A 105- to 94-kilodalton protein in the epididymal fluids of domestic mammals is angiotensin I-converting enzyme (ACE); evidence that sperm are the source of this ACE

SDS-PAGE analysis of luminal fluid from the ram testis and epididymis revealed a protein of about 105 kDa in the fluid in the caput epididymal region. The molecular mass of this fluid protein shifted from 105 kDa to 94 kDa in the distal caput epididymidis and remained at 94 kDa in the lower regions of the epididymis. The possible sperm origin of this protein was suggested by the decrease in intensity of a 105-kDa compound on the sperm plasma membrane extract and by its total disappearance from the fluid of animals with impaired sperm production caused by scrotal heating. The 94-kDa protein was purified from ram cauda epididymal fluid, and a rabbit polyclonal antiserum was obtained. This antiserum showed that membranes of testicular sperm and sperm from the initial caput were positive for the presence of an immunologically related antigen. The protein was immunolocalized mainly on the flagellar intermediate piece, whereas in some corpus and caudal sperm, only the apical ridge of the acrosomal vesicle was labeled. The purified protein was microsequenced: its N-terminal was not found in the sequence database, but its tryptic fragments matched the sequence of the angiotensin I-converting enzyme (ACE). Indeed, the purified 94-kDa protein exhibited a carboxypeptidase activity inhibited by specific blockers of ACE. All the soluble seminal plasma ACE activity in the ram was attributable to the 94-kDa epididymal fluid ACE. The polyclonal antiserum also showed that a soluble form of ACE appeared specifically in the caput epididymal fluid of the boar, stallion, and bull. This soluble form was responsible for all the ACE activity observed in the fluid from the distal caput to the cauda epididymidis in these species. Our results strongly suggest that the epididymal fluid ACE derives from the germinal form of ACE that is liberated from the testicular sperm in a specific epididymal area.     

Localization of the antigotensin-converting enzyme activity in testis and epididymis

Angiotensin-converting enzyme (ACE) has been studied in different reproductive organs of the male rat, in somatic cell lines clonally derived from both rat and mouse testes, and in isolated spermatogenic cells of the mouse. Among the various reproductive organs only testis and epididymis show high levels of enzyme activity. The testicular activity is found mainly in the isolated germinal cells and residual bodies, whereas somatic cell lines contain negligible levels of activity even after addition of hormones and growth factors. Testicular homogenates, spermatogenic cells, epididymal spermatozoa, and spermatozoan cytoplasmic droplets, when fractionated by anion exchange chromatography, contain one major and one minor activity peak, whereas epididymal homogenates and epididymal secretions reveal an additional major activity peak together with the minor peak. All forms of ACE have a similar response to different modifiers, and are equally sensitive to the specific inhibitor N-[(S)-1-(ethoxycarbonyl)-3-phenylpropyl]-L-alanyl-L-proline (Enalapril). The testicular enzyme could provide a useful marker for spermatogenic maturation and/or cytoplasmic processes both in testis and epididymis. The separate epididymal peak is a secretory enzyme that may be responsible for the processing of spermatozoan plasma membrane constituents during epididymal transit, or may have a role in attacking some biologically active compounds.     

Molecular cloning of the cDNA for two major androgen-dependent secretory proteins of 18.5 kilodaltons synthesized by the rat epididymis

cDNA clones representing two closely related androgen-dependent secretory proteins of 18.5 kDa were selected by screening a rat epididymal cDNA library constructed in lambda gt 11 with affinity-purified antibody directed against the 18.5-kDa proteins. The entire amino acid sequence of the 18.5-kDa secretory proteins and a putative signal sequence of 18 amino acids was derived from 682 base pairs of the nucleotide sequence of overlapping cDNA clones. Confirmation of the identity of the cDNA clones was obtained by matching a partial amino acid sequence obtained for the N terminus of the pure protein with that of the sequence derived from the nucleotide code of the cDNA. Evidence is presented that the difference between the two closely related proteins may be associated with differential post-translational modification of the N terminus of the protein following cleavage of the signal sequence. Northern blot analysis revealed that the mRNA for the proteins is approximately 850 nucleotides long and that the concentration of the mRNA in the tissue is androgen-dependent. The proteins and their mRNAs were restricted to the epididymis as determined by Western and Northern blots, respectively, since signals were absent from the skin, brain, liver, kidney, heart, skeletal muscle, and testis. With the exception of a weak cross-reaction with mouse epididymis, the proteins were not detected by Western blots of extracts of guinea pig, rabbit, or bull epididymis. The two proteins account for a substantial proportion of the total protein in epididymal luminal fluid and become incorporated as components of the sperm plasma membrane where they may play a specific role in the post-testicular phase of sperm development.     

Immunolocalization of a 25-kilodalton protein in mouse testis and epididymis.

We have recently observed that a polyclonal antibody raised against a mouse epididymal luminal fluid protein (MEP 9) recognizes a 25-kDa antigen in mouse testis and epididymis [Rankin et al., Biol Reprod 1992; 46:747-766]. This antigen was localized by light and electron microscopic immunohistochemistry. The immunoreactivity in the testis was found in the residual cytoplasm of the elongated spermatids, in the residual bodies, and in the cytoplasmic droplets of spermatozoa. In the epididymis, the epithelial principal cells were stained from the distal caput to the distal cauda. Immunogold labeling in the principal cells showed diffuse distribution without preferential accumulation in either the endocytic or the secretory apparatus of the cells. In the epididymal lumen, the immunoreactivity was restricted to the sperm cytoplasmic droplets. No membrane-specific labeling was observed in luminal spermatozoa, cytoplasmic droplets, or isolated sperm plasma membranes. Three weeks after hemicastration or severance of the efferent ducts, a normal distribution of the immunoreactive sites was found in the epididymis. Immunoreactivity, was also detected in the epididymal epithelium of immature mice as well as in that of XXSxr male mice having no spermatozoa in the epididymis. These results suggest that the immunoreactivity seen in the principal cells originates from synthesis rather than endocytosis of the testicular protein from disrupted cytoplasmic droplets. Furthermore, these results suggest that the 25-kDa protein is synthesized independently by both testis and epididymis.     

Maturation of guinea pig sperm in the epididymis involves the modification of proacrosin oligosaccharide side chains

Proacrosin from guinea pig cauda epididymal sperm has a lower molecular weight compared with the testicular zymogen. In this study, we have examined the structural basis of this change and where the conversion in proacrosin molecular weight occurs during sperm maturation. Immunoblotting of trifluoromethanesulfonic acid-deglycosylated testicular and cauda epididymal sperm extracts with antibody to guinea pig testicular proacrosin demonstrated that the polypeptide backbones of proacrosins from the testis and cauda epididymal sperm had the same molecular weights (approximately 44,000). Keratanase, an endo-beta-galactosidase specific for lactosaminoglycans, partially digested testicular proacrosin but had no effect on proacrosin from cauda epididymal sperm. In extracts of testis, caput epididymis, and corpus epididymis analyzed by immunoblotting, anti-proacrosin recognized a major antigen with an apparent molecular weight (Mr) of 55,000, although a 50,000-Mr minor antigen began to appear in the corpus epididymis. By contrast, extracts of cauda epididymis, vas deferens, and cauda epididymal sperm had the 50,000 Mr protein as the only immunoreactive antigen. By enzymography following electrophoresis, the major bands of proteolytic activity in extracts of testis, caput epididymis, and corpus epididymis had 55,000 Mr. A band of protease activity with 55,000 Mr also appeared in extracts of the corpus epididymis. However, the most prominent bands of proteolytic activity in cauda epididymis, vas deferens, and cauda epididymal sperm had 50,000 Mr. In addition, two other major protease activities were detected with 32,000 and 34,000 Mr; the relationships of these proteases to proacrosin are unclear. From these results, we conclude that the oligosaccharides of proacrosin are altered during epididymal transit and that this modification occurs in the corpus epididymis.(ABSTRACT TRUNCATED AT 250 WORDS)     

In vivo secretion and association of clusterin (SGP-2) in luminal fluid with spermatozoa in the rat testis and epididymis.

Clusterin (sulfated glycoprotein-2) is a heterodimeric glycoprotein synthesized and secreted by rat Sertoli cells. An antigenically similar form is synthesized and secreted by the epididymis. The goal of this study was to define the epididymal regions in which clusterin is present and the regions in which clusterin is secreted and interacts with developing spermatozoa. Seminiferous tubule (STF), caput, corpus, and cauda fluids were collected by micropuncture and/or microperfusion and two-dimensional Western blot analysis was performed with a polyclonal antibody directed against Sertoli cell clusterin. Clusterin was found in both STF and epididymal fluid. STF contained predominantly the clusterin heavy chain (45 kd); however, a 70 Kd heterodimer was present under nonreducing conditions. Two subunits of clusterin with lower molecular weights (41 kd, heavy chain; 32 kd, light chain) and higher isoelectric points were present in the luminal fluid of all epididymal regions. The intraluminal levels of the heavy and light chains decreased from caput to cauda. Analysis by two-dimensional gel electrophoresis of proteins secreted directly into the epididymal luminal fluid revealed that clusterin was secreted by caput epithelium and not by the corpus and cauda epithelium. Western blots of membrane extracts from testicular, caput, and cauda spermatozoa revealed that testicular clusterin was associated with testicular sperm and epididymal clusterin with predominantly caput sperm. Our findings suggest that clusterin is secreted into the caput epididymal lumen, where it binds to sperm and then dissociates from sperm to be endocytosed by cells of the distal epididymal epithelium.     

Localization of a maturation-dependent epididymal sperm surface antigen recognized by a monoclonal antibody raised against a 135-kilodalton protein in porcine epididymal fluid.

A specific 135-kDa protein was purified from porcine cauda epididymal fluid. Analysis of its N-terminal amino acid sequence revealed it to be a new protein. Stable clones of hybridomas that produced monoclonal antibodies against the purified 135-kDa protein were established. A clone, B-11, reacting both with epididymal fluid and with sperm plasma membranes was selected and used in this study. Immunoblotting analysis showed that B-11 reacted only with a 135-kDa protein among epididymal fluid proteins. In contrast, B-11 did not recognize a similar 135-kDa sperm protein but did strongly react with a 27-kDa protein among sperm membrane proteins, extracted by NP-40 in the presence of protease inhibitors. B-11 also reacted only with a 27-kDa protein fragment among trypsin digests of the 135-kDa epididymal protein. The 135-kDa protein was first detected, by ELISA or immunoblotting analysis, at the beginning of the corpus epididymis. Maximal levels were reached in the distal corpus and levels were slightly decreased in the cauda epididymis. On the other hand, the surface of caput sperm were found to contain small amounts of antigen(s), the concentration of which gradually increased during epididymal transit. In immunocytochemical studies, the antigen was detectable in the epithelial cells from the initial segment to the corpus of the epididymis but not in the caudal cells. In the lumen, the presence of the 135 kDa protein was apparent in the corpus (at a maximum in the middle and distal corpus) and to a lesser degree in the caudal lumen. The 27-kDa protein was distributed all over the equatorial region of the acrosome of less than 10% of caput epididymal sperm. As sperm passed through the corpus epididymis, the percentage of immunoreactive cells increased and the protein was restricted to specific domains of the sperm head. Thus, on the mature sperm, antigen was localized in a crescent-shaped area of the equatorial segment just behind the anterior part of the acrosome and on the apical rim of the sperm head. This is the first observation of a sperm surface antigen derived from an epididymal protein as a proteolytic fragment that interacts with specific regions of the sperm membrane during the process of spermatozoa maturation.     

Identification of bovine CD52-like molecule by monoclonal antibody IVA-543: distribution of CD52-like molecule in the bull genital tract

The bovine maturation-associated sperm membrane antigen CD52-like molecule has been analysed using a mouse anti-sperm monoclonal antibody developed against bull spermatozoa. The antigen recognised by monoclonal antibody IVA-543 was detected on blood mononuclear cells (including lymphocytes and monocytes) and on a minor population of polymorphonuclear leukocytes. The bovine CD52-like molecule is secreted by the epididymal epithelium and then it is inserted into the sperm membrane during the epididymal transport in the distal part of epididymis. The CD52-like molecule was absent from spermatozoa derived from testes, and the highest proportion of IVA-543-reactive sperm was observed in the cauda epididymis (91.6%).This study has shown that the new molecule identified on bovine cells has properties analogous to those previously described for CD52 molecules in man, mouse, rat, monkey, and dog.     

Mapping of the human testicular proteome and its relationship with that of the epididymis and spermatozoa

The testis produces male gametes in the germinal epithelium through the development of spermatogonia and spermatocytes into spermatids and immature spermatozoa with the support of Sertoli cells. The flow of spermatozoa into the epididymis is aided by testicular secretions. In the epididymal lumen, spermatozoa and testicular secretions combine with epididymal secretions that promote sperm maturation and storage. We refer to the combined secretions in the epididymis as the sperm-milieu. With two-dimensional-PAGE matrix-assisted laser desorption ionization time-of-flight MS analysis of healthy testes from fertile accident victims, 725 unique proteins were identified from 1920 two-dimensional-gel spots, and a corresponding antibody library was established. This revealed the presence of 240 proteins in the sperm-milieu by Western blotting and the localization of 167 proteins in mature spermatozoa by ICC. These proteins, and those from the epididymal proteome (Li et al. 2010), form the proteomes of the sperm-milieu and the spermatozoa, comprising 525 and 319 proteins, respectively. Individual mapping of the 319 sperm-located proteins to various testicular cell types by immunohistochemistry suggested that 47% were intrinsic sperm proteins (from their presence in spermatids) and 23% were extrinsic sperm proteins, originating from the epididymis and acquired during maturation (from their absence from the germinal epithelium and presence in the epididymal tissue and sperm-milieu). Whereas 408 of 525 proteins in the sperm-milieu proteome were previously identified as abundant epididymal proteins, the remaining 22%, detected by the use of new testicular antibodies, were more likely to be minor proteins common to the testicular proteome, rather than proteins of testicular origin added to spermatozoa during maturation in the epididymis. The characterization of the sperm-milieu proteome and testicular mapping of the sperm-located proteins presented here provide the molecular basis for further studies on the production and maturation of spermatozoa. This could be the basis of development of diagnostic markers and therapeutic targets for infertility or targets for male contraception.     

Characterization of a 22 kDa protein with widespread tissue distribution but which is uniquely present in secretions of the testis and epididymis and on the surface of spermatozoa

The purification is reported of a 22 kDa protein which was first identified as one of the major components of the luminal secretion of the rat testis and epididymis. Antibodies against the 22 kDa protein cross-reacted with a protein of the same molecular weight in cytosolic extracts of other tissues from both male and female rats. However, since the protein could not be detected in blood, peritoneal fluid, saliva, milk, uterine fluid, seminal vesicle secretion, coagulating gland secretion or prostatic secretion, it would appear that the testis and epididymis may be unique in containing the protein in a soluble form within their luminal secretions. Proteins with slightly lower molecular weight were detected by the antibodies in cytosolic extracts of tissues from other animals (mice, rabbits, sheep, pigs, cattle), indicating that the protein may be conserved in a variety of species. However, in contrast to the rat, the protein was apparently not present in the testicular and epididymal secretions of these species. In addition to the occurrence of the 22 kDa protein as a soluble moiety in rat testicular and epididymal fluids, the protein was also located on sperm plasma membranes where its distribution was restricted to the surface of the flagellum. Amongst sperm surface proteins, the 22 kDa protein was the major protein containing sulphydryl groups and one of the major entities containing disulphide bonds. These properties may be of importance in the maintenance of sperm viability.     

A rat sperm flagellar surface antigen that originates in the testis and is expressed on the flagellar surface during epididymal transit.

We identified a rat sperm flagellar surface antigen using an IgG1 monoclonal antibody (MC31) against rat epididymal sperm. Avidin-biotin-peroxidase immunohistochemistry demonstrated that the antigen was first expressed in the cytoplasm of early primary spermatocytes, then gradually became restricted to the principal piece of the sperm flagellum during spermatogenesis. However, when the sperm reached the corpus epididymidis, the antigen was expressed on the surface of both the principal piece and the midpiece of the flagellum. The epithelial cells of the epididymis were not stained with MC31. Immunogold electron microscopy showed that the antigen was present on the surface of the sperm flagellar plasma membrane. Immunoblotting of Triton X-100 extracts of epididymal sperm after one-dimensional sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) under nonreducing conditions demonstrated that MC31 detected a major antigen of 26,000-28,000 daltons (26-28K). Two-dimensional isoelectric focusing and SDS-PAGE indicated that the 26-28K antigen had an isoelectric focusing point (pl) of 5.8-5.3; minor antigens were also detected from 26K (pl 5.8) to 35K (pl 5.0). These results indicate that the antigen recognized by MC31 is an acidic 26-35K protein that originates in the testis, is integrated into the sperm flagellar plasma membrane of the principal piece during spermatogenesis, and then is expressed on the entire flagellar surface during epididymal transit.     

Dipeptidase-inactivated tACE action in vivo: selective inhibition of sperm-zona pellucida binding in the mouse

The angiotensin-converting enzyme (ACE) plays a crucial role in male fertilization and is a key regulator of blood pressure. Testicular ACE (tACE), the germinal specific isozyme expressed on different promoters, exclusively carries out the role of ACE in fertility, although the site and mode of action are not well known. To investigate the contribution of tACE in fertilization, we produced transgenic mouse lines carrying a dipeptidase-inactivated mutant. Although the transgenic mice showed normal blood pressure, kidney morphology, and fertility, reduced fertilization was observed after in vitro fertilization (IVF). The sperm-zona pellucida (ZP) binding was exclusively impaired in these lines in a manner similar to that observed in an Ace knockout mouse. The dipeptidase activity was reduced in epididymal ingredients but not in the testis. Furthermore, direct application of mutant protein did not suppress sperm-ZP binding of intact sperm during IVF, implying that the dipeptidase-inactivated mutant affects sperm modification in the epididymis for ZP binding. Our results indicate that the dipeptidase-inactivated tACE acts in vivo, suggesting that tACE contributes to fertilization as a dipeptidase at least in the epididymis.     

Cyto-immunological studies of guinea pig sperm antigens

Summary Antigenic localization in guinea pig epididymal sperm and testicular imprints as well as in viable, motile guinea pig epididymal sperm was studied by means of fluorescent labelled antibody techniques. Globulins from rabbits and chickens immunized with guinea pig epididymal sperm were used in the direct procedure while sera from sheep and fowl injected with rabbit globulins were used in the indirect procedure. The main findings were: 1) spermatozoa from the distal portion of the epididymis displayed brilliant fluorescent acrosomes and less intensely stained midpieces and principal pieces when treated as dried smears in both the direct and indirect methods; 2) testicular spermatozoa were similarly stained but whereas in epididymal spermatozoa the whole acrosome stained intensely, the testicular spermatozoal acrosome displayed intense fluorescence of the inner acrosome; 3) protoplasmic droplets fluoresced strongly; 4) cross-reactivity was observed between human and guinea pig sperm but not between rat and guinea pig sperm, indicating an antigenic relationship between human and guinea pig but not between guinea pig and rat; 5) treatment of viable, motile guinea pig spermatozoa with fluorescent globulins resulted in agglutination and immobilization as well as formation of antigen-antibody aggregates adherent to the cell membrane of the head, midpiece and principal piece; the formation of such fluorescent aggregates in the medium surrounding the treated motile sperm was indicative of leaching of antigenic material from the sperm cells.This investigation was supported by funds from United States Public Health Service grant HE-05798-03, The Ford Foundation and National Science Foundation.     

Biochemical characterization of two ram cauda epididymal maturation-dependent sperm glycoproteins

Rabbit polyclonal antibodies were raised against ram cauda epididymal sperm proteins solubilized by N-octyl-beta-D-glucopy-ranoside (anti-CESP) and against proteins of the fluid obtained from the cauda epididymidis (anti-CEF). The anti-CESP polyclonal antibody reacted with several bands from 17 to 111 kDa with different regionalization throughout the epididymis. The strongest epitopes at 17 kDa and 23 kDa were restricted to the cauda epididymidis. The anti-CEF polyclonal antibody reacted mainly with a 17-kDa and a 23-kDa compound in the cauda sperm extract. These cauda epididymal 17- and 23-kDa proteins disappeared after orchidectomy, but they reappeared in the same regions after testosterone supplementation, indicating that they were secreted by the epithelium. The fluid and membrane 17- and 23-kDa antigens had a low isoelectric point and were glycosylated. The fluid 17- and 23-kDa proteins had hydrophobic properties: they were highly enriched in the Triton X-114 detergent phase and could be extracted from the cauda epididymal fluid by a chloroform-methanol mixture. These proteins were further purified, and their N-terminal sequences did not match any protein in current databases. A polyclonal antibody against the fluid 17-kDa protein recognized the protein in the cauda epididymal sperm extract and immunolocalized it on the sperm flagellum membrane and at the luminal border of all cells in the cauda epididymal epithelium. These results indicated that secreted glycoproteins with hydrophobic properties could be directly integrated in a specific domain of the sperm plasma membrane.     

小鼠和豚鼠精子在附睾成熟过程中Ca~(2 )分布变化规律的研究

小鼠附睾头精子,其头部Ca~(2 )在顶体前区顶体外膜内侧结合最多,Ca~(2 )沉淀反应颗粒于该处呈连续层状。附睾头豚鼠精子其头部结合Ca~(2 )含量很少,且主要结合于顶体前区腹面顶体外膜内侧。小鼠附睾体和附睾尾精子Ca~(2 )的分布特征基本上和附睾头精子相同。但豚鼠附睾尾精子顶体外膜内侧无Ca~(2 )结合。和附睾头、附睾尾的附睾液相比,附睾体附睾液基质内具有大量Ca~(2 )存在。附睾体柱状上皮细胞的微绒毛切面上也具有Ca~(2 )沉淀反应颗粒,微绒毛可能与附睾液Ca~(2 )含量的调节有关。精子尾部Ca~(2 )主要分布于线粒体内,在质膜内、外两侧和线粒体外膜外侧也结合有少量的Ca~(2 )。和小鼠精子相比,豚鼠精子尾部线粒体内具有大量的Ca~(2 )。     

Localization of sulfated glycoprotein-2 (clusterin) on spermatozoa and in the reproductive tract of the male rat

Sulfated glycoprotein-2 (SGP-2) is one of the major proteins secreted by rat Sertoli cells and epididymal cells in culture. The disulfide-linked dimeric protein secreted by Sertoli cells and found in seminiferous tubule fluid is composed of monomers of Mr 47 000 and 34 000 whereas the epididymal protein exhibits monomers of Mr 40 000 and 29 000. When both forms were chemically or enzymatically deglycosylated, they yielded proteins of similar molecular weight. No modification of the higher molecular weight testicular form by epididymal cells or fluids could be detected in incubation media. SGP-2 mRNA was localized in epididymal epithelium by in situ hybridization. Northern blot analysis indicated the testicular and epididymal mRNAs were of similar size. These findings suggest that the two forms of the protein occur because of tissue-specific post-translational modifications. The detergent-extracted protein from washed testicular spermatozoa is of the higher molecular weight form while epididymal sperm carry the lower molecular weight form. Immunohistochemical evidence suggests that the testicular form is removed prior to the initial segment of the epididymis and the epididymal form is applied in the proximal caput epididymidis. SGP-2 was immunolocalized to the sperm membrane at the ultrastructural level and was distinctly different from the immunolocalization of outer dense fiber proteins and fibrous sheath proteins.     

Isolation and characterization of a 25-kilodalton protein from mouse testis: sequence homology with a phospholipid-binding protein.

A 25-kDa epididymal secretory protein (MEP 9), isolated from mouse epididymal fluid, has recently been characterized in our laboratory [Rankin et al., Biol Reprod 1992; 46:747-766]. The polyclonal antibody raised against this protein was found to recognize a 25-kDa component in epididymal fluid and testicular extract. The 25-kDa testicular antigen (MTP) was purified by means of ammonium sulfate precipitation, gel filtration, and anion-exchange chromatography; MTP was found to be similar to MEP 9 in several properties including molecular mass (25 kDa), isoelectric point (pI 6.0), and immunoreactivity when the proteins were resolved in the presence of SDS (one-dimensional and two-dimensional PAGE). However, when the proteins were resolved under non-denaturing conditions, MTP showed strong immunoreactivity while MEP 9 did not. This observation suggests that although the 25-kDa antigens from the epididymal fluid and testicular extract are quite similar, they may have different immunological conformations. When analyzed for amino acid composition and partial amino acid sequence, the testicular antigen showed substantial homology (> 80%) with a phosphatidylethanolamine-binding protein characterized from bovine brain. MTP also showed phosphatidylethanolamine-binding activity (Kd = 1.95 x 10(-5) M, Bmax = 1.86 nmol/micrograms MTP), suggesting that the mouse 25-kDa protein is a member of the phospholipid-binding protein family and may have a role in lipid metabolism during sperm maturation.