Precursor-Product Relationship between Amphibian Vitellogenin and the Yolk Proteins, Lipovitellin and Phosvitin

Hormonal control of vitellogenin synthesis in avian liver

Estradiol induces the synthesis of vitellogenin in the avian liver. We describe the precursor-product relationship between vitellogenin and the yolk proteins phosvitin and lipovitellin. The high rate of vitellogenin synthesis is a consequence of the accumulation of a stable messenger RNA. We suggest that estradiol acts at the level of the genome by opening a hitherto non-transcribed gene.

Morphological and biochemical changes in the hepatic endoplasmic reticulum and golgi apparatus of male Xenopus laevis after induction of egg-yolk protein synthesis by oestradiol-17 beta

This report describes morphological and biochemical changes accompanying oestrogen induced synthesis of the egg-yolk protein precursor, vitellogenin, in male Xenopus liver. Extensive proliferation of the rough and smooth endoplasmic reticulum and the Golgi apparatus occurs between 3 and 9 days after administration of oestradiol-17 beta. Subcellular fractionation showed that microsomal fractions have an increased number of ribosomes available for protein synthesis, hormone treatment enhances the in vitro protein synthetic capacity per unit of RNA; both in microsome and ribosome preparations. Polypeptides synthesized in vitro by ribosome preparations show an enrichment in serine content after hormone treatment and an increased proportion of ribosomes can be immunoprecipitated by antibodies directed against vitellogenin. Our data are consistent with the proposal that vitellogenin is synthesized on the ribosomes of the rough endoplasmic reticulum and processed and packaged for secretion in the smooth endoplasmic reticulum and Golgi apparatus. Response to hormonal induction of vitellogenin involves an early phase in which membrane proliferation occurs in order to increase the cellular capacity to synthesize, process and secrete large quantities of egg-yolk protein precursor.

Amphibian embryo protease inhibitor. III. Binding studies on the trypsin inhibitor and properties of its yolk-bound form

Amphibian embryo trypsin inhibitor (ATI) is shown to be a temporary inhibitor in which full trypsin activity can eventually be restored. Lineweaver-Burk plots and other binding data indicate a reversible change from competitive to non-competitive inhibition during incubation of enzyme with inhibitor in the absence of substrate. The inhibitor is active while sequestered in the yolk, and the sequestered inhibitor co-purifies with lipovitellin when the yolk is fractionated. It is further demonstrated that the inhibitor slowly leaches from the yolk in a soluble form, and it is suggested that this leakage is the result of a negative feedback controlled proteolysis within the yolk. Possible interactions between ATI and endogenous trypsin-like proteases are discussed with particular regard to the potential involvement of those materials in the control of certain developmental events; notably melanogenesis.

Organization of vitellogenin polysomes, size of the mRNA and polyadenylate fragment

A heavy polysome fraction containing vitellogenin mRNA was isolated from the liver of oestradiol-treated chicks. As determined by urea-polyacrylamide gel electrophoresis, the molecular weight of vitellogenin mRNA is about 2.5 x 10(6). The mRNA contains a polyadenylate segment of about 220 nucleotides at the 3' end. The remaining 7000 nucleotides are sufficient to code for a polypeptide of Mr about 270000. Combining 'run off' experiments of heavy polysomes in vitro together with radioimmunoprecipitation and polyacrylamide gel electrophoresis of the translation product, we concluded that vitellogenin mRNA is probably monocistronic and the 2.5 x 10(6)-Mr mRNA codes for two polypeptides, Mr 30000 and 240000. The largest polypeptide is, in our cell-free system and liver homogenate, readily cleaved into smaller peptides.

Characterization of polysomes from Xenopus liver synthesizing vitellogenin and translation of vitellogenin and albumin messenger RNA's in vitro

1. Conditions are described for the isolation of polysomes from the liver of Xenopus laevis. The method involves homogenization of liver in 0.2 M Tris-HCl pH 8.5, treatment with 2% Triton X-100 and subsequent sucrose density gradient fractionation of polysomes from a 10000 X g supernatant. 2. Vitellogenin synthesis was induced in male Xenopus liver by oestradiol treatment. Polysomes were isolated and vitellogenin-synthesizing polysomes characterized by their association with monospecific 125 I-labelled rabbit anti-vitellogenin antibody and by reaction with rabbit anti-vitellogenin immunoglobulins followed by indirect immunoprecipitation with goat anti-rabbit antibody. 3. Changes in liver polysome content following oestrogen treatment of male Xenopus are correlated with the appearance of vitellogenin synthesis using an organ culture assay. 4. RNA extracted from livers of oestradiol-treated male Xenopus and from purified polysomes is shown to code for the synthesis of vitellogenin-specific immunoprecipitable polypeptides in a rabbit reticulocyte cell-free protein-synthesizing system, a major component having a molecular weight of 210000. Xanopus liver RNA is also shown to code for the synthesis of an albumin-specific immunoprecipitable polypeptide of 74000 molecular-weight which coelectrophoresed with Xenopus albumin.