Molecular cloning and sequencing of calnexin-t. An abundant male germ cell-specific calcium-binding protein of the endoplasmic reticulum

Structural Analysis of Calreticulin, an Endoplasmic Reticulum-Resident Molecular Chaperone

Calreticulin (Calr) is an endoplasmic reticulum (ER) chaperone involved in protein quality control, Ca²⁺ regulation and other cellular processes. The structure of Calr is unusual, reflecting different functions of the protein: a proline-rich β-hairpin arm and an acidic C-terminal tail protrude from a globular core, composed of a β-sheet sandwich and an α-helix. The arm and tail interact in the presence of Ca²⁺ and cover the upper β-sheet, where a carbohydrate-binding site gives the chaperone glycoprotein affinity. At the edge of the carbohydrate-binding site is a conserved, strained disulphide bridge, formed between C¹⁰⁶ and C¹³⁷ of human Calr, which lies in a polypeptide-binding site. The lower β-sheet has several conserved residues, comprised of a characteristic triad, D¹⁶⁶-H¹⁷⁰-D¹⁸⁷, Tyr¹⁷² and the free C¹⁶³. In addition to its role in the ER, Calr translocates to the cell surface upon stress and functions as an immune surveillance marker. In some myeloproliferative neoplasms, the acidic Ca²⁺-binding C-terminal tail is transformed into a polybasic sequence.

RNA Seq analysis of the Eimeria tenella gametocyte transcriptome reveals clues about the molecular basis for sexual reproduction and oocyst biogenesis

Background

The protozoan Eimeria tenella is a common parasite of chickens, causing avian coccidiosis, a disease of on-going concern to agricultural industries. The high prevalence of E. tenella can be attributed to the resilient oocyst stage, which is transmitted between hosts in the environment. As in related Coccidia, development of the eimerian oocyst appears to be dependent on completion of the parasite’s sexual cycle. RNA Seq transcriptome profiling offers insights into the mechanisms governing the biology of E. tenella sexual stages (gametocytes) and the potential to identify targets for blocking parasite transmission.

Results

Comparisons between the sequenced transcriptomes of E. tenella gametocytes and two asexual developmental stages, merozoites and sporozoites, revealed upregulated gametocyte transcription of 863 genes. Many of these genes code for proteins involved in coccidian sexual biology, such as oocyst wall biosynthesis and fertilisation, and some of these were characterised in more depth. Thus, macrogametocyte-specific expression and localisation was confirmed for two proteins destined for incorporation into the oocyst wall, as well as for a subtilisin protease and an oxidoreductase. Homologues of an oocyst wall protein and oxidoreductase were found in the related coccidian, Toxoplasma gondii, and shown to be macrogametocyte-specific. In addition, a microgametocyte gamete fusion protein, EtHAP2, was discovered.

Conclusions

The need for novel vaccine candidates capable of controlling coccidiosis is rising and this panel of gametocyte targets represents an invaluable resource for development of future strategies to interrupt parasite transmission, not just in Eimeria but in other Coccidia, including Toxoplasma, where transmission blocking is a relatively unexplored strategy.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-1298-6) contains supplementary material, which is available to authorized users.

The function of chaperone proteins in the assemblage of protein complexes involved in gamete adhesion and fusion processes

The remarkable complexity of the molecular events governing adhesion and fusion of the male and female gametes is becoming apparent. Novel research suggests that these highly specific cellular interactions are facilitated by multiprotein complexes that are delivered to and/or assembled on the surface of the gametes by molecular chaperones in preparation for sperm-egg interaction. While the activation of these molecular chaperones and the mechanisms by which they shuttle proteins to the surface of the cell remain the subject of ongoing investigation, a compelling suggestion is that these processes are augmented by dynamic membrane microdomains or lipid rafts that migrate to the apical region of the sperm head after capacitation. Preliminary studies of the oocyte plasma membrane have also revealed the presence of lipid rafts comprising several molecular chaperones, raising the possibility that similar mechanisms may be involved in the activation of maternal fusion machinery and the regulation of oocyte plasma membrane integrity. Despite these findings, the analysis of oocyte surface multiprotein complexes is currently lacking. Further analyses of the intermediary proteins that facilitate the expression of key players in sperm-egg fusion are likely to deliver important insights into this unique event, which culminates in the cytoplasmic continuity of the male and female gametes.

Calnexin phosphorylation: linking cytoplasmic signalling to endoplasmic reticulum lumenal functions

Calnexin is an abundant integral membrane phosphoprotein of the endoplasmic reticulum (ER) of eukaryotic cells. The role of the luminal domain as an N-glycoprotein specific lectin has been well-established. Cytosolic C-terminal domain phosphorylation of calnexin has recently been elucidated in glycoprotein folding and quality control. Signalling of the presence of unfolded proteins from the lumen of the ER is mediated by the three ER membrane sensor proteins Ire1, ATF6 and PERK. The observation that the C-terminus of calnexin is differentially phosphorylated when glycoproteins are misfolded initiated our search for functional roles of calnexin phosphorylation. Recent studies have defined a role for phosphorylation at a proline-directed kinase site (Ser563) in ER protein quality control, while phosphorylation at a casein kinase 2 site (Ser534, Ser544) may be linked to transport functions. There are also four other abundant integral membrane phosphoproteins in the ER, and these may be components of other signalling pathways that link and coordinate other ER functions with the rest of the cell.

Comparative contribution of the aryl hydrocarbon receptor gene to perinatal stage development and dioxin-induced toxicity between the urogenital complex and testis in the mouse

TCDD (2,3,7,8-tetrachlorodebenzo-p-dioxin) requires the presence of the aryl hydrocarbon receptor (Ahr) gene for its toxic effects, such as reproductive disorders in male offspring of maternally exposed rats and mice. To study the involvement of the Ahr gene in producing the toxic phenotype with respect to testicular development, we administered a relatively high dose of TCDD to mice with three different maternally derived Ahr genotypic traits, and then compared several Ahr-dependent alterations among male reproductive systems on Postnatal Day 14. Reduction in anogenital distance and expression of prostatic epithelial genes in the urogenital complex (UGC) were detected in Ahr(+/+) and Ahr(+/-) mice exposed to TCDD, whereas no difference was observed in Ahr(-/-) mice. In situ hybridization revealed the absence of probasin mRNA expression in the prostate epithelium, despite the obvious development of prostatic lobes in TCDD-exposed mice. In contrast to obvious prostatic dysfunction and induction of cytochrome P450 (CYP) family genes in the UGC by TCDD, no alterations in testicular functions were observed in germ cell/Sertoli cell/interstitial cell marker gene expression or CYP family induction. No histopathological changes were observed among the three genotypes and between control and TCDD-exposed mice. Therefore, mouse external genitalia and prostatic development are much more sensitive to TCDD treatment than testis. Further, the Ahr gene, analyzed in this study, does not significantly contribute to testicular function during perinatal and immature stages, and the developing mouse testis appears to be quite resistant to TCDD exposure.

The Ca2+/Mn2+ pumps in the Golgi apparatus

Recent evidence highlights the functional importance of the Golgi apparatus as an agonist-sensitive intracellular Ca(2+) store. Besides Ca(2+)-release channels and Ca(2+)-binding proteins, the Golgi complex contains Ca(2+)-uptake mechanisms consisting of the well-known sarco/endoplasmic reticulum Ca(2+)-transport ATPases (SERCA) and the much less characterized secretory-pathway Ca(2+)-transport ATPases (SPCA). SPCA supplies the Golgi compartments and, possibly, the more distal compartments of the secretory pathway with both Ca(2+) and Mn(2+) and, therefore, plays an important role in the cytosolic and intra-Golgi Ca(2+) and Mn(2+) homeostasis. Mutations in the human gene encoding the SPCA1 pump (ATP2C1) resulting in Hailey-Hailey disease, an autosomal dominant skin disorder, are discussed.

Effects of 3,3',4,4',5-pentachlorobiphenyl, a coplanar polychlorinated biphenyl congener, on cultured neonatal mouse testis

3,3',4,4',5-Pentachlorobiphenyl (PCB126), a congener with a planar configuration, has been established to have relatively strong toxicities similar to those of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) via aryl hydrocarbon receptors. We investigated the effects of this coplanar PCB on mammalian early spermatogenesis and steroidogenesis in a mouse neonatal testicular organ culture system. Testes collected from newborn mice were subjected to organ culture in medium containing 0, 10, 100 or 1000 nM PCB126. Histochemical analysis revealed that the BrdU-labeling indices of both spermatogenic cells and Sertoli cells were unchanged in all testis specimens exposed to the coplanar PCB. CYP1A1 and steroidogenic enzymes (P450scc, P450c17, 3beta-HSD and 17beta-HSD) mRNA levels were determined by semiquantitative RT-PCR. The CYP1A1 mRNA level in cultured testis was significantly increased by PCB126 in a dose-dependent manner. Although mRNA levels of 3beta-HSD and 17beta-HSD were unchanged, the P450scc mRNA level was significantly down-regulated by PCB126 in a dose-dependent manner. In contrast, the P450c17 mRNA level was significantly higher in 1000 nM PCB126-exposed testis than in control testis. These results suggest that the coplanar PCB does not alter the proliferative activity of spermatogenic cells and Sertoli cells in neonatal testis, but that it directly affects the expression of steroidogenic enzyme genes.

Major histocompatibility class I folding, assembly, and degradation: a paradigm for two-stage quality control in the endoplasmic reticulum

Protein folding in living cells is a complex process involving many interdependent factors. The primary site for folding of nascent proteins destined for secretion is the endoplasmic reticulum (ER). Several disease states, including cystic fibrosis, are brought about because of irregularities in protein folding. Under normal cellular conditions, "quality control" mechanisms ensure that only correctly folded proteins are exported from the ER, with incorrectly folded or incompletely assembled proteins being degraded. Quality control mechanisms can be divided into two broad processes: (1) Primary quality control involves general mechanisms that are not specific for individual proteins; these monitor the fidelity of nascent protein folding in the ER and mediate the destruction of incompletely folded proteins. (2) Partially folded or assembled proteins may be subject to secondary quality control mechanisms that are protein- or protein-family-specific. Here we use the folding and assembly of major histocompatibility complex (MHC) class I as an example to illustrate the processes of quality control in the ER. MHC class I, a trimeric complex assembled in the ER of virally infected or malignant cells, presents antigenic peptide to cytotoxic T lymphocytes; this mediates cell killing and thereby prevents the spread of infection or malignancy. The folding and assembly of MHC class I is subjected to both primary and secondary quality control mechanisms that lead either to correct folding, assembly, and secretion or to degradation via a proteasome-associated mechanism.

Calreticulin, a calcium-binding molecular chaperone, is required for stress response and fertility in Caenorhabditis elegans

Calreticulin (CRT), a Ca(2+)-binding protein known to have many cellular functions, including regulation of Ca(2+) homoeostasis and chaperone activity, is essential for heart and brain development during embryogenesis in mice. Here, we report the functional characterization of Caenorhabditis elegans calreticulin (crt-1). A crt-1 null mutant does not result in embryonic lethality but shows temperature-dependent reproduction defects. In C. elegans CRT-1 is expressed in the intestine, pharynx, body-wall muscles, head neurons, coelomocytes, and in sperm. crt-1 males exhibit reduced mating efficiency and defects late in sperm development in addition to defects in oocyte development and/or somatic gonad function in hermaphrodites. Furthermore, crt-1 and itr-1 (inositol triphosphate receptor) together are required for normal behavioral rhythms. crt-1 transcript level is elevated under stress conditions, suggesting that CRT-1 may be important for stress-induced chaperoning function in C. elegans.

Cytosolic phosphorylation of calnexin controls intracellular Ca(2+) oscillations via an interaction with SERCA2b

Calreticulin (CRT) and calnexin (CLNX) are lectin chaperones that participate in protein folding in the endoplasmic reticulum (ER). CRT is a soluble ER lumenal protein, whereas CLNX is a transmembrane protein with a cytosolic domain that contains two consensus motifs for protein kinase (PK) C/proline- directed kinase (PDK) phosphorylation. Using confocal Ca(2+) imaging in Xenopus oocytes, we report here that coexpression of CLNX with sarco endoplasmic reticulum calcium ATPase (SERCA) 2b results in inhibition of intracellular Ca(2+) oscillations, suggesting a functional inhibition of the pump. By site-directed mutagenesis, we demonstrate that this interaction is regulated by a COOH-terminal serine residue (S562) in CLNX. Furthermore, inositol 1,4,5-trisphosphate- mediated Ca(2+) release results in a dephosphorylation of this residue. We also demonstrate by coimmunoprecipitation that CLNX physically interacts with the COOH terminus of SERCA2b and that after dephosphorylation treatment, this interaction is significantly reduced. Together, our results suggest that CRT is uniquely regulated by ER lumenal conditions, whereas CLNX is, in addition, regulated by the phosphorylation status of its cytosolic domain. The S562 residue in CLNX acts as a molecular switch that regulates the interaction of the chaperone with SERCA2b, thereby affecting Ca(2+) signaling and controlling Ca(2+)-sensitive chaperone functions in the ER.

Calnexin family members as modulators of genetic diseases

The endoplasmic reticulum (ER) is an intracellular compartment devoted to the synthesis, segregation and folding of soluble and membrane secretory proteins. Some mutations in these proteins lead to their incorrect or incomplete folding in the ER. The ER has a quality control system which detects misfolded proteins and then specifies their fate. Some mutated proteins are retained in the ER wherein they accumulate (Russell bodies for misfolded immunoglobulin heavy chains, the PiZZ for alpha 1-antitrypsin), others are retrotranslocated from the ER and degraded by the cytosolic proteasomal system, and yet other proteins are eventually secreted (in AZC-treated cells). In this review we summarize the role of ER resident proteins in quality control of mutated secretory proteins.

In vitro reconstitution of calreticulin-substrate interactions

Calreticulin is a soluble, endoplasmic reticulum-resident protein and a molecular chaperone for glycoproteins. We have reconstituted the binding of recombinant calreticulin to two glycoprotein substrates, vesicular stomatitis virus G protein and influenza hemagglutinin, in vitro. The binding was found to be direct and to require monoglucosylated, asparagine-linked oligosaccharides on the substrate glycoprotein but no other cellular factors. The binding could be modulated in vitro by incubation of substrate with purified preparations of the glycan modifying enzymes glucosidase II and the UDP-glucose:glycoprotein glucosyltransferase, thus recapitulating the regulation of calreticulin-binding by glycan modification that occurs in vivo. Using the purified ER enzymes and the recombinant calreticulin, an assay was established for reconstituting a complex, multicomponent chaperone binding cycle in vitro. We demonstrated, moreover, that the acidic C-terminal 62 residues of calreticulin are dispensable for substrate binding whereas further deletions inhibit substrate binding.

Molecular chaperone calmegin localization to the endoplasmic reticulum of meiotic and post-meiotic germ cells in the mouse testis

Calmegin is a testis-specific Ca(2+)-binding protein that is homologous to calnexin. Recently, sperm from transgenic mice lacking calmegin have been shown to be infertile. To further characterize calmegin, we analyzed the precise stage of expression and the intracellular localization of this protein in germ cells during mouse spermatogenesis by an immunoperoxidase technique using the anti-calmegin monoclonal antibody TRA369. Light microscopic immunocytochemistry showed that calmegin appeared in early pachytene spermatocytes, with the highest expression in round and elongating spermatids, and disappeared in the maturation phase of spematids at step 15. Immunoelectron microscopy showed that selective localization was found at the endoplasmic reticulum membrane and the nuclear envelope of spermatogenic cells. During the maturation phase, a dramatic reduction in calmegin occurred in the endoplasmic reticulum of the spermatids, suggesting that the major function of calmegin has been completed by the time spermatids reach step 14. In addition, although the immunoreactivity was completely absent in the calmegin-deficient mutant mouse testis, ultrastructural analysis showed that mature sperm from the knockout mice were normal. This suggests that calmegin is not required for the morphogenesis of male germ cells. Thus, our results suggest that calmegin has a major role in mouse spermatogenesis, and also indicate that this protein would be useful as a maker molecule to study the functional role of the endoplasmic reticulum in the process of spermatid differentiation.

Characterization of domains in mice of calnexin-t, a putative molecular chaperone required in sperm fertility, with use of glutathione S-transferase-fusion proteins

Calnexin-t (calmegin) is a male germ cell-specific variant of calnexin, a membrane bound-molecular chaperone in the endoplasmic reticulum (ER). Although it is temporally expressed during spermatogenesis, it has recently been shown to be highly involved in sperm fertility. To investigate the biochemical states of calnexin-t during spermatogenesis, we produced a series of glutathione S-transferase-fusion proteins with several specific coding domains of calnexin-t. Immunostaining and 45Ca2+ overlay assays clearly showed that the internal proline-rich repeat region has Ca2+-binding ability and contains an epitope recognized by monoclonal antibody 1C9. Western blot analysis of protein extracts from the testes of 10-, 18-, 26-, and 60-day-old mice revealed only a single 101-kDa protein during testicular development by 1C9. Anti-C, a cytoplasmic domain-specific antibody generated by immunization with recombinant protein, produced the same results, indicating that the 101-kDa form of calnexin-t is prevalent at all stages of spermatogenesis expressing calnexin-t. In paraffin sections of mouse testis, Anti-C stained spermatocytes and spermatids intensely, whereas 1C9 stained spermatocytes only slightly but spermatids intensely, suggesting that the affinity of 1C9 for its epitope is lower in pachytene spermatocytes than in spermatids. Acid phosphatase treatment of the 101-kDa form generated a 93-kDa band that in turn could be recovered to the 101-kDa form by incubation with HeLa cell S100 fraction, indicating that the 101-kDa form is a phosphorylated type of calnexin-t. The sites of phosphorylation were shown to be restricted to the cytoplasmic domain. Our results suggest that the structure of the ER luminal domain of calnexin-t is likely to differ in middle pachytene versus haploid germ cell phases. In addition, the cytoplasmic domain of calnexin-t was shown to be highly phosphorylated immediately after protein synthesis and constitutively phosphorylated during spermatogenesis.

Differential modulation of SERCA2 isoforms by calreticulin

In Xenopus laevis oocytes, overexpression of calreticulin suppresses inositol 1,4,5-trisphosphate-induced Ca2+ oscillations in a manner consistent with inhibition of Ca2+ uptake into the endoplasmic reticulum. Here we report that the alternatively spliced isoforms of the sarcoendoplasmic reticulum Ca2+-ATPase (SERCA)2 gene display differential Ca2+ wave properties and sensitivity to modulation by calreticulin. We demonstrate by glucosidase inhibition and site-directed mutagenesis that a putative glycosylated residue (N1036) in SERCA2b is critical in determining both the selective targeting of calreticulin to SERCA2b and isoform functional differences. Calreticulin belongs to a novel class of lectin ER chaperones that modulate immature protein folding. In addition to this role, we suggest that these chaperones dynamically modulate the conformation of mature glycoproteins, thereby affecting their function.

Oligosaccharide binding characteristics of the molecular chaperones calnexin and calreticulin

Calnexin and calreticulin are homologous molecular chaperones of the endoplasmic reticulum. Their binding to newly synthesized glycoproteins is mediated, at least in part, by a lectin site that recognizes the early N-linked oligosaccharide processing intermediate, Glc1Man9GlcNAc2. We compared the oligosaccharide binding specificities of calnexin and calreticulin in an effort to determine the basis for reported differences in their association with various glycoproteins. Using mono-, di-, and oligosaccharides to inhibit the binding of Glc1Man9GlcNAc2 to calreticulin and to a truncated, soluble form of calnexin, we show that the entire Glc alpha 1-3Man alpha 1-2Man alpha 1-2Man structure, extending from the alpha 1-3 branch point of the oligosaccharide core, is recognized by both proteins. Furthermore, analysis of the binding of monoglucosylated oligosaccharides containing progressively fewer mannose residues suggests that for both proteins the alpha 1-6 mannose branch point of the oligosaccharide core is also essential for recognition. Consistent with their essentially identical substrate specificities, calnexin and calreticulin exhibited the same relative affinities when competing for binding to the Glc1Man9GlcNAc2 oligosaccharide. Thus, differential glycoprotein binding cannot be attributed to differences in the lectin specificities or binding affinities of calnexin and calreticulin. We also examined the effects of ATP, calcium, and disulfide reduction on the lectin properties of calnexin and calreticulin. Whereas oligosaccharide binding was only slightly enhanced for both proteins in the presence of high concentrations of a number of adenosine nucleotides, removal of bound calcium abrogated oligosaccharide binding, an effect that was largely reversible upon readdition of calcium. Disulfide reduction had no effect on oligosaccharide binding by calnexin, but binding by calreticulin was inhibited by 70%. Finally, deletion mutagenesis of calnexin and calreticulin identified a central proline-rich region characterized by two tandem repeat motifs as a segment capable of binding oligosaccharide. This segment bears no sequence homology to the carbohydrate recognition domains of other lectins.

Cloning and characterization of the human Calmegin gene encoding putative testis-specific chaperone

The putative chaperone Calmegin is required for sperm fertility in mouse and the relevance of the gene to certain cases of human male infertility has been suggested. In the present paper, we have isolated and characterized the human homolog cDNA of the mouse germ cell-specific Calmegin. The entire coding region of the human cDNA showed 80% identity with the previously reported mouse Calmegin. The predicted amino acid sequence showed strong conservation of the two sets of internal repetitive sequences (Ca2+ binding motif), and the hydrophilic COOH terminus, which corresponds to the putative endoplasmic reticulum (ER) retention motif. Our finding will support diagnosis of male infertility. Northern blotting analysis of various human tissues showed that the transcript was 3 kb in length and was expressed exclusively in the testis. Using the fluorescence in situ hybridization (FISH) technique, human Calmegin gene was mapped to chromosome 4q28.3-q31.1.

The putative chaperone calmegin is required for sperm fertility

The proper folding of newly synthesized membrane proteins in the endoplasmic reticulum (ER) is required for the formation of functional mature proteins. Calnexin is a ubiquitous ER chaperone that plays a major role in quality control by retaining incompletely folded or misfolded proteins. In contrast to other known chaperones such as heat-shock proteins, BiP and calreticulin, calnexin is an integral membrane protein. Calmegin is a testis-specific ER protein that is homologous to calnexin. Here we show that calmegin binds to nascent polypeptides during spermatogenesis, and have analysed its physiological function by targeted disruption of its gene. Homozygous-null male mice are nearly sterile even though spermatogenesis is morphologically normal and mating is normal. In vitro, sperm from homozygous-null males do not adhere to the egg extracellular matrix (zona pellucida), and this defect may explain the observed infertility. These results suggest that calmegin functions as a chaperone for one or more sperm surface proteins that mediate the interactions between sperm and egg. The defective zona pellucida-adhesion phenotype of sperm from calmegin-deficient mice is reminiscent of certain cases of unexplained infertility in human males.

Analysis of a 103 kbp cluster homology region from the left end of Saccharomyces cerevisiae chromosome I

The DNA sequence and preliminary functional analysis of a 103-kbp section of the left arm of yeast chromosome I is presented. This region, from the left telomere to the LTE1 gene, can be divided into two distinct portions. One portion, the telomeric 29 kbp, has a very low gene density (only five potential genes and 21 kbp of noncoding sequence), does not encode any "functionally important" genes, and is rich in sequences repeated several times within the yeast genome. The other portion, with 37 genes and only 14.5 kbp of noncoding sequence, is gene rich and codes for at least 16 "functionally important" genes. The entire gene-rich portion is apparently duplicated on chromosome XV as an extensive region of partial gene synteney called a cluster homology region. A function can be assigned with varying degrees of precision to 23 of the 42 potential genes in this region; however, the precise function is know for only eight genes. Nineteen genes encode products presently novel to yeast, although five of these have homologs elsewhere in the yeast genome.

Recent concepts in the management of male infertility

This review outlines the current treatment of many causes of male infertility, including idiopathic spermatogenic disorder, varicocele, obstruction of the seminal tract, and emission disorders. Recommendations regarding future treatments are presented.

Calnexin: its molecular cloning and expression in the liver of the frog, Rana rugosa

A 2.2-kb cDNA clone encoding the endoplasmic reticulum (ER) resident protein, calnexin (CLX), was isolated from the frog Rana rugosa liver cDNA library and sequenced. The sequence encoded 622 amino acids and was 77% similar to mammalian CLXs and 56% to mouse CLX-t. In the phylogenetic tree, mouse CLX-t was clearly diverged from other CLXs including frog. The amino acid sequence of CLX showed regions similar to those in frog calreticulin (CLT), although CLX, but not CLT, contained a histidine-rich region at the NH2-terminus. CLX gene expression was observed only in the liver among various tissues examined. Additionally, its expression was strong in the liver in 2-month post-metamorphosis frogs, but very weak in adults. The results suggest that the CLX gene is expressed in a tissue- and stage-dependent manner in the frog, R. rugosa, and that CLX is widely distributed in eukaryotic organisms.

Isolation and characterization of two monoclonal antibodies that recognize different epitopes of the human c-kit receptor

After immunizing mice with a human megakaryoblastic leukemia cell line, M-MOK, we obtained two monoclonal antibodies which recognize the human c-kit receptor. The monoclonal antibodies, designated MTK1 and MTK2, were found to specifically recognize Balb/3T3 cells transfected with human c-kit cDNA and not parent Balb/3T3 cells while showing different immunological, biochemical and biological behaviors. Both allowed visualization of the 140 kDa c-kit protein by Western blot analysis, but MTK1 detected only positive band with non-reducing conditions for sodium dodecyl sulfate-polyacrylamide gel electrophoresis. MTK1 partially inhibited the stem cell factor (SCF) induced proliferation of M-MOK cells, whereas, MTK2 was without effect. MTK1 also inhibited the bone marrow derived colony forming unit granulocyte/macrophage (CFU-GM) formed by granulocyte-macrophage colony stimulating factor (GM-CSF) and SCF. Not only anti-CD34 antibodies (HPCA-1) but also MTK1 could be shown to concentrate bone marrow CFU-GM and burst forming unit erythroid (BFU-E) effectively. The presently described monoclonal antibodies may therefore be useful for functional analysis of the ligand binding domain of the human c-kit receptor, as well as for further classification of hematopoietic stem cells in addition to the CD34 positive cells.