The fate of Xenopus and locust vitellogenins made in Xenopus oocytes. An export-import processing model

The oocyte as an endocytic cell

Oocytes of Xenopus laevis grow primarily by sequestering vitellogenin (VTG) selectively from the maternal bloodstream. Morphological observations have demonstrated that an endocytic system is responsible for VTG uptake. Binding studies indicate the presence of 2-28 X 10(10) surface VTG receptors per oocyte. These are continuously internalized into endosomes whether or not they are occupied by VTG, and other macromolecules may become trapped in the process. VTG-containing endosomes give rise to dense transitional yolk bodies; these fuse with yolk platelets only after the cleavage of vitellogenin. In the absence of VTG, endosomes appear to fuse directly with yolk platelets. From these observations it is postulated that receptor occupancy can act as a transmembrane signal which directs the postendocytic compartmentation of proteins. Yolk platelet proteins do not undergo subsequent turnover, whereas adventitiously incorporated protein is gradually lost from the oocyte by a dual mechanism which may involve both lysosomal proteolysis and secretion from the oocyte as a consequence of membrane recycling. Although these observations may not apply to all growing oocytes, the X. laevis oocyte nevertheless appears to be a particularly attractive experimental system for studies of endocytic compartmentation and membrane receptor recycling.

Human estrogen receptor introduced into the Xenopus oocyte represses expression from an artificial frog estrogen response element

Although the estrogen responsiveness and estrogen receptors of Xenopus hepatocytes have been well described, oocytes of this species have not previously been shown to contain estrogen receptors (ER). Recombinant human ER (HER) was expressed in oocytes in a dose dependent fashion as measured by [35S]methionine incorporation into newly synthesized proteins. Chloramphenicol acetyl transferase (CAT) reporter plasmids, driven by a herpes simplex thymidine kinase promotor with or without a 17 base pair estrogen response element (ERE) from the vitellogenin A2 gene, were also injected into oocytes. When injected without the accompanying HER sequences, the construct containing the ERE expressed 10-fold more CAT activity, and this response was saturable as demonstrated by injecting increasing amounts of reporter plasmid. These results suggest either the activity of small amounts of a Xenopus ER (measured here by LH-20 assay), or the presence of some endogenous oocyte protein other than the ER that can interact with this ERE. When HER was co-expressed with ERECAT, CAT expression was suppressed over a wide range of HER concentrations. This unexpected repression may be due to displacement of an estrogen receptor or other endogenous oocyte regulatory protein on the ERE. HER's positive regulatory activity may require transcription factors that are lacking or insufficient in the oocyte. Alternatively the simple 17 base pair ERE may not provide DNA binding sites for such transcription factors.

A role for the beta-subunit in the expression of functional Na+-K+-ATPase in Xenopus oocytes

In all cellular systems studied so far, the catalytic alpha- and the glycosylated beta-subunit of Na+-K+-ATPase are coordinately synthesized and are assembled into stoichiometric alpha, beta-complexes. In contrast to these data, in this study we show that the fully grown oocyte of Xenopus laevis synthesizes much less beta-subunit than alpha-subunit. The alpha-subunit produced in excess over the beta-subunit is membrane associated but highly trypsin sensitive and can be compared with the immature alpha-subunit population identified in epithelial cells immediately after synthesis (K. Geering, J. P. Kraehenbuhl, and B.C. Rossier, J. Cell Biol. 105: 2613-2619, 1987). The Xenopus oocyte thus turns out to be a unique system to study the functional role of the beta-subunit. Injection of beta-subunit-specific mRNA transcribed in vitro from a beta-cDNA clone (derived from Xenopus kidney, A6 cells) into oocytes results in translation of a glycosylated beta-subunit. The synthesis of this exogenous beta-subunit increases significantly the proportion of trypsin-resistant oocyte alpha-subunits able to perform cation-dependent conformational changes. In addition, 25-65% more ouabian binding sites are expressed at the plasma membrane in beta-mRNA-injected oocytes. In contrast, newly synthesized alpha-subunit translated after injection of size-fractionated mRNA enriched in alpha-mRNA remains trypsin sensitive as the oocyte alpha-subunit. These data suggest that association of the beta-subunit to the alpha-subunit provokes a structural rearrangement of the alpha-subunit that might be a first step toward the functional maturation of the Na+-K+-ATPase and its expression at the plasma membrane.

Structure of the lamprey yolk lipid-protein complex lipovitellin-phosvitin at 2.8 A resolution

The X-ray crystallographic structure of the lipid-protein complex lipovitellin-phosvitin has been determined with the multiple isomorphous replacement method using four heavy-atom derivatives. Lamprey yolk lipovitellin-phosvitin is a dimeric molecule of molecular weight 352,000. The monomer consists of three polypeptide chains. The smallest is known as phosvitin and has an extremely high phosphoserine content. The monomeric unit also contains about 16% (w/w) of non-covalently bound lipid, probably in a monolayer or bilayer-like configuration. Within each monomer is a "cavity" or region of low electron density. The cavity has a volume of about 68,000 A3 and is believed to contain the lipid in a presumably disordered state. The cavity is roughly conical in shape and is lined on two sides by seven and eight-stranded antiparallel beta-sheets. The base of the cavity opens away from the intersubunit interface, but appears partially closed off from solvent regions by additional antiparallel beta-sheet structure. The beta-sheets lining the sides of the cavity are surrounded by a shell of two curved layers of 16 interconnected helices. The helices in either layer of the shell are all roughly parallel to each other and antiparallel to all of the helices of the other layer. The connectivity of the helices resembles a "superhelix" and is different from the connectivities seen in proteins containing four-helix bundles. There are an estimated 1300 amino acids in lamprey lipovitellin-phosvitin and almost 1000 alanine residues have been modeled into electron density. The remaining residues are assumed to be disordered.

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.

Injection of partially purified estrogen receptor protein from Xenopus liver nuclei into oocytes activates the silent vitellogenin locus

Injection of extracts from Xenopus liver nuclei that are enriched 2000 times in estradiol receptor into Xenopus oocytes induces transcription of the silent vitellogenin locus, which is activated in liver by estradiol, but not of the albumin locus, which is active in liver but suppressed by high levels of estradiol. Transcription initiates within the 5'-end region of the gene we have studied and probably continues into the 3' third. The activation seems to be very efficient, but most of the primary transcripts are probably rapidly and inaccurately processed. New proteins are also made and secreted by the oocytes.