Homology of Drosophila yolk proteins and the triacylglycerol lipase family

Finishing the egg

Gamete development is a fundamental process that is highly conserved from early eukaryotes to mammals. As germ cells develop, they must coordinate a dynamic series of cellular processes that support growth, cell specification, patterning, the loading of maternal factors (RNAs, proteins, and nutrients), differentiation of structures to enable fertilization and ensure embryonic survival, and other processes that make a functional oocyte. To achieve these goals, germ cells integrate a complex milieu of environmental and developmental signals to produce fertilizable eggs. Over the past 50 years, Drosophila oogenesis has risen to the forefront as a system to interrogate the sophisticated mechanisms that drive oocyte development. Studies in Drosophila have defined mechanisms in germ cells that control meiosis, protect genome integrity, facilitate mRNA trafficking, and support the maternal loading of nutrients. Work in this system has provided key insights into the mechanisms that establish egg chamber polarity and patterning as well as the mechanisms that drive ovulation and egg activation. Using the power of Drosophila genetics, the field has begun to define the molecular mechanisms that coordinate environmental stresses and nutrient availability with oocyte development. Importantly, the majority of these reproductive mechanisms are highly conserved throughout evolution, and many play critical roles in the development of somatic tissues as well. In this chapter, we summarize the recent progress in several key areas that impact egg chamber development and ovulation. First, we discuss the mechanisms that drive nutrient storage and trafficking during oocyte maturation and vitellogenesis. Second, we examine the processes that regulate follicle cell patterning and how that patterning impacts the construction of the egg shell and the establishment of embryonic polarity. Finally, we examine regulatory factors that control ovulation, egg activation, and successful fertilization.

The ABCs of the atypical Fam20 secretory pathway kinases

The study of extracellular phosphorylation was initiated in late 19th century when the secreted milk protein, casein, and egg-yolk protein, phosvitin, were shown to be phosphorylated. However, it took more than a century to identify Fam20C, which phosphorylates both casein and phosvitin under physiological conditions. This kinase, along with its family members Fam20A and Fam20B, defined a new family with altered amino acid sequences highly atypical from the canonical 540 kinases comprising the kinome. Fam20B is a glycan kinase that phosphorylates xylose residues and triggers peptidoglycan biosynthesis, a role conserved from sponges to human. The protein kinase, Fam20C, conserved from nematodes to humans, phosphorylates well over 100 substrates in the secretory pathway with overall functions postulated to encompass endoplasmic reticulum homeostasis, nutrition, cardiac function, coagulation, and biomineralization. The preferred phosphorylation motif of Fam20C is SxE/pS, and structural studies revealed that related member Fam20A allosterically activates Fam20C by forming a heterodimeric/tetrameric complex. Fam20A, a pseudokinase, is observed only in vertebrates. Loss-of-function genetic alterations in the Fam20 family lead to human diseases such as amelogenesis imperfecta, nephrocalcinosis, lethal and nonlethal forms of Raine syndrome with major skeletal defects, and altered phosphate homeostasis. Together, these three members of the Fam20 family modulate a diverse network of secretory pathway components playing crucial roles in health and disease. The overarching theme of this review is to highlight the progress that has been made in the emerging field of extracellular phosphorylation and the key roles secretory pathway kinases play in an ever-expanding number of cellular processes.

The Golgi 'casein kinase' Fam20C is a genuine 'phosvitin kinase' and phosphorylates polyserine stretches devoid of the canonical consensus

Egg yolk phosvitins, generated through the fragmentation of vitellogenins (VTGs), are among the most heavily phosphorylated proteins ever described. Despite the early discovery in 1900 that chicken phosvitin is a phosphoprotein and its subsequent employment as an artificial substrate for a number of protein kinases, the identity of the enzyme(s) responsible for its phosphorylation remained a matter of conjecture until present. Here, we provide evidence that phosvitin phosphorylation is catalyzed by a family with sequence similarity 20, member C (Fam20C), an atypical protein kinase recently identified as the genuine casein kinase and responsible for the phosphorylation of many other secreted proteins at residues specified by the S-x-E/pS consensus. Such a conclusion is grounded on the following observations: (a) the levels of Fam20C and phosphorylated VTG rise in parallel upon treatment of zebrafish with oestrogens; (b) zebrafish phosvitin is readily phosphorylated upon coexpression in U2OS cells with Fam20C, but not with its catalytically inactive mutant; (c) a peptide reproducing a stretch of 12 serines, which are phosphorylated in chicken phosvitin despite lacking the C-terminal priming motif S-x-E, is efficiently phosphorylated by both recombinant and native Fam20C. The last finding expands the repertoire of potential targets of Fam20C to include several proteins known to harbor (p-Ser)n clusters not specified by any known kinase consensus.

Targeted delivery of CRISPR-Cas9 ribonucleoprotein into arthropod ovaries for heritable germline gene editing

Cas9-mediated gene editing is a powerful tool for addressing research questions in arthropods. Current approaches rely upon delivering Cas9 ribonucleoprotein (RNP) complex by embryonic microinjection, which is challenging, is limited to a small number of species, and is inefficient even in optimized taxa. Here we develop a technology termed Receptor-Mediated Ovary Transduction of Cargo (ReMOT Control) to deliver Cas9 RNP to the arthropod germline by injection into adult female mosquitoes. We identify a peptide (P2C) that mediates transduction of Cas9 RNP from the female hemolymph to the developing mosquito oocytes, resulting in heritable gene editing of the offspring with efficiency as high as 0.3 mutants per injected mosquito. We demonstrate that P2C functions in six mosquito species. Identification of taxa-specific ovary-specific ligand-receptor pairs may further extend the use of ReMOT Control for gene editing in novel species.

Lipoprotein lipase: from gene to obesity

Lipoprotein lipase (LPL) is a multifunctional enzyme produced by many tissues, including adipose tissue, cardiac and skeletal muscle, islets, and macrophages. LPL is the rate-limiting enzyme for the hydrolysis of the triglyceride (TG) core of circulating TG-rich lipoproteins, chylomicrons, and very low-density lipoproteins (VLDL). LPL-catalyzed reaction products, fatty acids, and monoacylglycerol are in part taken up by the tissues locally and processed differentially; e.g., they are stored as neutral lipids in adipose tissue, oxidized, or stored in skeletal and cardiac muscle or as cholesteryl ester and TG in macrophages. LPL is regulated at transcriptional, posttranscriptional, and posttranslational levels in a tissue-specific manner. Nutrient states and hormonal levels all have divergent effects on the regulation of LPL, and a variety of proteins that interact with LPL to regulate its tissue-specific activity have also been identified. To examine this divergent regulation further, transgenic and knockout murine models of tissue-specific LPL expression have been developed. Mice with overexpression of LPL in skeletal muscle accumulate TG in muscle, develop insulin resistance, are protected from excessive weight gain, and increase their metabolic rate in the cold. Mice with LPL deletion in skeletal muscle have reduced TG accumulation and increased insulin action on glucose transport in muscle. Ultimately, this leads to increased lipid partitioning to other tissues, insulin resistance, and obesity. Mice with LPL deletion in the heart develop hypertriglyceridemia and cardiac dysfunction. The fact that the heart depends increasingly on glucose implies that free fatty acids are not a sufficient fuel for optimal cardiac function. Overall, LPL is a fascinating enzyme that contributes in a pronounced way to normal lipoprotein metabolism, tissue-specific substrate delivery and utilization, and the many aspects of obesity and other metabolic disorders that relate to energy balance, insulin action, and body weight regulation.

vLIP, a viral lipase homologue, is a virulence factor of Marek's disease virus

The genome of Marek's disease virus (MDV) has been predicted to encode a secreted glycoprotein, vLIP, which bears significant homology to the alpha/beta hydrolase fold of pancreatic lipases. Here it is demonstrated that MDV vLIP mRNA is produced via splicing and that vLIP is a late gene, due to its sensitivity to inhibition of DNA replication. While vLIP was found to conserve several residues essential to hydrolase activity, an unfavorable asparagine substitution is present at the lipase catalytic triad acid position. Consistent with structural predictions, purified recombinant vLIP did not show detectable activity on traditional phospholipid or triacylglyceride substrates. Two different vLIP mutant viruses, one bearing a 173-amino-acid deletion in the lipase homologous domain, the other having an alanine point mutant at the serine nucleophile position, caused a significantly lower incidence of Marek's disease in chickens and resulted in enhanced survival relative to two independently produced vLIP revertants or parental virus. These data provide the first evidence that vLIP enhances the replication and pathogenic potential of MDV. Furthermore, while vLIP may not serve as a traditional lipase enzyme, the data indicate that the serine nucleophile position is nonetheless essential in vivo for the viral functions of vLIP. Therefore, it is suggested that this particular example of lipase homology may represent the repurposing of an alpha/beta hydrolase fold toward a nonenzymatic role, possibly in lipid bonding.

Cloning and expression of the yolk protein of the tsetse fly Glossina morsitans morsitans

Two major families of nutritional proteins exist in insects, namely the vitellogenins and the yolk proteins. While in other insects only vitellogenins are found, cyclorraphan flies only contain yolk proteins. Possible sites of yolk protein synthesis are the fat body and the follicle cells surrounding the oocyte. We report the cloning of the yolk protein of the tsetse fly Glossina morsitans morsitans, a species with adenotrophic viviparity. The tsetse fly yolk protein could be aligned with other dipteran yolk proteins and with some vertebrate lipases. In contrast to the situation in most fly species, only a single yolk protein gene was found in the tsetse fly. Northern blot analysis showed that only the ovarian follicle cells, and not the fat body represents the site of yolk protein synthesis.

A mammalian-like lipase gene is expressed in the female reproductive accessory glands of the sand fly Phlebotomus papatasi (Diptera, Psychodidae)

A lipase-like protein (PhpaLIP) was identified as the major protein component in the secretion of the female reproductive accessory glands of the sand fly Phlebotomus papatasi. The full-length cDNA encoding this protein was isolated and its nucleotide sequence determined. The deduced translational product of the gene contains a GFSFG motif, consistent with a GXSXG consensus, which is shared by most bacterial and eukaryotic hydrolases. Transcriptional analysis of the PhpaLIP gene showed that its expression is female-specific, and is also detectable in districts other than accessory glands, suggesting that it might play different functions. Taken together with the observation of sequence similarity shared by PhpaLIP and mammalian lipases, the demonstration of the presence of lipase activity in the accessory gland secretion suggests a possible biological role of PhpaLIP gene product.

Fat body expressed yolk protein genes in Hyphantria cunea are related to the YP4 follicular epithelium yolk protein subunit gene of pyralid moths

cDNA clones for two of the yolk proteins, YP1 and YP2, produced by the fat body of the moth, Hyphantria cunea, were sequenced and found to be homologous to the follicular epithelium yolk proteins of pyralid moths. Both cDNA clones coded for polypeptides of 290 residues and the deduced amino acid sequence identity between YP1 and YP2 was very high (79.0%). Analysis of the secondary structure of the predicted polypeptides suggests that YP1 and YP2 do not form heteromeric proteins because of differences in secondary structure due to the lack of alpha helices in YP1. Northern blot analysis showed that the transcripts for YP1 (1.2 kb) and YP2 (1.1 kb) were present primarily in the female fat body with only trace levels detectable in the ovary of the adult female. In a developmental study, the YP1 and YP2 transcripts were first detectable in 10-day-old pupae and increased into the adult stage. These results suggest that the YP1 and YP2 genes in H. cunea have been recruited to replace the vitellogenin gene as the primary source of yolk proteins. During this process they have acquired a modified pattern of expression that is different from homologous genes reported in pyralid moths. The assessment of the evolution of proteinaceous yolk in these moths should serve as an excellent model for the evolution of gene recruitment.

Occurrence of vertebrate steroids, estradiol 17beta and progesterone in the reproducing females of the mud crab Scylla serrata

In crustaceans, vitellogenesis is known to be controlled by eyestalk neuropeptides, biogenic amines, ecdysteroids and a juvenile hormone-like compound, methyl farnesoate. In recent years, the occurrence of vertebrate steroid hormones, estradiol 17beta (E2) and progesterone (PG) has also been reported in a few decapods, although their precise role in female reproduction is yet to be determined. The levels of E2 and PG in the ovary, hepatopancreas and the hemolymph of the red mud crab, Scylla serrata were analyzed in different vitellogenic stages in order to establish a correlation between hormone profile and stages of vitellogenesis. It was observed that the levels of both the steroids increased steeply in the tissues at the onset of vitellogenesis (vitellogenic stage I). Maximum levels of estradiol were present in the hepatopancreas whereas the highest concentration of progesterone was seen in the ovary, suggesting dichotomous roles for these hormones in vitellogenesis. Furthermore, levels of these hormones were estimated in different embryonic stages of the eggs of the sand crab Emerita asiatica and mud crab S. serrata. Their levels fluctuated, following a definite pattern in the different stages, suggesting a possible functional role as morphogenetic hormones. This study, in addition, also reports the presence of E2 and PG on lipovitellin purified from ovary and eggs as well as vitellogenin purified from the hemolymph implicating a role for these lipoproteins as steroid carriers.

Invasion genetics of the Mediterranean fruit fly: variation in multiple nuclear introns

Biological invasions generally start from low initial population sizes, leading to reduced genetic variation in nuclear and especially mitochondrial DNA. Consequently, genetic approaches for the study of invasion history and population structure are difficult. An extreme example is the Mediterranean fruit fly, Ceratitis capitata (Medfly), for which successive invasions during this century have resulted in a loss of 60% of ancestral genetic variation in isozymes and 75% of variation in mitochondrial DNA. Using Medflies as an example, we present a new approach to invasion genetics that measures DNA sequence variation within introns from multiple nuclear loci. These loci are so variable that even relatively recently founded Medfly populations within California and Hawaii retain ample genetic diversity. Invading populations have only lost 35% of the ancestral genetic variation. Intron variation will allow high-resolution genetic characterization of invading populations in both natural and managed systems, although non-equilibrium methods of analysis may be necessary if the genetic diversity represents sorting ancestral polymorphism.

The Drosophila melanogaster lipase homologs: a gene family with tissue and developmental specific expression

We report the molecular cloning of Drosophila genes encoding putative lipase homologs, Dm lip1, lip2 and lip3, the definition of their structure and the expression patterns during development. These Drosophila lipases are related to acid lipases, with a common GHSQG motif, within a more general consensus GXSXG, identified as the active site shared by all the members of lipase superfamily. The lip1 and lip3 genes are transcribed in different tissues and developmental stages, suggesting that they have different functions. The lip1 gene, coding for a protein similar to digestive lipases, is expressed in ovaries and early embryos and, with a different sized transcript, in all the other developmental stages. The lip3 gene, whose translation product is more similar to lysosomal acid lipases, is expressed only during the larval period. The lip2 gene seems non-functional. The Drosophila putative lipases do not show similarity with the Drosophila yolk proteins that are reported to have sequence similarity with lipoprotein lipases, but share a consistent similarity with lepidopteran proteins reported as egg specific or yolk proteins, probably corresponding to lipase homologs. The results reported here are discussed in relation to the evolution and functions of lipases within the between species.

5' coding region of the follicular epithelium yolk polypeptide 2 cDNA in the moth, Plodia interpunctella, contains an extended coding region

The 5' region of YP2 cDNA, a follicular epithelium yolk protein subunit in the moth, Plodia interpunctella, shows that the polypeptide contains an extended internal coding region. Partial cDNA clones for YP2 were isolated from a pharate adult female ovarian cDNA expression library in Lambda Zap II by screening with antigen selected YP2 antiserum. The 5' sequence of the YP2 transcript was determined by 5' RACE PCR of ovarian mRNA using YP2 sequence-specific nested primers. The combined cDNA and 5' RACE sequencing showed the YP2 transcript to be 1971 bp in length up to the poly(A) tail with a single open reading frame for a predicted polypeptide of 616 amino acids. Northern analysis showed a single YP2 transcript to be present in ovarian RNA that was approximately 2 kb in length. The predicted amino acid sequence for YP2 from P. interpunctella is most closely related to egg specific protein (ESP) from Bombyx mori and the partial YP2 sequence from Galleria mellonella. YP2 from P. interpunctella also is similar to vertebrate lipases and contains a conserved lipid binding region. However, the 5' coding region of YP2 from P. interpunctella contains an in-frame insert of approximately 438 bp that had replaced an approximately 270-bp region as compared with ESP from B. mori and YP2 of G. mellonella. This suggests that the insert occurred by a recombinational event internal to the YP2 structural gene of P. interpunctella.

Developmental regulation of yolk protein gene expression in Anastrepha suspensa

A partial cDNA clone for the 48,000 dalton yolk polypeptide gene from Anastrepha suspensa was isolated from a cDNA expression library using a yolk polypeptide antibody probe and hybridization to the Drosophila melanogaster yolk protein 1 gene. The sequenced DNA has greatest homology to the yolk protein genes from Ceratitis capitata, D. melanogaster, and Calliphora erythrocephala and, similar to these genes, shares amino acid sequence domains with those from lipases. RNA hybridization studies indicated that the yolk protein gene expression is completely female-specific and limited to the ovaries, without apparent regulation by 20-hydroxyecdysone or juvenile hormone. This is in contrast to an earlier study which suggested, based on immunological probes, that a very low level of yolk protein synthesis occurred in fat body and was not sex-specific.

Isolation, characterization and complete nucleotide sequence of a Galleria mellonella cDNA coding for the follicle cell-specific yolk protein, YP4

An ovariole-specific yolk polypeptide cDNA from Galleria mellonella was isolated from a cDNA library constructed using vitellogenic ovariole poly (A)+ RNA, by differential screening with the ovariole and day-0 male pupal poly (A)+ RNA. The Ov11 cDNA hybridized to a 1.1 kb message from the ovariole but not from male pharate adults, ovariectomized pharate adults or day-5 last instar larvae. Developmental Northern analysis showed that this message is expressed only during vitellogenic stages. In situ hybridization of digoxigenin-dUTP labeled Ov11 cDNA to the day-8 pharate adult ovariole showed that only follicle cells express the Ov11 gene. The 960 bp Ov11 cDNA is nearly full-length, containing a single open reading frame coding for a 286 amino acid polypeptide with a hydrophobic signal sequence and two potential N-glycosylation and tyrosine sulfation sites. The fact that Ov11 cDNA codes for the 37 kDa ovariole-specific yolk protein, YP4, was confirmed by the N-terminal amino acid sequence analysis of purified YP4. Genomic Southern analysis suggests that the yp4 gene is a single copy gene and PCR analysis using the 5' and 3' end primers of the cDNA indicates that the yp4 gene is intronless within the coding region.

Fundulus heteroclitus vitellogenin: the deduced primary structure of a piscine precursor to noncrystalline, liquid-phase yolk protein

We have cloned and sequenced a cDNA encoding a vitellogenin (Vtg) from the mummichog, Fundulus heteroclitus, an estuarine teleost. We constructed a liver cDNA library against RNA from estrogen-treated male mummichogs. Five overlapping cDNA clones totalling 5,197 bp were isolated through a combination of degenerate oligonucleotide probing of the library and PCR. The cDNA sequence contains a 5,112 bp open reading frame. The predicted primary structure of the deduced 1,704-amino-acid protein is 30-40% identical to other documented chordate Vtgs, establishing this Vtg as a member of the ancient Vtg gene family. Of the previously reported chordate Vtg sequences (Xenopus laevis, Gallus domesticus, Ichthyomyzon unicuspis, and Acipenser transmontanus), all four act as precursor proteins to a yolk which is eventually rendered insoluble under physiological conditions, either as crystalline platelets or as noncrystalline granules. The yolk of F. heteroclitus, on the other hand, remains in a soluble state throughout oocyte growth. The putative F. heteroclitus Vtg contains a polyserine region with a relative serine composition that is 10-20% higher than that observed for the other Vtgs. The trinucleotide repeats encoding the characteristic polyserine tracts of the phosvitin region follow a previously reported trend: TCX codons on the 5' end and AGY codons toward the 3' end. Whether the difference in Vtg primary structure between F. heteroclitus and that of other chordates is responsible for the differences in yolk structure remains to be elucidated. As the first complete teleost Vtg to be reported, these data will aid in designing nucleotide and immunological probes for detecting Vtg as a reproductive status indicator in F. heteroclitus and other piscine species.

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.

cDNA cloning and expression of Rhodnius prolixus vitellogenin

It was shown that Rhodnius prolixus vitellogenin (Vg) is synthesized as precursors of 205 and 190 kDa. Each Vg subunit is antigenically related to a domain in the precursor molecules. Since Vg has been previously detected in R. prolixus male adults, protein synthesis by fat bodies from 5th instar male nymphs was investigated and no Vg synthesis could be detected. Also, a 6.1 Kb RNA is present in female adults but not in 5th instar male nymphs. Therefore, cDNAs from female adult and 5th instar male fat bodies were used for differential screening of a female fat body cDNA library leading to the isolation of several female specific clones. All the clones hybridizing to the female specific 6.1 Kb RNA species were identical. We also describe the construction of new expression vectors, pGex-A and pGex-B, derived from the previously described plasmid pGex-1N. The new vectors, together with pGex-3X, comprise a set of expression plasmids with cloning sites in all three possible reading frames that give a fusion polypeptide with the glutathione S-transferase. This carrier protein can be cleaved by digestion with factor Xa in all three plasmids; one of the Vg cDNA clones was subcloned in pGex-A. Antibodies affinity purified from the fusion protein Vg/glutathione S-transferase recognized both large Vg subunits, suggesting an antigenic relationship between them. Furthermore, the small Vg subunits were not recognized, indicating that they may be localized at the N-terminal region of Vg precursors.

Proteolytic processing of the vitellogenin precursor in the boll weevil, Anthonomus grandis

The soluble proteins of the eggs of the coleopteran insect Anthonomus grandis Boheman, the cotton boll weevil, consist almost entirely of two vitellin types with M(r)s of 160,000 and 47,000. We sequenced their N-terminal ends and one internal cyanogen bromide fragment of the large vitellin and compared these sequences with the deduced amino acid sequence from the vitellogenin gene. The results suggest that both the boll weevil vitellin proteins are products of the proteolytic cleavage of a single precursor protein. The smaller 47,000 M(r) vitellin protein is derived from the N-terminal portion of the precursor adjacent to an 18 amino acid signal peptide. The cleavage site between the large and small vitellins at amino acid 362 is adjacent to a pentapeptide sequence containing two pairs of arginine residues. Comparison of the boll weevil sequences with limited known sequences from the single 180,000 M(r) honey bee protein show that the honey bee vitellin N-terminal exhibits sequence homology to the N-terminal of the 47,000 M(r) boll weevil vitellin. Treatment of the vitellins with an N-glycosidase results in a decrease in molecular weight of both proteins, from 47,000 to 39,000 and from 160,000 to 145,000, indicating that about 10-15% of the molecular weight of each vitellin consists of N-linked carbohydrate. The molecular weight of the deglycosylated large vitellin is smaller than that predicted from the gene sequence, indicating possible further proteolytic processing at the C-terminal of that protein.

The boll weevil vitellogenin gene: nucleotide sequence, structure, and evolutionary relationship to nematode and vertebrate vitellogenin genes

Boll weevil (Anthonomus grandis) eggs contain two yolk proteins, YP47 and YP160. Using anti-YP160 antiserum as probe, a partial-length complementary DNA (cDNA) was isolated from a lambda gt11 adult female cDNA library. A second partial-length cDNA was isolated from a lambda gt10 adult female cDNA library by differential screening with male vs. female cDNAs. Northern blot analysis showed that each cloned cDNA hybridized to a 6-kb female-specific transcript. These cDNAs were used to probe a genomic library, and two overlapping genomic clones were obtained that span the boll weevil vitellogenin gene. The entire transcription unit was sequenced, and introns were mapped by a combination of primer extension experiments, S1 nuclease protection experiments, and polymerase chain reaction-mediated synthesis of two additional cDNA clones. Based on these data, the vitellogenin mRNA is 5511 nucleotides [plus a poly(A) tail of undetermined length] and specifies a provitellogenin of 1790 amino acids. The deduced protein has a Glu+Gln content of 16.3%, which is a relatively high value that is typical of most vitellogenins. Protein sequence similarities including Cys clusters conserved between boll weevil vitellogenin and Xenopus laevis A2 or Caenorhabditis elegans vit-5 vitellogenins indicated that the boll weevil protein is a member of the ancient nematode-vertebrate vitellogenin family. Moreover, the six introns in the boll weevil vitellogenin gene interrupt the coding region at positions closely or exactly corresponding to a subset of the positions of the 34 vertebrate vitellogenin introns, further supporting the argument for a common evolutionary relationship. This report represents the first complete nucleotide sequence and structural analysis of a nondipteran insect vitellogenin gene.

Lipoprotein lipase: recent contributions from molecular biology

Lipoprotein lipase (LPL) is a glycoprotein enzyme that is produced in several cells and tissues. LPL belongs to a large lipase gene family that includes, among others, hepatic lipase and pancreatic lipase. After secretion, LPL becomes anchored on the luminal surface of the capillary endothelial cells. There it hydrolyzes triglycerides in triglyceride-rich lipoproteins, generating free fatty acids that can serve either as a direct energy source or can be stored. Through this action LPL plays a pivotal role both in energy and in lipoprotein metabolism. LPL production is regulated in a tissue-specific fashion by developmental, hormonal, and nutritional factors. The recent availability of the regulatory sequences of the LPL gene will greatly facilitate these regulatory studies in the future. In man, several mutations resulting in familial LPL deficiency have been delineated at a molecular level. The study of these mutations is not only very beneficial from a clinical point of view but also contributes in a major way to our understanding of the structure-function relationship of LPL and other lipases. In this review major attention is given to molecular studies relating to the regulation of LPL production, to the defects underlying LPL deficiency, and to structure-function relationship of the lipases.

Lipoprotein lipases and vitellogenins in relation to the known three-dimensional structure of pancreatic lipase

A 106-residue region of high similarity between lipoprotein/pancreatic/hepatic lipases and Drosophila vitellogenins encompasses four beta-strands with all residues but one strictly conserved or conservatively replaced between the structures, and enclosing the putative active site Ser-152. The properties suggest a common folding pattern but the region probably does not function as an 'interface recognition site' in the lipases, although it might well bind fatty acid esters of ecdysteroids or single lipid molecules in the vitellogenins. C-terminally of this 106-residue region, a surface loop ('flap') covers the active site. No residue within this loop is conserved through all lipases, but adjacent segments exhibit 60-70% residue identity. Hepatic and lipoprotein lipases probably hydrolyze both soluble and emulsified substrates at the same site. They lack residues corresponding to a second active site postulated in pancreatic lipase to account for hydrolysis of soluble substrates. In addition, due to structural differences the flap could prevent entry of soluble substrate molecules into the active site of pancreatic lipase.

Vitellogenin motifs conserved in nematodes and vertebrates

Caenorhabditis elegans vitellogenins are encoded by a family of six genes, one of which, vit-5, has been previously sequenced and shown to be surprisingly closely related to the vertebrate vitellogenin genes. Here we report an alignment of the amino acid sequences of vitellogenins from frog and chicken with those from three C. elegans genes: vit-5 and two newly sequenced genes, vit-2 and vit-6. The four introns of vit-6 are all in different places from the four introns of vit-5, but three of these eight positions are identical or close to intron locations in the vertebrate vitellogenin genes. The encoded polypeptides have diverged from one another sufficiently to allow us to draw some conclusions about conserved positions. Many cysteine residues have been conserved, suggesting that vitellogenin structure has been maintained over a long evolutionary distance and is dependent upon disulfide bonds. In addition, a 20-residue segment shows conservation between the vertebrate and the nematode vitellogenins. This sequence may play a highly conserved role in vitellogenesis, such as specific recognition by oocytes. On the whole, however, selection may be acting more strongly on amino acid composition and codon usage than on amino acid sequence, as might be expected for abundant storage proteins: The amino acid compositions of vit-2, vit-5, and vit-6 products are remarkably similar, despite the fact that the sequence of the vit-2 protein is only 22% and 50% identical to the sequences of vit-6 and vit-5 proteins, respectively.