Synthesis of rat prostatic binding protein in Xenopus oocytes and in wheat germ

The biosynthesis of biologically active proteins in mRNA-microinjected Xenopus oocytes

The basic properties of mRNA-injected Xenopus oocytes as a heterologous system for the production of biologically active proteins will be reviewed. The advantages and limitations involved in the use of this in ovo system will be discussed, as compared with in vitro cell-free translation systems and with in vivo microinjected mammalian cells in culture. The different assay systems that have been utilized for the identification of the biological properties of oocyte-produced proteins will be described. This section will review the determination of properties such as binding of natural ligands, like heme or alpha-bungarotoxin; immunological recognition by antibodies; subcellular compartmentalization and/or secretion; various enzymatic catalytic activities; and induction in ovo of biological activities that affect other living cells in culture, such as those of interferon and of the T-cell receptor. The limitations involved in interpretation of results obtained using mRNA-injected oocytes will be critically reviewed. Special attention will be given to the effect of oocyte proteases and of changes in the endogenous translation rate on quantitative measurements of oocyte-produced proteins. In addition, the validity of the various measurement techniques will be evaluated. The various uses of bioassays of proteins produced in mRNA-injected Xenopus oocytes throughout the last decade will be reviewed. Nuclear and cytoplasmic injections, mRNA and protein turnover measurements and abundance calculations, and the use of in ovo bioassays for molecular cloning experiments will be discussed in this section. Finally, potential future uses of the oocyte system in various fields of research, such as immunology, neurobiology, and cell biology will be suggested.

Synthesis of androgen-dependent secretory proteins by rat seminal vesicles

Androgenic steroids control the synthesis and secretion of several proteins by the seminal vesicles of the male rat. Prominent among them are 2 basic proteins, S and F, previously used as markers of androgen action. These proteins are not found among translation products of a wheat-germ protein-synthesising system supplied with mRNA from seminal vesicles of normal rats. Instead, higher molecular weight forms, S' and F', are formed which are nonetheless related antigenically to S and F respectively. Injected into Xenopus Laevis oocytes, seminal vesicle mRNA does direct synthesis and secretion of polypeptides S and F. Partial peptide analysis confirms that S' and F' have extensive amino acid sequence homology with S and F respectively. We conclude that S' and F' are precursor forms of the secreted proteins and thus at least 2 abundant proteins of this male accessory tissue are secreted by a mechanism common to a wide number of secreted proteins.

Effect of testosterone propionate on protein synthesis by two-dimensional electrophoresis in rat ventral prostate

Two-dimensional polyacrylamide gel electrophoresis has been used to map the pattern of protein synthesis in rat ventral prostate from both castrate and testosterone treated animals. Proteins were detected by fluorography of 35S-methionine incorporation. In the soluble or cytosol fraction, four major proteins that have molecular weights less than 25,000 and isoelectric points (pI) less then 6.0 and one than has a molecular weight above 30,000 and pI 6.6 are under androgen control. One of these proteins with molecular weight 12,000 and pI 5.4 also appears in the microsomal fraction. Using nonequilibrium pH gradient electrophoresis, soluble proteins of pI greater than 7.0 were investigated. One major protein of 20-25,000 daltons and pI 8.0 appears to be androgen dependent. The ventral prostate from castrate synthesized proteins which are not detectable in androgen stimulated castrates.

On the presence of prostatic secretion protein in rat seminal fluid

The copulating plug collected from the tip of the penis from rats immediately after decapitation contains a protein very similar and probably identical to PSP (prostatic secretion protein); this protein has earlier been purified from rat prostatic cytosol and characterized. The protein present in the copulating plug interacts with [3H]estramustine and binds to the antibody raised against rat PSP. The concentration of the protein in the copulating plug is 400 ng/mg of total protein, when measured using the radioimmunoassay technique developed earlier for measurement of PSP in rat prostate. The [3H]estramustine-protein complex formed in a preparation of the copulating plug has an apparent molecular weight of about 50,000 and a sedimentation coefficient of about 3S when analyzed using sucrose density gradient centrifugation. The complex was retained on Concanavalin-A Sepharose indicating that the protein is a glycoprotein. Binding of the complex was also observed on hydroxylapatite and DEAE-Sephadex columns, from which it was eluted at 0.18 M KCl. Light microscope autoradiograms of rat sperms incubated with 125I-labeled PSP indicated that PSP is bound to all parts of the sperms. A macromolecule interacting with the PSP-antibodies is also present in human seminal fluid but at a concentration considerably lower than in rat seminal fluid. The present study shows that a macromolecule probably identical to prostatic secretion protein is present in the copulating plug from the rat. The biological role of this protein in normal male fertility is discussed.

The Xenopus oocyte as a surrogate secretory system. The specificity of protein export

Combining messenger RNA from one kind of secretory cell with the cytoplasm of another such cell can reveal the nature and specificity of protein export mechanisms. We show that messenger RNAs from secretory cells of chickens, rats, mice, frogs, guinea-pigs, locusts and barley plants, when injected into Xenopus oocytes, direct the synthesis and export of proteins. Chicken ovalbumin, Xenopus albumin, mouse thyroid-stimulating hormone, locust vitellin and guinea-pig milk proteins were identified using specific antibodies, whilst chicken lysozyme and ovomucoid, rat albumin, Xenopus vitellogenin and rat seminal vesicle basic proteins were identified provisionally from their molecular weights. Certain endogenous proteins are sequestered and secreted although most oocyte proteins are not exported. Similarly the major polyoma viral protein and the simian virus 40 and polyoma tumour antigens are retained within the oocyte. Radioactive proteins exported by oocytes programmed with chicken oviduct or Xenopus liver RNA are not re-exported in detectable amounts when injected into fresh oocytes, nor is there secretion of chicken oviduct or guinea-pig mammary gland primary translation products prepared using wheat germ extracts. Thus the export of secretory proteins from oocytes cannot be explained by leakage and may require a cotranslational event. The secretory system of the oocyte is neither cell-type nor species-specific yet is highly selective. We suggest that the oocyte can be used as a general surrogate system for the study of gene expression, from transcription through translation to the final subcellular or extracellular destination of the processed protein.

Purification and characterisation of prostatic binding protein and its subunits

The prostatic binding protein, previously described in rat ventral prostate, was isolated. The purified protein binds pregnenolone with an affinity of 1.2 X 10(6) M-1 and contains an average of 0.84 binding site per molecular. Its carbohydrate content is 3.2%. Its Mr, estimated by gel filtration, is 51 000 but in the presence of 6 M guanidine hydrochloride or 0.1% dodecylsulfate it dissociates into two subunits (S and F), which can be separated by polyacrylamide gel electrophoresis or by chromatography on hydroxyapatite. The Mr of these subunits is about 17 000, when estimated by gel filtration in 6 M guanidine hydrochloride, or 19 000 for subunit F and 20 000 for subunit S, when measured by dodecylsulfate/polyacrylamide gel electrophoresis. Their isoelectric points, estimated by isoelectric focusing in 8 M urea, are 4.6 for subunit F and 4.9 for subunit S. Prostatic binding protein and both subunits have a very similar amino acid composition. Upon reduction of disulfide bridges each subunit dissociates further into two components: one of these components is the same in both subunits.