These two libraries contain 3513 putative gene entries, including ubiquitously expressed genes. Thus, it is possible that a number of spermatogenic cell-specific genes are absent from these data sets. In the present study, we analyzed all mouse testis UniGene libraries and identified 13 authentic genes as being putatively testis-specific or -predominant. Our RT-PCR analysis confirmed that all of the selected genes are testis-and germ cell-specific. Consistent with previous findings that male germ cell-specific genes characteristically show developmental regulation during the meiotic and postmeiotic phases [1,2], the expression patterns observed for the 13 selected genes during postnatal testicular development were indicative of developmental regulation. With the exception of Mm.159422/4930505A04Rik, all of the genes were found to be expressed from pachytene and diplotene spermatocytes. We integrated the available in silico information on the genomic characteristics and encoded transcripts of the 13 selected genes. We identified human orthologs for 12 of the mouse genes, all of which are found in regions of conserved synteny. With the exception of Mm.276332/Morn5 and Mm.23509/Pebp4, all of the human orthologs were predicted to be testis-specific or -predominant in the human UniGene detected as a loading control. All of the gene products showed specific expression in testis. B. The stage-specific expressions of the novel proteins were determined from mouse testes obtained on days 7, 14, 21, 28, 35, 42, 49, and 56 after birth. The anti-α-tubulin antibody was used as a control. All of the proteins were expressed in late meiotic or postmeiotic germ cells. https://doi.org/10.1371/journal.pone.0182038.g004 Germ cell-specific genes and sperm-tail proteins , and mature sperm (S) were blotted with antibodies against the novel proteins. α-Tubulin was detected as a control. The expressions of the selected proteins were found to be developmentally regulated during sperm maturation. B. Sperm from the cauda epididymis and vas deferens were separated into head (H) and tail (T) fractions, and subjected to Western blot analysis. ADAM2 and α-tubulin were used to database. We thus propose that the functions of these genes may be conserved between human and mouse. The proteins encoded by the 13 selected genes all possess conserved domains with unknown function. The MORN5 protein is known to be expressed in chick craniofacial structures and has been shown to be involved in bone morphogenetic protein (BMP) signaling [14]. PEBP4 is a secreted and glycosylated protein that appears to be required for Protein kinase B activation [15]. Thus, MORN5 and PEBP4 may function in signaling pathways during the meiotic and postmeiotic phases. The proteins encoded by Mm.87624/Fam71f1, Mm.258841/Fam71e1, and Mm.46148/Fam209 are predicted to localize in the nucleus, and a previous LC-MS/MS study identified the human homologs of FAM71A and FAM71B in the human sperm nucleus [16].
We corroborated the authenticity of seven of the selected genes at the protein level, and further investigated the encoded proteins. Specific antibodies generated against the novel proteins clearly recognized distinct bands of the expected sizes in the testis. Immunoblotting confirmed that the seven proteins were testis-specific. Our analysis of the stage-specific expression of the novel pro- The most remarkable finding of the present work was our discovery that three of the novel proteins, Mm.276332/MORN5, Mm.271255/1700013F07Rik, and Mm.159422/4930505A04Rik, were restricted to the tail region of mature sperm. Notably, Mm.271255/1700013F07Rik was observed to exist at the neck and midpiece regions of sperm tail. The flagellum of a mammalian sperm is divided into the middle, principal, and end pieces. The central core of the flagellum consists of a cytoskeletal structure called the axoneme, and the region between the axoneme and the plasma membrane harbors accessory structures, such as mitochondria, ODFs and the FS. The latter two structures are believed to stiffen the sperm tail while allowing elastic bending. The proteins that comprise ODFs and FS commonly resist non-ionic detergents, and FS proteins are insoluble in 6 M urea [13]. Our Western blot analysis showed that our three identified sperm tail proteins were insoluble in the non-ionic detergents, NP-40 and Triton X-100. Mm.276332/MORN5 was solubilized only in 6 M urea. Mm.271255/1700013F07Rik and Mm.159422/4930505A04Rik gradually solubilized as the concentration of urea increased. These results suggest that Mm.276332/ MORN5 is intrinsic component of the FS, while Mm.271255/1700013F07Rik and Mm.159422/ 4930505A04Rik are ODF components or are weakly associated with the FS.
Finally, we found that the antibody against mouse Mm.276332/MORN5 recognized a protein band corresponding to the expected size of MORN5 in human sperm, suggesting that this protein might be functionally conserved in mouse and human sperm. MORN5 contains three MORN (Membrane Occupation and Recognition Nexus) motifs, which contribute to the plasma membrane-binding capacity of junctophilin type 1 [17]. MORN1 may function as a confirm the head and tail fractions, respectively. All of the selected proteins were found to be expressed in the sperm tail. https://doi.org/10.1371/journal.pone.0182038.g005 Germ cell-specific genes and sperm-tail proteins Characterization of three sperm tail proteins. A. Sperm from the epididymis and vas deferens were treated with 1% NP-40 or 1% Triton X-100 and then centrifuged. Soluble and insoluble fractions were subjected to immunoblot analysis. ADAM2 and α-tubulin were used to verify the soluble and insoluble fractions, respectively. The tested proteins failed to solubilize with these detergents. S, supernatant after centrifugation; P, pellet after centrifugation. B. Sperm were treated with 2, 3, 4, or 6 M urea and then centrifuged. Soluble and insoluble fractions were subjected to Western blot analysis, with α-tubulin detected as a loading control. MORN5 and Mm.271255 were found to be insoluble in urea.
https://doi.org/10.1371/journal.pone.0182038.g007 linker protein between certain membrane regions and cytoskeleton of the parasites such as Toxoplasma gondii and other Apicomplexa [18]. MOPT and MORN3 were found to be expressed in the acrosomal region of elongating spermatids [19,20], and a recent study suggested that MORN5 is both regulated by and required for BMP signaling during craniofacial development in chicken [14]. Based on these previous findings, we speculate that MORN5 may function in signaling during sperm development, and that it may participate in the tight interaction that occurs between the plasma membrane and cytoskeletal structures of a mature sperm flagellum. and 161 amino acids in mouse and human, respectively, and contain three MORN motifs (gray boxes). The antigenic region used to raise the mouse MORN5 antibody is indicated by a red line. This region showed 85% identity between the mouse and human protein sequences (whole protein: 79% identity). https://doi.org/10.1371/journal.pone.0182038.g008 Germ cell-specific genes and sperm-tail proteins More than a hundred proteins have been identified in mature sperm tail (S2 Table). Our summary of the results from localization and functional studies of these sperm proteins (Table 1) reveals that 73 of the 111 were restricted to the tail region of mature sperm, and more than half of these tail proteins showed region-specific distributions thought to reflect their molecular functions in the sperm tail. GO analysis showed that these sperm tail proteins participated in various processes and structures, including voltage-gated calcium channel activity, cilium, sperm axoneme assembly, transmembrane transport, sperm capacitation, and sperm motility (S3 Table). Notably, only 21 of these proteins have been characterized in terms of their functions in tail formation and fertilization. For example, ODF1 [21], ODF2 [22], meiosis-specific nuclear structural protein 1 (MNS1) [23], and solute carrier family 22 member 14 (SLC22A14) [24] are reportedly related with sperm structure, while the cation channel sperm associated (CatSper) proteins [25,26], plasma membrane Ca 2+ -ATPase 4 (PMCA4) [27], and rhophilin associated tail protein 1 (ROPN1) [28] function in sperm motility.
In this study, we identified and characterized authentic genes specifically expressed in male germ cells through integrative analyses including genomic, transcript, and protein approaches. In particular, the three sperm-tail proteins are noteworthy because of their potential functions in mature sperm. However, the precise functions of the proteins are currently unknown. Future studies are needed to investigate the functions of our identified tail proteins and their relationships with the other known proteins. This should provide new insights into the role of various proteins in sperm development, motility, and fertilization.
Supporting information S1 Fig. Protein characteristics of seven candidates. The GenBank accession numbers of the cDNA sequences predicted for the novel genes are listed. The numbers of amino acids, hydrophobicities, and expected molecular weights were predicted from the deduced coding regions of these cDNA sequences. The bars indicate regions corresponding to the antigens used for