RING fingers and B-boxes: zinc-binding protein-protein interaction domains

The cysteine-rich zinc-binding motifs known as the RING and B-box are found in several unrelated proteins. Structural, biochemical, and biological studies of these motifs reveal that they mediate protein-protein interactions. Several RING-containing proteins are oncoproteins and recent data indicate that proapoptotic activities can be mediated through the RING. 1H NMR methods were used to determine the structures of RINGs and a B-box domain and to monitor the conformational changes these motifs undergo upon zinc ligation. This review discusses in detail the structural features of the RING and B-box domains. Further, possible structure function relationships for these motifs particularly in their role as protein interaction domains are discussed.

The role of calcium on the activity of ERcalcistorin/protein-disulfide isomerase and the significance of the C-terminal and its calcium binding. A comparison with mammalian protein-disulfide isomerase

ERcalcistorin/protein-disulfide isomerase (ECaSt/PDI) shows a 55% identity with mammalian protein-disulfide isomerase (PDI) (Lucero, H. A., Lebeche, D., and Kaminer, B. (1994) J. Biol. Chem. 269, 23112-23119) is a high capacity low affinity Ca2+-binding protein and behaves as a Ca2+ storage protein in the ER of a living cell (Lucero, H. A., Lebeche, D., and Kaminer, B. (1998) J. Biol. Chem. 273, 9857-9863). Here we show that recombinant ECaSt/PDI bound 26 mol of Ca2+/mol and a C-terminal truncated mutant bound 14 mol of Ca2+/mol, both with a Kd of 2.8 mM in 50 mM KCl and 5.2 mM in 150 mM KCl. The percentage reduction in Ca2+ binding in the mutant corresponded with the percentage reduction of deleted pairs of acidic residues, postulated low affinity Ca2+-binding sites. 5 mM Ca2+ moderately increased the PDI activity of both ECaSt/PDI and the C-terminal truncated mutant on reduced RNase and insulin. Surprisingly, ECaSt/PDI in the absence of Ca2+ prevented the spontaneous reactivation of reduced bovine pancreatic trypsin inhibitor. In the presence of 1-5 mM Ca2+ (or 10 microM polylysine) ECaSt/PDI augmented the bovine pancreatic trypsin inhibitor reactivation rate. In contrast, the C-terminal truncated ECaSt/PDI augmented rBPTI reactivation in the absence of Ca2+ and 1-5 mM Ca2+ further accelerated the reactivation rate, responses similar to those obtained with mammalian PDI.

The structure of vitellogenin provides a molecular model for the assembly and secretion of atherogenic lipoproteins

The assembly of atherogenic lipoproteins requires the formation in the endoplasmic reticulum of a complex between apolipoprotein (apo)B, a microsomal triglyceride transfer protein (MTP) and protein disulphide isomerase (PDI). Here we show by molecular modelling and mutagenesis that the globular amino-terminal regions of apoB and MTP are closely related in structure to the ancient egg yolk storage protein, vitellogenin (VTG). In the MTP complex, conserved structural motifs that form the reciprocal homodimerization interfaces in VTG are re-utilized by MTP to form a stable heterodimer with PDI, which anchors MTP at the site of apoB translocation, and to associate with apoB and initiate lipid transfer. The structural and functional evolution of the VTGs provides a unifying scheme for the invertebrate origins of the major vertebrate lipid transport system.

Identification of a novel mRNA-associated protein in oocytes of Pleurodeles waltl and Xenopus laevis

Amphibian oocytes accumulate a large pool of mRNA molecules for future embryonic development. Due to their association with specific proteins the stored maternal RNAs are translationally repressed. The identification of these RNA-binding proteins and the characterization of their functional domains may contribute to the understanding of the translational repression mechanisms and the subsequent activation processes during early embryogenesis. Here we present the complete Pleurodeles cDNA sequence of a cytoplasmic protein which is present in oocytes, eggs, and very early cleavage stage embryos but undetectable in postcleavage embryo and adult tissues. The predicted molecular mass of the protein is 55 kDa and the apparent molecular mass as determined by SDS-PAGE, 68 kDa. The deduced amino acid sequence reveals proline- and serine-rich domains in the aminoterminal part as well as two RGG boxes which represent characteristic motifs of several RNA-binding proteins. No distinct homologies to the consensus RNA recognition motif were found. The 55-kDa protein was recovered in cytoplasmic ribonucleoprotein (RNP) particles containing poly(A)+ RNA. It was therefore termed RAP55 for mRNA-associated protein of 55 kDa. However, a direct interaction of RAP55 with mRNA could not be demonstrated by UV-crosslinking experiments, indicating that it is bound to mRNP complexes via protein-protein interactions. RAP55 is evolutionarily conserved since antibodies raised against a recombinant Pleurodeles RAP55 fragment recognize the protein from Pleurodeles and Xenopus. The expression pattern and intracellular distribution of RAP55 suggest that it is part of those mRNP particles which are translationally repressed during oogenesis and become activated upon progesterone-induced oocyte maturation.

Two hexameric cyanoprotein subunits from an insect, Riptortus clavatus. Sequence, phylogeny and developmental and juvenile hormone regulation

Hemolymph of a hemipteran insect, Riptortus clavatus, contains four distinct hexameric proteins (cyanoproteins) composed of two distinct subunits, CP alpha and CP beta, which show profiles of abundance depending on developmental stage, diapause status controlled by juvenile hormone, and sex. We have isolated cDNA clones encoding the two subunits and determined the complete sequences. The nucleotide sequences predict polypeptides of 693 and 691 residues for CP alpha and CP beta, respectively, including identical 16-residue signal peptides. The deduced amino acid sequences of both CP alpha and CP beta have significant similarity to other hemocyanin-related proteins, indicating that the cyanoproteins represent hexamerins. Phylogenetic analyses show that the cyanoprotein subunits can be grouped together with other hexamerins from exopterygote insects. Developmental Northern-blot analyses suggest that the expression of the cyanoproteins is regulated at the level of transcript abundance. In addition, the expression of the CP alpha subunit in female adults has been shown to be enhanced by juvenile hormone (JH) while the expression of the CP beta subunit is suppressed by the same hormone. To our knowledge, the CP alpha subunit of R. clavatus is the first case of a JH-enhanceable hexamerin whose sequence has been determined.

Transport of ER vesicles on actin filaments in neurons by myosin V

Axoplasmic organelles in the giant axon of the squid have been shown to move on both actin filaments and microtubules and to switch between actin filaments and microtubules during fast axonal transport. The objectives of this investigation were to identify the specific classes of axoplasmic organelles that move on actin filaments and the myosin motors involved. We developed a procedure to isolate endoplasmic reticulum (ER) from extruded axoplasm and to reconstitute its movement in vitro. The isolated ER vesicles moved on exogenous actin filaments adsorbed to coverslips in an ATP-dependent manner without the addition of soluble factors. Therefore myosin was tightly bound and not extracted during isolation. These vesicles were identified as smooth ER by use of an antibody to an ER-resident protein, ERcalcistorin/protein disulfide isomerase (EcaSt/PDI). Furthermore, an antibody to squid myosin V was used in immunogold EM studies to show that myosin V localized to these vesicles. The antibody was generated to a squid brain myosin (p196) that was classified as myosin V based on comparisons of amino acid sequences of tryptic peptides of this myosin with those of other known members of the myosin V family. Dual labeling with the squid myosin V antibody and a kinesin heavy chain antibody showed that the two motors colocalized on the same vesicles. Finally, antibody inhibition experiments were performed with two myosin V-specific antibodies to show that myosin V motor activity is required for transport of vesicles on actin filaments in axoplasm. One antibody was made to a peptide in the globular tail domain and the other to the globular head fragment of myosin V. Both antibodies inhibited vesicle transport on actin filaments by greater than 90% compared to controls. These studies provide the first direct evidence that ER vesicles are transported on actin filaments by myosin V. These data confirm the role of actin filaments in fast axonal transport and provide support for the dual filament model of vesicle transport.

Studies on sulfhydryl groups during cell division of sea urchin egg. III. SH groups of KC1-soluble proteins and their change during cleavage

Sea urchin egg proteins extracted with KCl are mostly TCA-soluble and, conversely, those extracted with TCA are KCl-soluble. Both groups are water-insoluble and show fluctuations in—SH content during the division cycle. The fluctuation of the—SH groups of the KCl-soluble protein of the whole egg is due to a —SH⇌—S—S— interchange within the freely reacting groups and not within the sluggish and masked —SH groups of the protein. The —SH content of the KCl-soluble protein of the egg cortex also fluctuates in a similar way.

Evolution of oogenesis: the receptor for vitellogenin from the rainbow trout

Receptors that transport vitellogenin (VTG) into oocytes are of vital importance to egg-laying species because they mediate a key step in oocyte development. Here we describe the cloning of the first piscine oocyte-specific receptor cDNA, i.e., that encoding the VTG receptor from the rainbow trout (Oncorhynchus mykiss). The receptor, a 826-residue type-I membrane protein, is a member of the low density lipoprotein receptor (LDLR) superfamily. It closely resembles the mammalian so-called very low density lipoprotein receptors, in that its aminoterminal ligand binding domain consists of a cluster of 8 cysteine-rich repeats. The short intracellular portion contains the internalization signal typical for the LDLR superfamily, Phe-Glu-Asn-Pro-Val-Tyr. Notably, the receptor lacks a domain with a high density of potential O-glycosylation sites often found in somatic cell-specific members of the LDLR family. A specific transcript of 3.9 kb is abundant in ovary, but undetectable in muscle and heart, which are the major sites of expression of very low density lipoprotein receptors in mammals. In vitro translation of the full-length cDNA produced a 97-kDa protein, and transient expression in COS-1 cells showed that the cDNA encodes a protein of the same size that binds vitellogenin in ligand blots. As revealed by in situ hybridization, transcripts are present in previtellogenic oocytes, indicating that production of receptor protein precedes the phase of yolk deposition. Our results in fish, together with those in birds (Bujo, H., et al. 1994. EMBO J. 13: 5165-5175) suggest that vitelogenesis provides a prime model for the study of ligand/receptor systems designed to sustain reproduction.

The structural basis of lipid interactions in lipovitellin, a soluble lipoprotein

BACKGROUND

The conformation and assembly of lipoproteins, protein containing large amounts of noncovalently bound lipid, is poorly understood. Lipoproteins present an unusual challenge as they often contain varying loads of lipid and are not readily crystallized. Lipovitellin is a large crystallizable oocyte protein of approximately 1300 residues that contains about 16% w/w lipid. Lipovitellin contains two large domains that appear to be conserved in both microsomal triglyceride transfer protein and apolipoprotein B-100. To gain insight into the conformation of a lipoprotein and the potential modes of binding of both neutral and phospholipid, the crystal structure of lamprey lipovitellin has been determined.

RESULTS

We report here the refined crystal structure of lipovitellin at 2.8 A resolution. The structure contains 1129 amino acid residues located on five peptide chains, one 40-atom phosphatidylcholine, and one 13-atom hydrocarbon chain. The protein contains a funnel-shaped cavity formed primarily by two beta sheets and lined predominantly by hydrophobic residues.

CONCLUSIONS

Using the crystal structure as a template, a model for the bound lipid is proposed. The lipid-binding cavity is formed primarily by a single-thickness beta-sheet structure which is stabilized by bound lipid. This cavity appears to be flexible, allowing lipid to be loaded or unloaded.

Molecular characteristics of insect vitellogenins and vitellogenin receptors

The recent cloning and sequencing of several insect vitellogenins (Vg), the major yolk protein precursor of most oviparous animals, and the mosquito Vg receptor (VgR) has brought the study of insect vitellogenesis to a new plane. Insect Vgs are homologous to nematode and vertebrate Vgs. All but one of the insect Vgs for which we know the primary structure are cleaved into two subunits at a site [(R/K)X(R/K)R or RXXR with an adjacent beta-turn] recognized by subtilisin-like proprotein convertases. In four of the Vgs, the cleavage site is near the N-terminus, but in one insect species, it is near the C-terminus of the Vg precursor. Multiple alignments of these Vg sequences indicate that the variation in cleavage location has not arisen through exon shuffling, but through local modifications of the amino acid sequences. A wasp Vg precursor is not cleaved, apparently because the sequence at the presumed ancestral cleavage site has been mutated from RXRR to LYRR and is no longer recognized by convertases. Some insect Vgs contain polyserine domains which are reminiscent of, but not homologous to, the phosvitin domain in vertebrate Vgs. The sequence of the mosquito VgR revealed that it is a member of the low-density lipoprotein receptor (LDLR) family. Though resembling chicken and frog VgRs, which are also members of the LDLR family, it is twice as big, carrying two clusters of cysteine-rich complement-type (Class A) repeats (implicated in ligand-binding) instead of one like vertebrate VgRs and LDLRs. It is very similar in sequence and domain arrangement to the Drosophila yolk protein receptor (YPR), despite a non-vitellogenin ligand for the latter. Though vertebrate VgRs, insect VgR/YPRs, and LDLR-related proteins/megalins all accommodate one cluster of eight Class A repeats, fingerprint analysis of the repeats in these clusters indicate they are not directly homologous with one another, but have undergone differing histories of duplications, deletions, and exon shuffling so that their apparent similarity is superficial. The so-called epidermal growth factor precursor region contains two types of motifs (cysteine-rich Class B repeats and YWXD repeats) which occur independently of one another in diverse proteins, and are often involved in protein-protein interactions, suggesting that they potentially are involved in dimerization of VgRs and other LDLR-family proteins. Like the LDLR, but unlike vertebrate VgRs and the Drosophila YPR, the mosquito VgR contains a putative O-linked sugar region on the extra-cellular side of the transmembrane domain. Its function is unclear, but may protect the receptor from membrane-bound proteases. The cytoplasmic tail of insect VgR/YPRs contains a di-leucine (or leucine-isoleucine) internalization signal, unlike the tight-turn tyrosine motif of other LDLR-family proteins. The importance of understanding the details of yolk protein uptake by oocytes lies in its potential for exploitation in novel insect control strategies, and the molecular characterization of the proteins involved has made the development of such strategies a realistic possibility.

Molecular characterization and expression of two ovarian lipoprotein receptors in the rainbow trout, Oncorhynchus mykiss

Partial cDNAs encoding two lipoprotein receptors were isolated and sequenced from the ovary in the rainbow trout (rt), Oncorhynchus mykiss. One of the cDNAs (rt-LPR) contained the 5 domains characteristic of receptors belonging to the low-density lipoprotein receptor superfamily. The second cDNA (rt-LP[OS]R) was similar to rt-LPR but lacked 105 base pairs encoding the O-linked sugar domain. The deduced amino acid sequences of the rt-LPR and rt-LP[OS]R had between 75% and 80% identity with very-low-density lipoprotein and vitellogenin receptors of other species. The rt-lipoprotein receptor mRNAs were approximately 3.5 kilobases in size. The rt-LPR was expressed in both the ovary and somatic tissues, whereas the rt-LP[OS]R was ovary-specific. Messenger RNA for the lipoprotein receptor(s) was expressed at high levels in both pre-vitellogenic (< 0.3 mm) and early vitellogenic (up to 1 mm) follicles. Using reverse transcription-polymerase chain reaction, expression of rt-LPR and rt-LP[OS]R mRNA was also detected in larger vitellogenic follicles (up to 2.5 mm in diameter) but not in follicles in late vitellogenesis or in ovulated eggs. The sequence, ovary specificity, and pattern of ovarian expression of the rt-LPR mRNA suggest that it encodes the receptor that mediates vitellogenin uptake into the ovary.

ERcalcistorin/protein-disulfide isomerase acts as a calcium storage protein in the endoplasmic reticulum of a living cell. Comparison with calreticulin and calsequestrin

ERcalcistorin/protein-disulfide isomerase (ECaSt/PDI), a high capacity low affinity Ca2+-binding protein in the endoplasmic reticulum of sea urchin eggs (Lebeche, D., and Kaminer, B. (1992) Biochem. J. 287, 741-747), shares 55% sequence identity with mammalian PDI and has PDI activity (Lucero, H., Lebeche, D., and Kaminer, B. (1994) J. Biol. Chem. 269, 23112-23119). We report on ECaSt/PDI functioning as a Ca2+ storage protein in the endoplasmic reticulum (ER) of a living cell and compare it with calsequestrin and calreticulin, high capacity low affinity Ca2+-binding proteins in the sarcoplasmic reticulum and ER, respectively. Stably transfected Chinese hamster ovary cell clones expressed these proteins, which were localized in the ER of the cell. Microsomes from cells expressing ECaSt/PDI, calreticulin, and calsequestrin accumulated 17.2 +/- 0.27, 20.0 +/- 0.82, and 38.0 +/- 0.28 nmol of Ca2+/mg of protein, respectively; control microsomes accumulated from 2.6 +/- 0.17 to 2.9 +/- 0.14 nmol of Ca2+/mg of protein. The initial rate of Ca2+ uptake was similar in microsomes from transfected and control cells. Microsomes containing an ECaSt/PDI mutant in which 45% of the acidic residue pairs in the C terminus were truncated had a reduced Ca2+ storage capacity. This supports our previous hypothesis that the degree of low affinity Ca2+ binding is dependent on the number of pairs of carboxyl groups in the molecule. The maximal Ca2+ accumulation by microsomes containing the expressed ECaSt/PDI, C-terminally truncated ECaSt/PDI, calreticulin, or calsequestrin correlates approximately with the Ca2+ binding capacity of the respective proteins.

Evidence that phenylalanine hydroxylation rates are overestimated in neonatal subjects receiving total parenteral nutrition with a high phenylalanine content

Recent publications have indicated that the parenterally fed neonate has a substantial ability to hydroxylate phenylalanine. Examination of these data suggests that, at high phenylalanine intakes, estimated rates of hydroxylation exceed rates of intake. This implies significant net tissue breakdown. However, the quantitative validity of the estimates of phenylalanine hydroxylation cannot be assessed without nitrogen balance data. We have recently developed a parenterally fed neonatal piglet model and have used this to study aromatic amino acid metabolism in piglets fed different amino acid solutions. Reappraisal of the data from these studies has allowed us to estimate both phenylalanine hydroxylation and tissue protein accretion. Piglets were parenterally fed Vamin [292 micromol of Phe x kg(-1) x h(-1), 26 micromol of Tyr x kg(-1) x h(-1)], Vaminolact + Phe [VLP, 277 micromol of Phe x kg(-1) x h(-1), 26 micromol Tyr x kg(-1) x h(-1)], or Vaminolact + glycyl-L-tyrosine [VLGT, 152 micromol of Phe x kg(-1) x h(-1), 159 micromol of Tyr x kg(-1) x h(-1)] for 8 d. Nitrogen balance was measured over the last 5 study d, and aromatic amino acid kinetics were determined using a primed continuous infusion of L-[1-4C]phenylalanine on d 8. Average body protein gain, derived from nitrogen balance, was 11 g x kg(-1) x d(-1). For the Vamin and VLP groups, the rates of phenylalanine hydroxylation were estimated to be 139 and 90% of intake, respectively. However, phenylalanine hydroxylation was only 16% of intake for the VLGT group. In view of the tissue protein accretion data, it appears that the rate of phenylalanine hydroxylation may be overestimated in neonates fed high phenylalanine parenteral nutrition. The extent to which the parenterally fed neonate can adapt to a high phenylalanine intake, by increasing the rate of phenylalanine hydroxylation, remains to be determined.

Ligand-binding domains in vitellogenin receptors and other LDL-receptor family members share a common ancestral ordering of cysteine-rich repeats

Insect vitellogenin and yolk protein receptors (VgR/YPR) are newly discovered members of the low-density lipoprotein receptor (LDLR) family, which is characterized by a highly conserved arrangement of repetitive modular elements homologous to functionally unrelated proteins. The insect VgR/YPRs are unique in having two clusters of complement-type cysteine-rich (class A) repeats or modules, with five modules in the first cluster and seven in the second cluster, unlike classical LDLRs which have a single seven-module cluster, vertebrate VgRs and very low density lipoprotein receptors (VLDLR) which have a single eight-module cluster, and LDLR-related proteins (LRPs) and megalins which have four clusters of 2-7, 8, 10, and 11 modules. Alignment of clusters across subfamilies by conventional alignment programs is problematic because of the repetitive nature of the component modules which may have undergone rearrangements, duplications, and deletions during evolution. To circumvent this problem, we "fingerprinted" each class A module in the different clusters by identifying those amino acids that are both relatively conserved and relatively unique within the cluster. Intercluster reciprocal comparisons of fingerprints and aligned sequences allowed us to distinguish four cohorts of modules reflecting shared recent ancestry. All but two of the 57 modules examined could be assigned to one of these four cohorts designated A, B, C, and D. Alignment of clusters based on modular cohorts revealed that all clusters are derived from a single primordial cluster of at least seven modules with a consensus arrangement of CDCADBC. All extant clusters examined are consistent with this consensus, though none matches it perfectly. This analysis also revealed that the eight-module clusters in vertebrate VgRs, insect VgR/YPRs, and LRP/megalins are not directly homologous with one another. Assignment of modules to cohorts permitted us to properly align 32 class A clusters from all four LDLR subfamilies for phylogenetic analysis. The results revealed that smaller one-cluster and two-cluster members of the family did not originate from the breakup of a large two-cluster or four-cluster receptor. Similarly, the LRP/megalins did not arise from the duplication of a two-cluster insect VgR/YPR-like progenitor. Rather, it appears that the multicluster receptors were independently constructed from the same single-cluster ancestor.

Intestinal uptake of lipovitellin from brine shrimp (Artemia franciscana) by larval inland silversides (Menidia beryllina) and striped bass (Morone saxatilis)

Intestinal uptake of lipovitellin (LV) from brine shrimp (Artemia franciscana) in larval inland silversides (Menidia beryllina) and striped bass (Morone saxatilis) was described using immunocytochemistry. Polyclonal antisera were raised against two subunits of LV (LV68 and LV190). When tested by immunocytochemistry, anti-LV68 showed cross-reactivity with some of the pancreatic cells especially in inland silversides. Therefore anti-LV190 was used to localize immunoreactive LV. Inland silversides at 14 days after hatching were fed Artemia nauplii and then sampled 4, 8, 12 hr after feeding. Similar experiments were carried out by using striped bass at 5 days and 15 days of age. They were sampled at 2, 4, 8, and 12 hr after feeding. Anterior enterocytes showed no evidence of uptake; however, the brush border of the cells of inland silversides reacted with the antiserum. Posterior enterocytes took up the LV and/or, possibly, their immunoreactive breakdown products. The pattern of uptake included accumulation in supranuclear vacuoles and digestion in supranuclear vacuoles, as suggested by the decay of the immunoreactivity over time. Thus, the posterior intestine of these larval fishes is the site of uptake and digestion of LV, an important nutritive component in the food of many larval fishes; this supports earlier findings using non-nutritive marker proteins.

Differential regulation of two genes implicated in fish reproduction: vitellogenin and estrogen receptor genes

In rainbow trout as well as in other species, variability of estrogen receptor (ER) gene expression according to the cell type and the physiological state reflects a differential cell and gene sensitivity to estrogen. We previously demonstrated that expression of the rainbow trout estrogen receptor (rtER) and vitellogenin (Vg) genes were induced differently by estrogens in rainbow trout liver. Therefore, these two genes offered a suitable model to study the influence of ER concentration on gene transcriptional activities. In the present study we show that the transcription rate of rtER and Vg genes during an estrogenic treatment are affected differently by variation of cellular ER concentration. We demonstrate that rtER gene exhibits a low threshold response to loaded estrogen receptor, and increasing ER amounts do not affect the transcriptional response of this gene during an estrogenic stimulation. On the contrary, Vg gene expression requires the presence of a higher loaded estrogen receptor level to be induced, and its transcriptional response is directly proportional to the amount of synthesised ER.

xnf7 functions in dorsal-ventral patterning of the Xenopus embryo

Xenopus nuclear factor 7 (xnf7) is a maternally expressed nuclear protein that is retained in the cytoplasm from oocyte maturation until the midblastula transition (MBT). Mutations of the xnf7 phosphorylation sites to glutamic acids (dnxnf7) resulted in the retention of the endogenous protein in the cytoplasm past the MBT, indicating that cytoplasmic retention is a phosphorylation dependent process. In addition, dnxnf7 acted as a dominant negative mutant by keeping the endogenous xnf7 protein in the cytoplasm past the MBT. Overexpression of dnxnf7 in future dorsal blastomeres resulted in a ventralized or posteriorized phenotype in which the embryos lacked anterior structures, while overexpression in ventral blastomeres resulted in dorsalized embryos. dnxnf7 also affected the expression of both dorsal and ventral mesodermal markers. These data suggest that xnf7 functions in dorsal/ventral patterning and that the movement of the protein from the cytoplasm to the nucleus at the MBT is critical for the execution of a genetic program conferring a dorsal or ventral identity to the mesoderm.

Internalization and recycling of vitellogenin receptor in the mosquito oocyte

The major yolk protein precursor in mosquito oocytes, vitellogenin (Vg), is internalized by a 205-kDa membrane-bound receptor (VgR). Recently, VgR has been isolated permitting the production of polyclonal anti-VgR antibodies. To elucidate the pathway of VgR internalization and recycling in mosquito oocytes during Vg uptake, we carried out an immunogold electron-microscopic study, labeling both Vg and VgR in ultrathin frozen sections of ovarian tissue. VgR immunolabeling demonstrated that the oocyte plasma membrane was subdivided into microdomains, with VgR being located between and at the lower portions of the oocyte microvilli. During the early stages of internalization, Vg and VgR were observed together in coated pits, coated vesicles, and early endosomes. Fusion of early endosomes created transitional yolk bodies (TYB) in which Vg and VgR became segregated. VgR label was present in the numerous tubular compartments that protruded from the TYBs. These tubular organelles extended to and fused with the plasma membrane, suggesting that they represented the vehicle for VgR recycling. Vg label was not observed in the tubular compartments. Instead, Vg accumulated in the core of the TYB, a region free of VgR label. Mature yolk bodies (MYB) were heavily labeled for Vg, but completely lacked any VgR label, indicating that MYB are storage compartments that do not participate in receptor recycling. Thus, our immunocytochemical data clearly visualize the steps in Vg/VgR internalization, dissociation, sorting, and recycling of the receptor to the plasma membrane.

The Drosophila melanogaster RAD54 homolog, DmRAD54, is involved in the repair of radiation damage and recombination

The RAD54 gene of Saccharomyces cerevisiae plays a crucial role in recombinational repair of double-strand breaks in DNA. Here the isolation and functional characterization of the RAD54 homolog of the fruit fly Drosophila melanogaster, DmRAD54, are described. The putative Dmrad54 protein displays 46 to 57% identity to its homologs from yeast and mammals. DmRAD54 RNA was detected at all stages of fly development, but an increased level was observed in early embryos and ovarian tissue. To determine the function of DmRAD54, a null mutant was isolated by random mutagenesis. DmRADS4-deficient flies develop normally, but the females are sterile. Early development appears normal, but the eggs do not hatch, indicating an essential role for DmRAD54 in development. The larvae of mutant flies are highly sensitive to X rays and methyl methanesulfonate. Moreover, this mutant is defective in X-ray-induced mitotic recombination as measured by a somatic mutation and recombination test. These phenotypes are consistent with a defect in the repair of double-strand breaks and imply that the RAD54 gene is crucial in repair and recombination in a multicellular organism. The results also indicate that the recombinational repair pathway is functionally conserved in evolution.

The mutation masculinizer (man) defines a sex-determining gene with maternal and zygotic functions in Musca domestica L

In Musca domestica, the primary signal for sex determination is the dominant factor M, which is assumed to regulate a postulated female-determining gene F. Presence of M prevents expression of F so that male development ensues. In the absence of M, F can become active, which dictates the female pathway. The existence of F is inferred from FD. a dominant factor that is epistatic to M. We describe a new mutation masculinizer, which has all the properties expected for a null or strongly hypomorphic allele of F: (1) it maps to the same chromosomal location as FD, (2) homozygous man/man animals develop as males, (3) homozygous man/man clones generated in man/+ female larvae differentiate male structures, (4) man has a sex-determining maternal effect. About a third of the morphological males synthesize yolk proteins, which indicates that they are intersexual in internal structures. The maternal effect of man is complete in offspring that derive from homozygous man/man pole cells transplanted into female hosts. In this case, all man/+ progeny become fertile males that do not produce yolk proteins A sex-determining maternal effect has previously been demonstrated for FD. Like F, maternal man' is needed for zygotic man' to become active, providing further evidence that man is a loss-of-function allele of F.

Distinct stress-inducible and developmentally regulated heat shock transcription factors in Xenopus oocytes

The presence of a maternal pool of heat shock factor (HSF) in Xenopus oocytes has been suggested by two lines of evidence from previous studies. First, heat shock response element (HSE)-binding activity is induced in heat-shocked eggs and embryos prior to expression of zygotic HSF. Second, expression from microinjected heat shock protein promoters in oocytes is induced upon heat shock. To date, however, endogenous oocyte HSF molecules have not been detected, nor has induction of HSE-binding activity been directly demonstrated. Here we report the detection of distinct stress-inducible and developmentally regulated HSE-binding activities of endogenous oocyte factors. Exposure of defolliculated oocytes to heat, cadmium, and arsenite resulted in the formation of an HSE-specific complex detectable by gel mobility shift assay. Induction of HSE-binding activity by each of these stressors corresponded to increased expression from a microinjected hsp70 promoter. The stress-inducible HSE-binding complex was recognized by antiserum against mammalian HSF1, but not by HSF2 antiserum, suggesting that a Xenopus homologue of HSF1 is the major component of this activity. The HSE-binding activity of HSF1 was induced by stress treatments of stage I through VI oocytes, an indication that it is responsive to stress throughout oogenesis. During recovery from heat shock, the HSF1-HSE complex rapidly declined to control levels, but was induced for prolonged periods in oocytes exposed to continuous stress, a pattern unlike the transient activation previously observed in fertilized eggs or embryos. The kinetics of HSF1 activation in oocytes suggests that a key protein(s) regulating attenuation of the stress response is present at exceedingly low levels or is somehow modified during preembryonic development. We also detected an unusual constitutive HSE-binding complex in unstressed stage I and II oocytes, but not in later stage oocytes, eggs, developing embryos, or A6 cells. This constitutive complex was unaffected by heat or chemical treatments and was not recognized by either HSF1 or HSF2 antiserum. Appearance of the constitutive HSE-binding activity during oogenesis corresponded closely with peak levels of hsp70 mRNA detected by Northern blot analysis of RNA from staged oocytes. We suggest that the constitutive HSE-binding activity in early oocytes is formed by a unique developmentally regulated heat shock factor that may play a role in the expression of heat shock proteins during early stages of oogenesis.

Ovorubin, a chromoprotein from the eggs of the gastropod mollusc Pomacea canaliculata

The eggs of Pomacea canaliculata australis (d’Orbigny), an amphibious freshwater prosobranch snail, have, as the most important nitrogenous constituent of the jelly surrounding the ovum, a red glycoprotein with a carotenoid prosthetic group. This protein, to which the name ovorubin has been given, has a high stability to denaturation by heat and by adsorption at interfaces. It is partially utilized during development of the ovum, although about two-thirds of the original ovorubin content of the egg is found in the visceral hump of the newly hatched animal. The carotenoid component is probably an ester or ether of astaxanthin, highly labile to alkali in the cold. The minimum molecular weight, calculated from the carotenoid content, lies in the region of 330 000. The carbohydrate component represents about 20% of the molecule. The carotenoid and the glycoprotein may readily be separated and recombined. Experiments on the apo-glycoprotein and the reconstituted caroteno-protein indicate that the carotenoid stabilizes the native configuration of the protein structure. It is suggested that stabilization of the configurations of protein molecules may be one of the roles of carotenoids in nature.

Trace mineral metabolism in the avian embryo

Trace mineral metabolism in the developing avian embryo begins with the formation of the egg and the trace mineral stores contained within it. Vitellogenin, the yolk precursor protein, serves as a trace mineral transporting protein that mediates the transfer of these essential nutrients from stores within the liver of the hen to the ovary and developing oocyte, and hence, to the yolk of the egg. Lipovitellin and phosvitin, derived from intraoocytic proteolytic processing of vitellogenin, are also trace mineral binding proteins that form important storage sites within the granule subfraction of yolk. The mobilization and uptake of egg trace mineral stores is mediated by the extra-embryonic membranes, principally the yolk sac membrane. The yolk sac also serves as a short-term storage site for trace minerals. Because it is an important site of plasma protein synthesis, the yolk sac has the ability to regulate the export of trace minerals to the embryo during development. Within the embryo, specific metaloproteins function in the interorgan transport cellular uptake, and intracellular storage of trace minerals. Thus, embryonic trace mineral homeostasis is established through the coordinated actions of the yolk sac, which mobilizes and exports trace minerals derived from egg stores; the vitelline circulation, which transports them to the embryo; and the liver, which accumulates trace minerals and distributes them to the rest of the tissues of the embryo via the embryonic circulation.

Identification of vitellogenin receptors in the ovary of a teleost fish, the Mediterranean sea bass (Dicentrarchus labrax)

The characterization of vitellogenin (VTG) receptors in ovarian membranes from vitellogenic female sea bass (Dicentrarchus labrax) is described. Incubation of membrane proteins with radiolabeled VTG (125I-VTG) after SDS-electrophoresis showed specific binding of 125I-VTG to a protein band of 100 kDa. Filter binding assays showed that binding of 125I-VTG to membrane receptors was saturable with increasing amounts of 125I-VTG. Scatchard analysis of the saturation data revealed a single class of binding sites with an apparent KD of 1.04 x 10(-8) M. The specificity of the VTG receptors was tested in competition assays; binding of 125I-VTG to ovarian membranes was completely abolished with an excess of purified sea bass VTG (cold VTG, VTG degree) or plasma from estradiol (E2)-treated fish, while the addition of control male plasma (without VTG) caused negligible effect.

YOLK PROTEIN UPTAKE IN THE OOCYTE OF THE MOSQUITO AEDES AEGYPTI. L

Yolk proteins are thought to enter certain eggs by a process akin to micropinocytosis but the detailed mechanism has not been previously depicted. In this study the formation of protein yolk was investigated in the mosquito Aedes aegypti L. Ovaries were fixed in phosphate-buffered osmium tetroxide, for electron microscopy, before and at intervals after a meal of blood. The deposition of protein yolk in the oocyte was correlated with a 15-fold increase in 140 mµ pit-like depressions on the oocyte surface. These pits form by invagination of the oocyte cell membrane. They have a 20 mµ bristle coat on their convex cytoplasmic side. They also show a layer of protein on their concave extracellular side which we propose accumulates by selective adsorption from the extraoocyte space. The pits, by pinching off from the cell membrane become bristle-coated vesicles which carry the adsorbed protein into the oocyte. These vesicles lose the coat and then fuse to form small crystalline yolk droplets, which subsequently coalesce to form the large proteid yolk bodies of the mature oocyte. Preliminary radioautographs, and certain morphological features of the fat body, ovary, and midgut, suggest that the midgut is the principal site of yolk protein synthesis in the mosquito.

Application of lipovitellin-salt-mannitol agar for screening, isolation, and presumptive identification of Staphylococcus aureus in a teaching hospital

Lipovitellin-salt-mannitol (LSM) plate medium was examined for its ability to directly isolate, recover, and presumptively identify Staphylococcus aureus from 418 clinical specimens. The criteria for medium evaluation included colony morphology reactions, selectivity, and ease of isolation. For 298 specimens used for screening, LSM agar medium was compared with the other conventional media used, mannitol salt agar (MSA), 5% horse blood agar (HBA), and phenolphthalein phosphate agar (PPA), to detect and recover S. aureus and methicillin-resistant S. aureus. The results indicated that LSM agar is more effective than MSA, HBA, or PPA for the recovery and isolation of S. aureus and methicillin-resistant S. aureus. On a replicator multipoint inoculation system, we compared the reactions on LSM agar, MSA, and DNase agar of 227 different strains of staphylococci, which included 178 different strains of S. aureus and 49 different strains of coagulase-negative staphylococci isolated from clinical specimens. By using the lipovitellin precipitation activity and mannitol fermentation characteristics, LSM agar gave a 100% correlation in presumptively identifying S. aureus. LSM agar may be an alternative plate medium for large hospital extensive screening for the detection and isolation of S. aureus.

Preferential recognition of the very low-density lipoprotein receptor ligand binding site by antibodies from phage display libraries

Screening of a phage library displaying single chain fragments of the variable regions of human immunoglobulins (scFv) for binding to the ovarian chicken very low-density lipoprotein/vitellogenin receptor (OVR) led to the isolation of several antibody fragments with high affinity. As for the natural ligands of OVR, receptor binding of all antibody fragments is strictly Ca(2+)-dependent and is prevented by receptor-associated protein (RAP). Moreover, attachment of human rhinovirus serotype 2 (HRV2) to this receptor is inhibited by all scFvs. In contrast to conventional immunization, the in vitro selection method thus exclusively led to antibodies that attach to or close to the ligand binding site and thereby block the receptor-ligand interaction.

Characterization of a vitellogenin receptor in white perch (Morone americana)

Receptors for white perch vitellogenin (wVTG) were characterized using wVTG, labeled in vivo with [3H]leucine or in vitro with 125I, and semipurified ovarian membranes. Specific binding of wVTG to the membranes was temperature-dependent, proportional to the amount of membrane, and saturable. Scatchard analyses revealed a single class of binding sites of low maximum binding capacity (MBC; approximately 35 pmol VTG/mg membrane protein) and high affinity (Kd approximately 400 nM), consistent with wVTG levels (540-2700 nM) circulating in maturing females. Ligand blotting revealed a receptor protein of M(r) approximately 157000 and a smaller protein, possibly its degradation product. Striped bass vitellogenin, chicken egg yolk very low density lipoprotein, and suramin displaced wVTG from its receptor, but BSA did not. No change in Kd was noted over the course of vitellogenesis in maturing perch, and MBC increased only slightly very late in the gametogenic cycle. The wVTG bound specifically to membranes prepared from liver, muscle, and mesenteric fat, but not to erythrocyte membranes. The Kd for ovary (394 nM) and liver (345 nM) were similar, but the Kd for muscle (1440 nM) was much higher.

Molecular characterization of the mosquito vitellogenin receptor reveals unexpected high homology to the Drosophila yolk protein receptor

The mosquito (Aedes aegypti) vitellogenin receptor (AaVgR) is a large membrane-bound protein (214 kDa when linearized) that mediates internalization of vitellogenin, the major yolk-protein precursor, by oocytes during egg development. We have cloned and sequenced two cDNA fragments encompassing the entire coding region of AaVgR mRNA, to our knowledge the first insect VgR sequence to be reported. The 7.3-kb AaVgR mRNA is present only in female germ-line cells and is abundant in previtellogenic oocytes, suggesting that the AaVgR gene is expressed early in oocyte differentiation. The deduced amino acid sequence predicts a 202.7-kDa protein before posttranslational processing. The AaVgR is a member of the low density lipoprotein receptor superfamily, sharing significant homology with the chicken (Gallus gallus) VgR and particularly the Drosophila melanogaster yolk protein receptor, in spite of a very different ligand for the latter. Distance-based phylogenetic analyses suggest that the insect VgR/yolk protein receptor lineage and the vertebrate VgR/low density lipoprotein receptor lineage diverged before the bifurcation of nematode and deuterostome lines.

cDNA cloning and expression of the Xenopus laevis vitellogenin receptor

A Xenopus laevis oocyte cDNA library was screened with a PCR-generated X. laevis vitellogenin (VTG) receptor probe and a 3.6 kb cDNA clone containing the entire open reading frame, and 5' and 3' noncoding regions were isolated. The deduced amino acid sequence was 72% homologous to the chicken VLDL/VTG receptor, and the characteristic domains were highly conserved. Ligand binding analysis confirmed that the cloned receptor was Xenopus VTG-specific. Although Northern blotting analysis revealed that this gene was expressed as a major transcript of 3.6 kb in Xenopus ovary, weak but significant expression was observed in other tissues by RT-PCR analysis. The fact that major expression of the gene occurs in the ovary suggests that it has an important function in this organ.

Estrogen receptor affinity and location of consensus and imperfect estrogen response elements influence transcription activation of simplified promoters

We have examined the ability of estrogen receptor (ER) to bind and bend DNA fragments containing the Xenopus laevis vitellogenin A2 estrogen response element (ERE), which contains a palindromic, consensus ERE sequence, the X. laevis vitellogenin B1 ERE2, which contains a 1-bp mismatch in the 5'-end of the half-palindrome, and the human pS2 ERE, which contains a 1-bp mismatch in the 3'-end of the half-palindrome. ER binding induced a 65 degrees bend in DNA fragments containing the consensus ERE, the vitellogenin B1 ERE2, or the pS2 ERE. However, ER affinity for the consensus ERE was 2-fold greater than for either the vitellogenin B1 ERE2 or the pS2 ERE. When Chinese hamster ovary (CHO) cells were transfected with reporter plasmids containing either the consensus ERE, the vitellogenin B1 ERE2, or the pS2 ERE separated from the TATA sequence by 26 helical turns, exposure to 10 nm 17 beta-estradiol increased transcription 12.7-, 2.4-, and 3.8-fold, respectively. Increasing the spacing between the ERE and TATA sequence to three helical turns decreased the ability of the consensus ERE to activate transcription by 55% and increased the ability of the pS2 ERE to activate transcription by 35% but had no significant effect on vitellogenin B1 ERE2 activity. Further increasing the distance between the ERE and TATA sequence to 3.6 helical turns restored the activity of promoters containing the consensus ERE and pS2 ERE but decreased the activity of the promoter containing the relatively weak vitellogenin B1 ERE2. These data support the idea that 1) the affinity of ER for the ERE, 2) the location of an ERE within the promoter, and 3) the magnitude and orientation of DNA bends induced by binding of ER or other proteins are important in transcription activation of estrogen-responsive genes.

X-ray absorption fine structure as a monitor of zinc coordination sites during oogenesis of Xenopus laevis

The x-ray absorption fine structure (XAFS) zinc K-edge steps for intact stages I,II and V,VI Xenopus laevis oocytes demonstrate that the zinc concentration is about 3 and 1 mM, respectively. However, the chi(k) function for the early stage oocytes differs markedly from that for the late one. Analysis of the XAFS data for stage I,II oocytes indicates that zinc is bound to 2.0 +/- 0.5 sulfur atoms at an average coordination distance of 2.29 +/- 0.02 angstroms and 2.0 +/- 0.5 nitrogen or oxygen (N/O) atoms at 2.02 +/- 0.02 angstroms. In marked contrast, in stage V,VI oocytes, zinc is bound to 4.1 +/- 0.4 N/O atoms at an average distance of 1.98 +/- 0.01 angstroms. Our previous studies demonstrated that 90% of the zinc in stage VI oocytes is sequestered within yolk platelets, associated with a single molecule, lipovitellin, the proteolytically processed product of vitellogenin. XAFS analysis of yolk platelets, lipovitellin, and vitellogenin demonstrates that zinc is bound to 4.0 +/- 0.5 N/O ligands at an average distance of 1.98 +/- 0.01 angstroms in each case, identical to that of stage V,VI oocytes. The higher shell contributions in the Fourier transforms indicate that two of the N/O zinc ligands are His in both stage V,VI and I,II oocytes. The results show that in stage I,II oocytes, there is a high concentration of a zinc protein whose zinc coordination site likely is composed of (His)2(Cys)2, such as, e.g., TFIIIA. As the oocytes develop, the predominant zinc species becomes one that exhibits the (His)2(N/0)2 zinc site found in lipovitellin. Hence, the ligands to the zinc atoms in intact oocytes and the changes that take place as a function of oogenesis and after their fertilization, during embryogenesis, now can be examined and explored.

Identification of four ovarian receptor proteins that bind vitellogenin but not other homologous plasma lipoproteins in the rainbow trout, Oncorhynchus mykiss

Membrane proteins from ovarian follicles, testis and somatic tissues of rainbow trout, Oncorhynchus mykiss, were extracted by ultracentrifugation, separated on sodium dodecyl sulphate gels and isolated on polyvinyl difluoride membranes. Vitellogenin receptor proteins were visualized using protein staining and hybridisation with 125I-vitellogenin. Four follicle-membrane proteins, with molecular masses of 220, 210, 110 and 100 kDa, showed a strong affinity for vitellogenin and were specific to the ovary. Other homologous lipoproteins (very low density lipoprotein, low density lipoprotein and high density lipoprotein) had a very limited ability to displace 125I-vitellogenin from its receptor, indicating that the ovarian receptor proteins were fairly specific for vitellogenin. Proteins with an affinity for very low density lipoprotein and low density lipoprotein were visualised in liver, spleen and muscle, eluting on sodium dodecyl sulphate gels with molecular masses of about 150 kDa. Peptides generated from trypsin digests of the receptor proteins with a high affinity for vitellogenin showed sequence homology with receptors in the lipoprotein family, including a sequence that is believed to act as the internalisation signal [Phe-Asp-Phe-Tyr-] and a sequence identity with the recently characterised chicken vitellogenin/very low density lipoprotein receptor [Ser-Glu-Leu-Tyr-Glu-Pro-Ala-]. Together, the ligand blotting and peptide sequence data support the contention that the four ovarian membrane proteins isolated are receptor proteins specific for vitellogenin and they do not bind other plasma lipoproteins to any significant degree.

Finely tuned regulation of cytoplasmic retention of Xenopus nuclear factor 7 by phosphorylation of individual threonine residues

Xenopus nuclear factor 7 (xnf7) is a maternal gene product that functi ons in dorsal/ventral patterning of the embryo. The xnf7 protein is stored in the oocyte nucleus germinal vesicle in a hypophosphorylated state. At oocyte maturation, xnf7 is hyperphosphorylated and released into the cytoplasm, where it is anchored until the midblastula stage, where it is dephosphorylated and enters the nucleus. We demonstrated that cytoplasmic anchoring of xnf7 was regulated by changes in the phosphorylation status of four threonines within two sites, site 1 (Thr-103) and site 2 (Thr-209, Thr-212, and Thr-218), which function in an additive manner. A mutant form of xnf7 (xnf7thr-glu) in which the threonines at sites 1 and 2 were mutated to glutamic acids to mimic a permanent state of phosphorylation was retained in the cytoplasm in oocytes and embryos through the gastrula stage. The cytoplasmic form of xnf7 was detected in a large 670-kDa protein complex probably consisting of xnf7 and several other unknown protein components. Anchoring of xnf7 was not dependent on association with either microtubule or microfilament components of the cytoskeleton, since treatment with cytochalasin B and nocodazole did not affect cytoplasmic retention. Both wild-type xnf7 and xnf7thr-glu form dimers in the yeast two-hybrid system; however, homodimerization was not required for cytoplasmic retention. We suggest that the cytoplasmic retention of xnf7 depends on the phosphorylation state of the protein whereas the cytoplasmic anchoring machinery appears to be constitutively present in oocytes and throughout development until the gastrula stage.

Vitellogenin receptors during vitellogenesis in the rainbow trout Oncorhynchus mykiss

Rainbow trout vitellogenin receptors have been characterized by ligand blotting and Scatchard analysis. Their evolution has been studied over a reproductive cycle in a broodstock of 2-year-old females. The receptors were prepared from ovarian membrane homogenates and were solubilized using n-octyl-beta-D-glucopyranoside. The visualization of the receptor by ligand blotting using 125iodine-vitellogenin after sodium dodecyl sulfate electrophoresis revealed the existence of one major binding component corresponding to a protein of 113 kDa. The Scatchard transformation of the binding data revealed a single class of binding sites with an apparent Kd of 1.8 x 10(-8) M/L. The variations of the binding characteristics (Kd and maximum binding) were investigated during vitellogenesis. This study revealed that the Kd was not affected by oocyte growth during vitellogenesis, but was highly decreased in ovulated eggs. The receptor number increased during the same period from 35 to 860 fM per oocyte, while the receptor number per mm2 of oocyte membrane surface was doubled during the same period.

Vitellogenin receptors: oocyte-specific members of the low-density lipoprotein receptor supergene family

Receptors that transport vitellogenin (VTG) into oocytes are of vital importance to egg-laying species, because they mediate a key step of oocyte maturation, a prerequisite to reproduction. Vitellogenins are lipophosphoglycoproteins that are produced under female hormonal control in large central organs (fat body in insects; liver in higher animals) and are transported in the circulation to the female gonads. VTG receptors localized in coated pits on the surface of growth-competent oocytes are able to accumulate in the yolk high concentrations of VTG and other ligands they recognize. The study of VTG receptors and their ligands has identified genes that specify related ligands, and a family of receptors. To date, all molecularly characterized VTG receptors belong to the low-density lipoprotein receptor supergene family, which ranges from a 600-kDa receptor in Caenorhabditis elegans to the 100-kDa so-called very-low-density lipoprotein receptors in mammals. These receptors, by and large, recognize ligands with similarities in structural elements first defined in the human apoplipoproteins B-100 and E. Recent studies on the receptor family have added VTG and lipoprotein lipase to the list of co-evolved ligands and have revealed that VTG receptors are able to interact with ligands other than VTG and also with some unrelated to lipoprotein metabolism. For example, the chicken VTG receptor also imports very-low-density lipoprotein, riboflavin-binding protein, and alpha-2-macroglobulin into growing oocytes. Such multifunctionality of receptors is likely the result of evolutionary pressure to provide the female germ cell with a highly economical machinery for vitellogenesis.

Novel topology of a zinc-binding domain from a protein involved in regulating early Xenopus development

Xenopus nuclear factor XNF7, a maternally expressed protein, functions in patterning of the embryo. XNF7 contains a number of defined protein domains implicated in the regulation of some developmental processes. Among these is a tripartite motif comprising a zinc-binding RING finger and B-box domain next to a predicted alpha-helical coiled-coil domain. Interestingly, this motif is found in a variety of protein including several proto-oncoproteins. Here we describe the solution structure of the XNF7 B-box zinc-binding domain determined at physiological pH by 1H NMR methods. The B-box structure represents the first three-dimensional structure of this new motif and comprises a monomer have two beta-strands, two helical turns and three extended loop regions packed in a novel topology. The r.m.s. deviation for the best 18 structures is 1.15 A for backbone atoms and 1.94 A for all atoms. Structure calculations and biochemical data shows one zinc atom ligated in a Cys2-His2 tetrahedral arrangement. We have used mutant peptides to determine the metal ligation scheme which surprisingly shows that not all of the seven conserved cysteines/histidines in the B-box motif are involved in metal ligation. The B-box structure is not similar in tertiary fold to any other known zinc-binding motif.

Towards molecular mechanisms for gamete adhesion and fusion during mammalian fertilization

The process of fertilization is fundamental to the maintenance of life. The ability of unfertilized eggs and free-swimming sperm to recognize, adhere to, and fuse with each other are vital aspects of the fertilization process. Significant progress has been made recently in identifying and characterizing (glyco)proteins that are associated with the surface of mammalian gametes and that support adhesion and fusion of eggs and sperm. As a result, there are currently reasons to believe that, in mammals, species-specific gamete adhesion is carbohydrate-mediated and gamete fusion is intergrin-mediated.

Xenopus lipovitellin 1 is a Zn(2+)- and Cd(2+)-binding protein

This report discusses the identification of a Zn(2+)- and Cd(2+)-binding protein of Xenopus laevis that is abundant in vitellogenic oocytes and in embryos from fertilization to stage 46. Oocyte or embryo homogenates were fractionated by SDS-PAGE, blotted onto nitrocellulose, and probed with 65Zn2+ or 109Cd2+. The resulting autoradiograms showed binding of both radionuclides to a protein, designated pCdZn. Freon extraction of oocyte and embryo homogenates showed pCdZn to be a yolk protein. When pCdZn was isolated from oocyte homogenates by ammonium sulfate precipitation, delipidation, and chromatography, it co-purified with lipovitellin 1. The amino acid composition of pCdZn closely resembled the reported composition of lipovitellin 1 and the molecular weight of purified pCdZn (approximately 115 kD) corresponded to reported values for lipovitellin 1 (111-121 kD). Amino acid sequence analyses of five peptides derived from pCdZn yielded 94% identity to the reported sequence of lipovitellin 1, deduced from the DNA sequence of the Xenopus vitellogenin A2 precursor gene. Based on these findings, pCdZn was identified as lipovitellin 1. This study suggests that lipovitellin 1 is the major storage protein for zinc in mature oocytes and developing embryos of Xenopus laevis.

Analysis of a vitellogenin gene of the mosquito, Aedes aegypti and comparisons to vitellogenins from other organisms

A genomic clone of the Aedes aegypti vitellogenin A1 gene was sequenced including 2015 bp of 5' untranscribed sequence, 6369 bp of open reading frame interrupted by two introns, and a short 3' untranslated region. Primer extension was used to identify the transcription initiation site. The amino termini of the large and small subunits were located by N-terminal sequencing of vitellin purified from eggs. The length of the signal sequence and the position of the cleavage site between the two subunits were also determined. Three sequential imperfect repeats were found near the beginning of the small subunit. The sequence of the coding region appears to be polymorphic. Comparison of the signal sequences of seven insect vitellogenin genes revealed several conserved leucines, and a conserved position of an intron. However, the signal sequences are not conserved between these genes and the yolk protein genes of Cyclorraphid Dipteran insects. The cleavage sites between the small and large subunits in the vitellogenins of the mosquito, A. aegypti, sawfly, Athalia rosae, boll weevil, Anthonomus grandis, and silkworm, Bombyx mori are flanked by sequences rich in serine. Pairwise dot matrix analysis at the protein level showed that the mosquito, boll weevil and silkworm vitellogenins are significantly related with approx. 50% similarity. One region of the three insect vitellogenin genes, near the N-terminal of the large subunit, showed the highest levels of similarity, from 57.5 to 64.4%. The position of cysteines in insect vitellogenins is conserved, particularly in the C-terminus of the large subunit. Dot matrix comparison of the mosquito vitellogenin with that of Xenopus laevis and Caenorhabditis elegans showed much lower, but still significant degrees of relationship. Pairwise comparisons of the mosquito vitellogenin and the Drosophila melanogaster yolk proteins did not show significant similarities. Potential regulatory regions in the mosquito VgA1 gene were identified by comparison to regulatory elements known from other organisms, especially D. melanogaster, which could provide useful information for further functional analysis.

Vitellogenin and lipovitellin: zinc proteins of Xenopus laevis oocytes

Xenopus laevis vitellogenin is a plasma protein that contains a total of 5 mol of metal/440 kDa dimer, 2 mol of zinc, and 3 mol of calcium (Montorzi et al. (1994) Biochem. Biophys. Res. Commun. 200, 1407-1413]. There are no other group IIB or transition metals in the molecule. The zinc atoms are removed instantaneously by 1,10-phenanthroline (OP) (pK 4.8). Once internalized by receptor-mediated endocytosis, vitellogenin is cleaved into multiple polypeptides, i.e., the two lipovitellin subunits (1 and 2) plus phosvitin; these are then stored as microcrystals within yolk platelets. We here show by metal analysis of the individual proteins generated by vitellogenin processing that zinc and calcium occur in different domains of the vitellogenin polypeptide chain. All of the vitellogenin zinc is present in lipovitellin, in amounts equal to 1 mol of zinc/141 kDa. Calcium, in contrast, is detected exclusively in phosvitin which, in addition, contains 3 mol of magnesium/35 kDa, apparently acquired following vitellogenin entry into the oocyte. The zinc in lipovitellin is removed by OP in a concentration-dependent manner with a pK of 4.8, identical to that obtained for vitellogenin, and by exposure to acidic conditions (below pH 5). Following removal of zinc, the two lipovitellin subunits remain associated, suggesting that zinc is not involved in their interaction. On exposure to 1% SDS, lipovitellin does dissociate into 106 and 33 kDa subunits. The presence of stoichiometric quantities of zinc in both vitellogenin and lipovitellin calls for the study of the hitherto unrecognized biochemistry and functions of these proteins in zinc metabolism and development of the frog oocyte and embryo.

Chromatin condensation activity and cortical activity during the first three cell cycles of a mouse embryo

One-cell parthenogenetic haploid embryos and blastomeres of the 2- and 4-cell diploid mouse embryos were observed in vitro for the occurrence of two cytoplasmic activities: the cortical activity and the chromatin condensation activity. For this purpose anucleated halves (AHs) and nucleated halves (NHs) were produced by bisection of one-cell embryos and of blastomeres. The cortical activity (manifested by surface deformations) was observed only during the first cleavage cycle. In AHs the surface activity began at the same time as in NHs and disappeared before the time of the cleavage division of nucleated halves. Anucleate fragments of blastomeres from 2- and 4-cell embryos did not exhibit any cortical activity. In the absence of the native nucleus the chromatin condensation activity (assayed by premature chromatin condensation of interphase thymocyte nuclei introduced into cytoplasts by cell fusion) could also have been detected only in the first cleavage cycle. In AHs this activity appeared at the time when NHs started to cleave and disappeared after the NHs finished the first cleavage division. AHs obtained from 2-cell and 4-cell stage blastomeres did not reveal condensation activity.

Mosquito vitellogenin receptor: purification, developmental and biochemical characterization

Vitellogenin receptors (VgRs) play a critical role in egg development of oviparous animals by mediating endocytosis of the major yolk protein precursor, vitellogenin. A modification of the method for extracting the mosquito (Aedes aegypti) VgR from ovary membranes resulted in an 11-fold higher yield and 56-fold increase in relative purity of the VgR, in turn permitting purification, antibody production, and microsequencing. A Kd of 15 nM was estimated from binding assays for the enriched VgR, indicating a very high affinity for its ligand. Immunoprecipitation of [14C]VgR using anti-VgR polyclonal antibodies followed by SDS-PAGE under reducing conditions and fluorography demonstrated that the 205 kDa VgR does not consist of subunits held together with disulfide bonds. However, an immunoblot of the native VgR suggests that it exists as an approximately 390 kDa noncovalent homodimer in its native state. Immunoblot assays confirmed that the VgR is present only in ovarian tissue. A quantitative immunoassay of VgR extracts showed that VgR was present in previtellogenic ovaries on the day of emergence, increasing from 2 ng to more than 10 ng per ovary by day 5. After initiation of vitellogenesis and onset of Vg uptake, VgR quantity increased rapidly between 8 and 24 h after a blood meal, then began to decline between 24 and 36 h. Immunocytochemistry confirmed the presence of substantial amounts of the VgR in 4-day-old previtellogenic oocytes. In both previtellogenic and vitellogenic ovaries, the VgR was present only in the oocyte, primarily in the cortex.

Characterization of the Japanese quail oocyte receptor for very low density lipoprotein and vitellogenin

The plasma membrane receptor for VLDL and vitellogenin from oocytes of Japanese quail (Coturnix coturnix japonica) was characterized and compared with that of another domestic fowl, the chicken (Gallus domesticus). When visualized by ligand blotting with biotinylated or 125I-labeled lipoproteins, the quail VLDL/vitellogenin receptor had an apparent M(r) of 95 kDa under nonreducing conditions, identical to that of the chicken receptor. Upon analysis by ligand blotting, binding of radiolabeled quail plasma VLDL to the quail oocyte receptor seemed to be saturable and exhibited high affinity (apparent Kd of 13.9 mg/L). Cross-reactivity, at the level of ligand recognition, was observed between quail and chicken VLDL/vitellogenin receptors, and immunological relatedness was demonstrated by Western blotting with a rabbit anti-chicken oocyte VLDL receptor antibody. In contrast, a species difference was observed in the apolipoprotein VLDL-II moiety of plasma VLDL. Chicken apolipoprotein VLDL-II, an 82-amino acid protein with a disulfide crosslink at residue 75 (the sole cysteine residue), existed as a homodimer of 9.5 kDa subunits and, to a lesser extent, as a monomer. Quail apolipoprotein VLDL-II existed only in monomeric form without reduction and lacked cysteine. The present results demonstrate that, despite a difference in an apolipoprotein moiety of VLDL, quail and chicken oocyte lipoprotein receptors share key structural and functional elements. This lends further support to the notion that receptor recognition is mediated by the common VLDL component, apolipoprotein B.

A double labeling procedure for lipoproteins: independent visualization of dual ligand-receptor interaction with colloidal gold- and 125I-labeled ligands

The analysis of multiple ligand binding to a single receptor molecule poses a methodological challenge. The chicken oocyte 95-kDa receptor for the uptake of the two major yolk lipoprotein precursors very low-density lipoprotein (VLDL) and vitellogenin (VTG) is such a multipotent transport receptor. Here we describe methods for rapid independent and simultaneous analysis of VLDL and VTG binding to this receptor, termed VLDL/VTG receptor. First, further development of a one-step labeling protocol for chicken lipoproteins with colloidal gold (Au) to visualize independently the binding of VLDL and VTG to the chicken VLDL/VTG receptor is reported. The advantage of this protocol is that the preparation of the Au-lipoprotein conjugates is rapid, and utilization of Au-lipoprotein complexes in ligand blots does not require their further purification, while signal enhancement by silver staining is still applicable. Second, the simultaneous use of 125I- and Au-labeled ligands in a one-step ligand blotting procedure facilitates the direct demonstration of the competitive interaction of VLDL and VTG with the receptor. Following sequential processing of the nitrocellulose strips, binding of differently labeled ligands can be studied independently. In summary, we describe a procedure for differential labeling of lipoproteins and its application toward the analysis of receptors that bind more than one ligand.

The Drosophila yolkless gene encodes a vitellogenin receptor belonging to the low density lipoprotein receptor superfamily

Sequence comparisons of vitellogenins from a wide range of organisms have identified regions of similarity not only to each other but also to vertebrate apolipoproteins (e.g. apoB-100 and apoE). Furthermore, the chicken vitellogenin receptor, which also binds apolipoproteins receptor (LDLR) superfamily [Bujo, H., Hermann, M., Kaderli, M. O., Jacobsen, L., Sugawara, S., Nimpf, J., Yamamoto, T. & Schneider, W. J. (1994) EMBO J. 13, 5165-5175]. The yolk proteins of higher dipterans are exceptional, however, and instead show similarity to lipoprotein lipases. The molecular characterization of the putative Drosophila melanogaster vitellogenin receptor gene, yolkless (yl), described in this report reveals that the protein it encodes (Yl), is also a member of the LDLR superfamily. The ovary-specific 6.5-kb yl RNA codes for a protein of approximately 210 kDa which contains all three motifs common to the LDLR class of proteins. Within this superfamily, Yl may be related more to the LDLR-related proteins (LRPs), which bind both apolipoproteins and lipoprotein lipases. The similarity of Yl to the other LDLR proteins is restricted to the putative extracellular domain. Most noticeably, the cytoplasmic domain of Yl lacks the typical NPXY sequence which is involved in receptor internalization.

Kinetics of compaction during lysozyme refolding studied by continuous-flow quasielastic light scattering

We recently developed an experiment, termed continuous-flow quasielastic light scattering (QLS), that is capable of monitoring the time evolution of the hydrodynamic diameter of macromolecules or macromolecular assemblies in solution. Here we report the use of this method to directly monitor the kinetics of compaction of the polypeptide chain of hen egg white lysozyme (HEWL) when protein refolding is initiated by 10-fold dilution from 5 M guanidine hydrochloride (GuHCl) at pH 1.5, 23 degrees C. Previously, such information could only be obtained indirectly, by analysis of the kinetics of binding ans release of a fluorescent probe dye. Refolding was also monitored by UV difference absorption spectroscopy to characterize the time scale of the formation of the native environment around the aromatic side chains under the same conditions used in the continuous-flow QLS experiments. We find that HEWL becomes compact within 1 s after the initiation of refolding, the shortest time that is accessible with our first-generation instrument. This time scale is shorter than that for the recovery of the native absorbances in the aromatic region. These results provide direct evidence that the intermediate on the folding pathway of lysozyme is compact. The implications of these results for models of protein folding are discussed.

Chicken oocyte growth is mediated by an eight ligand binding repeat member of the LDL receptor family

Deposition of the yolk mass components of chicken oocytes, very low density lipoprotein (VLDL) and vitellogenin (VTG), is mediated by a 95 kDa plasma membrane protein, termed VLDL/VTG receptor (VLDL/VTGR). Molecular characterization of the VLDL/VTGR revealed that it is a member of the LDLR gene superfamily, and harbours eight complement-type, cysteine-rich ligand binding repeats at the N-terminus. This ligand binding domain structure is the hallmark of the recently discovered mammalian so-called VLDLRs, whose true physiological function remains to be elucidated. Northern blot analysis revealed that this receptor is expressed almost exclusively in oocytes, with very much lower levels of hybridizing transcripts present in heart and skeletal muscle. Heterologous expression of the cloned receptor demonstrated its ability to bind both VLDL and VTG. The receptor gene is located on the avian sex chromosome Z, in agreement with the sex linkage of a single-gene defect in animals that fail to reproduce because of the lack of expression of functional VLDL/VTGR. In situ hybridization analysis of oocytes suggested that VLDL/VTGR mRNA may relocalize during oocyte growth. Thus, the current study has identified and characterized the first non-mammalian VLDLR. Its key role in avian reproduction and extremely high evolutionary conservation shed new light on VLDLR function in mammals, which also express the gene in ovaries.