35S-labeled Proteins Research Articles

The Arabidopsis cellulose synthase complex: a proposed hexamer of CESA trimers in an equimolar stoichiometry.

Cellulose is the most abundant renewable polymer on Earth and a major component of the plant cell wall. In vascular plants, cellulose synthesis is catalyzed by a large, plasma membrane-localized cellulose synthase complex (CSC), visualized as a hexameric rosette structure. Three unique cellulose synthase (CESA) isoforms are required for CSC assembly and function. However, elucidation of either the number or stoichiometry of CESAs within the CSC has remained elusive. In this study, we show a 1:1:1 stoichiometry between the three Arabidopsis thaliana secondary cell wall isozymes: CESA4, CESA7, and CESA8. This ratio was determined utilizing a simple but elegant method of quantitative immunoblotting using isoform-specific antibodies and (35)S-labeled protein standards for each CESA. Additionally, the observed equimolar stoichiometry was found to be fixed along the axis of the stem, which represents a developmental gradient. Our results complement recent spectroscopic analyses pointing toward an 18-chain cellulose microfibril. Taken together, we propose that the CSC is composed of a hexamer of catalytically active CESA trimers, with each CESA in equimolar amounts. This finding is a crucial advance in understanding how CESAs integrate to form higher order complexes, which is a key determinate of cellulose microfibril and cell wall properties.

Microinjection of RNA and Preparation of Secreted Proteins from Xenopus Oocytes

Microinjection of Xenopus oocytes or embryos with messenger RNA (mRNA) is a powerful technique for studying development, including protein expression during development. This protocol describes a method for the preparation of secreted (35)S-labeled proteins following mRNA microinjection into Xenopus oocytes.

Development of a new method for absolute protein quantification on 2‐D gels

With the development of systems biology projects aimed at modeling the cell, accurate and absolute measurements of cellular protein concentrations are increasingly important. However, methods for absolute quantification at the proteomic level remain rare. Using the yeast Saccharomyces cerevisiae, we propose a new method based on the radioactive labeling with an (35)S compound and 2-D PAGE. The principle is simple: cells are grown for more than four generations in the presence of a unique sulfur source labeled at a defined specific radioactivity, ensuring that more than 90% of the proteins are labeled at the same specific radioactivity as the sulfur source. After separation of (35)S-labeled proteins on 2-D gels, each protein is counted. The amount of each protein present in the gel is then calculated, from which is deduced the amount of each protein per cell. The method, limited to soluble and abundant proteins visible on 2-D gels, is simple, precise and reproducible and does not require an internal standard. We use it to compare the amounts of proteins in two growth conditions: 100 microM sulfate or 500 microM methionine. Up to now, we only had transcriptional data on the expression of these proteins in both conditions.

The STAT3 DNA-binding domain mediates interaction with NF-kappaB p65 and inducible nitric oxide synthase transrepression in mesangial cells.

Signal transducer and activator of transcription 3 (STAT3) and nuclear factor kappaB (NF-kappaB) are important transcription factors involved in glomerulonephritis and other inflammatory processes, including transcription of the inducible nitric oxide synthase (iNOS) gene. The ability of STAT3 to interact physically with NF-kappaB p65 in glomerular mesangial cells and thereby to inhibit NF-kappaB-mediated transactivation of the iNOS gene was demonstrated previously. STAT3 is a modular protein with several structurally and functionally defined domains. For defining STAT3 domains that interact with NF-kappaB p65, (35)S-labeled proteins that corresponded to each STAT3alpha domain were synthesized, and their ability to bind specifically a GST-NF-kappaB p65 fusion protein in GST pulldown assays was tested. The coiled-coil and DNA-binding domains were specifically retained by GST-NF-kappaB p65, whereas the N-terminal, linker domain, Src homology 2 domain, and transcriptional activation domain failed to interact with NF-kappaB p65. Deletion of the region L(358) through I(369) of the STAT3 DNA-binding domain greatly reduced binding to GST-NF-kappaB p65. Alanine substitution mutations at four highly conserved residues-L(358), N(359), K(363), and V(366)-in this region greatly abrogated the ability of STAT3 to bind NF-kappaB p65. Moreover, in contrast to the transrepression afforded by wild-type STAT3alpha, a STAT3alpha construct harboring these mutations, failed to suppress endogenous NO production and to transrepress iNOS promoter-reporter and kappaB element-reporter constructs in IL-beta-stimulated mesangial cells. These data reveal a novel role for the DNA-binding domain in the physical and functional coupling of STAT3 to NF-kappaB p65 that is important for regulating the transcriptional activity of iNOS and likely other NF-kappaB p65 responsive genes that are important for mesangial cell responses.

Detection of autoantibodies against the pituitary-specific proteins in patients with lymphocytic hypophysitis.

Several reports have described antipituitary antibodies by immunofluorescent or immunoblotting methods in patients with lymphocytic hypophysitis. However, with the exception of the pituitary hormones, individual antigens specific for the pituitary gland have not been studied. To understand the pathogenesis of lymphocytic hypophysitis and to diagnose this disease efficiently, we studied the presence of autoantibodies against three pituitary-specific proteins, GH and two novel pituitary-specific proteins, namely, pituitary gland specific factor 1a (PGSF1a) and PGSF2. Seventeen patients with lymphocytic hypophysitis, all of whom had pituitary enlargement (5 with lymphocytic adenohypophysitis and 12 with lymphocytic infundibuloneurohypophysitis, including 3 of the latter group proven by biopsy), and 14 patients with hypopituitarism without pituitary enlargement (10 with isolated ACTH deficiency and 4 with idiopathic TSH deficiency) were studied, and compared with 11 patients with non-functioning pituitary macroadenoma, 31 patients with other autoimmune diseases, and 36 healthy controls. The presence of each antibody was studied by radioligand assay using recombinant human (35)S-labeled protein. Three (18%) patients with lymphocytic hypophysitis having pituitary enlargement, five (36%) patients with hypopituitarism without pituitary enlargement and three (9.7%) patients with other autoimmune diseases were positive for one or more of the antibodies studied. Anti-human GH, anti-PGSF1a, and anti-PGSF2 antibodies were detected in patients with lymphocytic hypophysitis and other hypopituitarism, but were not detected in patients with non-functioning pituitary macroadenoma. Detection of these antibodies may be useful for the diagnosis of lymphocytic hypophysitis.

Prion protein is secreted in soluble forms in the epididymal fluid and proteolytically processed and transported in seminal plasma.

The presence of prion protein in sperm and fluids collected from different parts of the ram genital tract was investigated by immunoblotting with monoclonal antibodies. A slightly immunoreactive 25- to 30-kDa protein was recognized on Western blots of testicular and epididymal sperm extracts. Immunoreactivity increased on ejaculated sperm extracts and 2 other bands at 35 and 43 kDa also reacted. Seminal plasma showed several immunoreactive bands, the main bands being detected at 43 and 35 kDa, whereas less reactive bands were observed at 30, 25, 20, and <14 kDa. All these bands strongly decreased in the seminal plasma after vasectomy, indicating a testicular or an epididymal origin. Testicular fluid showed almost no reactivity, whereas caudal epididymal fluid contained the 2 strong immunoreactive bands at 43 and 35 kDa and in some cases a faint 30-kDa band. The 43-kDa band was also found in the fluid from the proximal caput, whereas the 35-kDa band appeared in the distal caput. Immunoprecipitation of (35)S-labeled proteins secreted in the epididymal fluid indicated that the 43-kDa form was synthesized in caput and caudal regions and the 35-kDa form in the distal caput to the distal corpus. Treatment of caudal fluid and seminal plasma by N-glycosidase resulted in the formation of 3 bands: 1 highly reactive at about 25 kDa, a second less reactive at about 28 kDa, and a third at approximately 20 kDa. The pattern of prion protein distribution in epididymal fluids was found to be similar in scrapie-infected rams to that of healthy rams. Cauda epididymal fluid and seminal plasma from infected animals could not be treated directly with proteinase K, because of the presence of protease inhibitors. However, the prion protein immunoprecipitated from these fluids was completely cleaved by proteinase K, whereas in the same conditions this from an infected sheep brain gave the usual resistant band pattern.

Selection and identification of proteins bound to DNA triple-helical structures by combination of 2D-electrophoresis and MALDI-TOF mass spectrometry.

Identification of proteins binding specifically to peculiar nucleic acid structures can lead to comprehension of their role in vivo and contribute to the discovery of structure-related gene regulation. This work was devoted to establishing a reliable procedure to select proteins on the basis of their interaction with a nucleic acid probe chosen to fold into a given structure. 2D-electrophoresis and mass spectrometry were combined for protein identification. We applied this procedure to select and identify triplex-binding activities in HeLa nuclear extracts. To achieve this, we used a panel of deoxyribonucleic probes adopting intramolecular triple-helices, varying in their primary sequence, structure or triple-helix motif. A limited number of spots was reproducibly revealed by South-western blotting. Spots of interest were localised among a complex population of (35)S-labelled proteins according to their (32)P-specific emission. Position of the same spots was extrapolated on a preparative gel coloured with Coomassie blue, allowing excision and purification of the corresponding proteins. The material was subjected to mass spectrometry upon trypsin digestion and MALDI-TOF peptide fingerprinting was used for research in databases: five of them were identified and found to belong to the hnRNP family (K, L, A2/B1, E1 and I). The identities of several of them were confirmed by comparing western and South-western blots on the same membrane using specific antibodies. The recognition specificity of most of these proteins is large, according to previous reports and our own experiments. It includes pyrimidine-rich DNA sequences in different contexts: single strand to a small extent, triplex and possibly other higher-order structures.

Fusion Proteins for Combined Analysis of Autoantibodies to the 65-kDa Isoform of Glutamic Acid Decarboxylase and Islet Antigen-2 in Insulin-dependent Diabetes Mellitus

Prediction, risk assessment, and diagnosis of autoimmune diseases often rely on detection of autoantibodies directed to multiple target antigens, such as the 65-kDa isoform of glutamic acid decarboxylase (GAD65-abs) and the tyrosine phosphatase-like protein islet antigen-2 (IA2-abs), the two major subspecificities of islet cell antibodies (ICAs) associated with insulin-dependent diabetes mellitus. We hypothesized that a combination of autoantigens in a fusion protein unifying the important immunodominant epitopes could provide an efficient target for cost-effective, one-step screening of sera. Chimeric proteins composed of GAD65 and IA2 residues were constructed, analyzed for their immune reactivity with monoclonal antibodies and sera, and used in a diagnostic assay with (35)S-labeled protein as antigen. Length and order of GAD65 and IA2 sequences were critical for conservation of the conformational epitopes in the fusion protein. Among four chimera tested, only IA2((606-979))/GAD65((1-585)) retained wild-type-like folding of GAD65 and IA2 domains and yielded a stable protein after baculovirus expression. Reactivity of GAD65 antibody- and IA2 antibody-positive sera from patients newly diagnosed with insulin-dependent diabetes mellitus, from ICA-positive prediabetics, and from ICA-positive first-degree relatives demonstrated conservation of the relevant autoreactive epitopes. The assay based on the in vitro translated fusion antigen had a sensitivity and specificity identical to those for detection of GAD65- and IA2-abs based on the two separate GAD65 and IA2 proteins. Autoantigens such as GAD65 and IA2 can be combined successfully in a fusion protein of similar immune reactivity. This allows simultaneous detection of GAD65- and IA2-abs in a one-step screening assay and cost-effective identification of positive individuals at risk of diabetes or at onset of disease.

Lipopolysaccharide-binding protein and phospholipid transfer protein release lipopolysaccharides from gram-negative bacterial membranes.

Although animals mobilize their innate defenses against gram-negative bacteria when they sense the lipid A moiety of bacterial lipopolysaccharide (LPS), excessive responses to this conserved bacterial molecule can be harmful. Of the known ways for decreasing the stimulatory potency of LPS in blood, the binding and neutralization of LPS by plasma lipoproteins is most prominent. The mechanisms by which host lipoproteins take up the native LPS that is found in bacterial membranes are poorly understood, however, since almost all studies of host-LPS interactions have used purified LPS aggregates. Using native Salmonella enterica serovar Typhimurium outer membrane fragments (blebs) that contained (3)H-labeled lipopolysaccharide (LPS) and (35)S-labeled protein, we found that two human plasma proteins, LPS-binding protein (LBP) and phospholipid transfer protein (PLTP), can extract [(3)H]LPS from bacterial membranes and transfer it to human high-density lipoproteins (HDL). Soluble CD14 (sCD14) did not release LPS from blebs yet could facilitate LBP-mediated LPS transfer to HDL. LBP, but not PLTP, also promoted the activation of human monocytes by bleb-derived LPS. Whereas depleting or neutralizing LBP significantly reduced LPS transfer from blebs to lipoproteins in normal human serum, neutralizing serum PLTP had no demonstrable effect. Of the known lipid transfer proteins, LBP is thus most able to transfer LPS from bacterial membranes to the lipoproteins in normal human serum.

The polyester polyurethanase gene (pueA) from Pseudomonas chlororaphis encodes a lipase.

A gene (pueA, polyurethane esterase A) encoding an extracellular polyurethanase (PueA) was cloned from Pseudomonas chlororaphis into Escherichia coli. The enzyme secreted from E. coli showed esterase activity when assayed with p-nitrophenyl acetate. Subcloning of a 3. 2-kb SalI-EcoRI fragment into a T7 RNA polymerase expression vector (pT7-6) produced a (35)S-labeled protein of 65 kDa. Nucleotide sequencing of pueA showed an open reading frame encoding a 65-kDa protein of 617 amino acid residues, with the serine hydrolase consensus sequence GXSXG. PueA was over-expressed using the pT7-6 vector transformed into E. coli BL21(DE3) and was purified in one step using Sephadex G-75.

Galectin 1 Modulates Attachment, Spreading and Migration of Cultured Vascular Smooth Muscle Cells via Interactions with Cellular Receptors and Components ofExtracellu lar Matrix

Galectin 1 (Gal-1), a lactose-binding lectin, is a component of vascular extracellular matrix and secreted by human vascular smooth muscle cells (SMCs). The purpose of this study was to investigate a possible role of Gal-1 in controlling adhesion and migration of cultured human vascular SMCs. Gal-1 co-localised with laminin and cellular fibronectin in extracellular matrix (ECM) secreted by cultured human vascular SMCs. Recombinant glutathione S-transferase (GST)-Gal-1 fusion protein bound to laminin and cellular fibronectin in ELISA. GST-Gal-1 inhibited SMC attachment to laminin via interactions with both SMCs and laminin. GST-Gal-1 inhibited SMC spreading on plastic or on laminin, but not on cellular fibronectin. GST-Gal-1 modulated SMC migration on laminin and inhibited migration on cellular fibronectin. GST-Gal-1 bound to several ³⁵S-labelled proteins in SMC extracts including laminin and α1β1 integrin, identified by depletion of SMC protein extracts with respective antibodies. We conclude that Gal-1 is able to modulate SMC attachment, spreading and migration via interactions with ECM proteins and α1β1 integrin.

Expression of stress proteins in cultured human cells as a sensitive indicator of metal toxicity.

Human cell lines cultured under standardized conditions are found to react after a wide variety of aggressive but sub-lethal treatments by expressing stress proteins, mainly of the HSP68, HSP70 and HSP90 families. The stress reaction is a repair mechanism which rapidly takes place after minute damages caused to cell structures. It was therefore proposed to develop a sensitive biomarker to monitor environmental pollution, especially related to metals, based on the evaluation of stress protein production. This was achieved by computer-assisted densitometric analysis of autoradiographies of electrophoretically separated (35)S-labelled proteins. The results indicate that our biological models consisting in HT29 and HepG2 cell-lines react to low concentrations of cadmium or nickel by a clear-cut increase of stress proteins expression. In most cases, this effect is much more significant and much more rapid to observe than changes in growth curves. It may constitute a reliable index of cell susceptibility to environmental aggressions.

Analysis of DNA‐Protein Interactions Using Proteins Synthesized In Vitro from Cloned Genes

To detect DNA binding activity, radiolabeled protein is incubated with specific DNA fragments, and protein-DNA complexes are separated from free protein by electrophoresis in native acrylamide gels. Unlike the more conventional mobility shift assay which utilizes (32)P-labeled DNA and unlabeled protein, the assay described here generally utilizes (35)S-labeled protein and unlabeled DNA. Major advantages of this method are that any desired mutant protein can be tested for its DNA-binding properties simply by altering the DNA template, and the subunit structure (e.g., dimer, tetramer) can be determined.

Influence of Temperature Stress on in Vitro Fertilization and Heat Shock Protein Synthesis in Maize (Zea mays L.) Reproductive Tissues

This study was conducted to investigate the response of maize (Zea mays) male and female mature reproductive tissues to temperature stress. We have tested the fertilization abilities of the stressed spikelets and pollen using in vitro pollination-fertilization to determine their respective tolerance to stress. The synthesis of heat shock proteins (HSPs) was also analyzed in male and female tissues using electrophoresis of (35)S-labeled proteins and fluorography, to establish a relationship between the physiological and molecular responses. Pollen, spikelets, and pollinated spikelets were exposed to selected temperatures (4, 28, 32, 36, or 40 degrees C) and tested using an in vitro fertilization system. The fertilization rate is highly reduced when pollinated spikelets are exposed to temperatures over 36 degrees C. When pollen and spikelets are exposed separately to temperature stress, the female tissues appear resistant to 4 hours of cold stress (4 degrees C) or heat stress (40 degrees C). Under heat shock conditions, the synthesis of a typical set of HSPs is induced in the female tissues. In contrast, the mature pollen is sensitive to heat stress and is responsible for the failure of fertilization at high temperatures. At the molecular level, no heat shock response is detected in the mature pollen.

Heat-shock proteins are associated with hnRNA in Drosophila melanogaster tissue culture cells

Ribonucleoprotein complexes of Drosophila melanogaster Kc tissue culture cells grown at 24 degrees C or heat-shocked at 37 degrees C were cross-linked in vivo by u.v. irradiation. Cross-linked heterogeneous nuclear ribonucleoprotein (hnRNP) complexes were fractionated by oligo(dT)-cellulose chromatography and CsCI density centrifugation. The hnRNP complexes of both 24 degrees C and 37 degrees C culture cells possess buoyant densities in CsCI between varrho = 1.38 g/cm and 1.43 g/cm. The S-labelled proteins bound to the hnRNA of 37 degrees C culture cells correspond in mol. wt. to the so-called heat-shock proteins of 70 K, 68 K, 27 K, 26 K, 23 K and 22 K. The 70 K and 68 K proteins are also present in hnRNP complexes of 24 degrees C culture cells. In addition, several other Drosophila hnRNPs of 140 K, 56 K, 45 K, 43 K, 38 K, 37 K and 34 K, whose synthesis is strongly repressed under heat-shock conditions, could be identified. The results demonstrate that the so-called heat-shock proteins possess a function as RNPs.

Tubulin from cultured tobacco cells: isolation and identification based on similarities to brain tubulin

The microtubule protein, tubulin, was isolated from most other proteins of cell suspension cultures of Nicotiana tabacum L. by its copolymerization with cow-brain tubulin. Cow-brain tubulin was added to the soluble protein fraction of extract from (35)S-labeled tobacco cells and subjected to two cycles of temperature-dependent assembly-disassembly (copolymerization). When analyzed by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) about 70% of the radioactivity in the twice copolymerized protein was found in a prominent doublet migrating close to the doublet of brain tubulin. When analyzed by two-dimensional isoelectric-focusing-SDS-PAGE the radioactive doublet behaved like the doublet of brain tubulin. Limited proteolysis of the individual polypeptides of the coublets showed that, while the peptide maps of the leading radioactive band and of the β-subunit of brain tubulin were virtually indistinguishable, the maps of the trailing radioactive band and of the α-subunit of brain tubulin, though similar, were not identical. Most of the copolymerized (35)S-labeled protein also behaved like brain tubulin during gel filtration and ion-exchange chromatography. It is concluded that the doublet of radioactive polypeptides isolated by copolymerization with brain tubulin are tobacco tubulin polypeptides that have, in their native as well as denatured forms, properties very similar to, but not identical with, cow brain tubulin. Apparently, tubulin has been highly conserved during evolution.

Differential Protein Composition and Gene Expression in Leaf Mesophyll Cells and Bundle Sheath Cells of the C4 Plant Digitaria sanguinalis (L.) Scop.

The distribution and molecular weights of cellular proteins in soluble and membrane-associated locations were analyzed using sodium dodecyl sulfate-polyacrylamide gel electrophoresis and Coomassie blue staining of leaf (Digitaria sanguinalis L. Scop.) extracts and isolated cell extracts. Leaf polypeptides also were pulse-labeled, followed by isolation of the labeled leaf cell types and analysis of the newly synthesized polypeptides in each cell type by electrophoresis and fluorography.Comparison of the electrophoretic patterns of crabgrass whole leaf polypeptides with isolated cell-type polypeptides indicated a difference in protein distribution patterns for the two cell types. The mesophyll cells exhibited a greater allocation of total cellular protein into membrane-associated proteins relative to soluble proteins. In contrast, the bundle sheath cells exhibited a higher percentage of total cellular protein in soluble proteins. Phosphoenolpyruvate carboxylase was the major soluble protein in the mesophyll cell and ribulose bisphosphate carboxylase was the major soluble protein in the bundle sheath cell. The majority of in vivo(35)S-pulse-labeled proteins synthesized by the two crabgrass cell types corresponded in molecular weight to the proteins present in the cell types which were detected by conventional staining techniques. The bundle sheath cell and mesophyll cell fluorograph profiles each had 15 major (35)S-labeled proteins. The major incorporation of (35)S by bundle sheath cells was into products which co-electrophoresed with the large and small subunits of ribulose bisphosphate carboxylase. In contrast, a major (35)S-labeled product in mesophyll cell extracts co-electrophoresed with the subunit of phosphoenolpyruvate carboxylase. Both cell types exhibited equivalent in vivo labeling of a polypeptide with one- and two-dimensional electrophoretic behavior similar to the major apoprotein of the light-harvesting chlorophyll a/b protein. Results from the use of protein synthesis inhibitors during pulse-labeling experiments indicated intercellular differences in both organelle and cytoplasmic protein synthesis. A majority of the (35)S incorporation by crabgrass mesophyll cell 70S ribosomes was associated with a pair of membrane-associated polypeptides of molecular weight 32,000 and 34,500; a comparison of fluorograph and stained gel profiles suggests these products resemble the precursor and mature forms of the maize chloroplast 32,000 dalton protein reported by Grebanier et al. (1978 J. Cell Biol. 28:734-746). In contrast, crabgrass bundle sheath cell organelle translation was directed predominantly into a product which co-electrophoresed with the large subunit of ribulose bisphosphate carboxylase.

Cell-free translation and partial characterization of proenkephalin messenger RNA from bovine striatum

Summary The biosynthesis of the primary precursor of the enkephalins (proenkephalin) was investigated by cell-free translation of bovine striatum polyadenylated RNA. 35 S-Labeled proteins encoded by striatum mRNA were digested with trypsin and carboxypeptidase B, and released [ 35 S]Met-enkephalin was identified by immunoprecipitation and high-performance liquid chromatography. The apparent molecular weight of putative proenkephalin synthesized in cell-free systems was found to be M r 31,000 ± 1000. Digestion of cell-free translated 3 H-labeled proteins of M r 31,000 ± 3000 yielded both [ 3 H]Metenkephalin and [ 3 H]Leu-enkephalin. Putative proenkephalin mRNA of striatum was found to be 1450 ± 200 nucleotides in length.

Electrophoretic evidence for the identity of the major zinc-binding polypeptides in the rat sperm tail.

Zinc, an important metallic constituent of mammalian spermatozoa, is incorporated during spermiogenesis into the dense fibers of the sperm tail. To test its relative affinity for sperm structural proteins, ‘5Zn was administered intratesticularly to mature Sprague-Dawley rats and recovered in cauda epididyrnidal spermatozoa 16-18 days postinjection, when its concentration in these cells was maximal. As expected, the “Zn was localized almost entirely within tail structures stabilized by -S-S- bonds (tail keratin), at least 75% being found within a fraction consisting mainly of dense fibers. After electrophoresis in dodecyl sulfate-polyacrylamide gels, tail keratin or dense fiber fractions labeled by intratesticular injection of 135S1-cysteine showed three peaks of “Slabeled protein, with apparent molecular weights of 24,000, 15,000 and 11,000. Only the first of these protein components retained substantial amounts of “Zn during co-electrophoresis of 35S and “Zn-labeled fractions. Following carboxymethylation, it was revealed to be heterogeneous, consisting of at least 2 polypeptides, whose unexpectedly low electrophoretic mobilities corresponded to molecular weights of 36,000 and 30,000. Efficient retention of “Zn by its unmodified precursor during electrophoresis required the presence of at least 40 mM dithiothreitol in the applied samples and was achieved equally well whether the “Zn was introduced in vitro or in vivo. Selective binding of “Zn by some, but not all, of the cysteine-rich polypeptides in the sperm tail extracts may be of physiological significance, particularly since those macromolecules that demonstrated the greatest affinity for zinc are believed to be the major constituents of dense fibers.