The Caenorhabditis elegans lin-12 gene encodes a transmembrane protein with overall similarity to Drosophila Notch

sur-2, a novel gene, functions late in the let-60 ras-mediated signaling pathway during Caenorhabditis elegans vulval induction

We describe here a new gene acting downstream of let-60 ras in the vulval signaling pathway of Caenorhabditis elegans. The sur-2 (suppressor of ras) gene is defined by eight mutations identified in a genetic screen for suppressors of the Multivulva phenotype of let-60(n1046), an activated let-60 ras mutation. sur-2 mutations result in pleiotropic, incompletely penetrant phenotypes that include a Vulvaless phenotype in hermaphrodites, defects in development of the male tail, gonadal abnormalities, and larval lethality, indicating a role for the sur-2 gene product in multiple developmental events. Genetic epistasis analyses suggest that sur-2 is required late in the vulval signaling pathway, downstream of let-60 Ras, and is likely to act downstream of the Raf/MAP Kinase cascade. We cloned the sur-2 gene by DNA-mediated transformation and have shown that it encodes a novel protein. We also show that a sur-2::lacZ transgene is expressed in the vulval precursor cells at the time of vulval determination.

Different levels of the C. elegans growth factor LIN-3 promote distinct vulval precursor fates

An invariant spatial pattern of three cell fates (3 degrees-3 degrees-2 degrees-1 degree-2 degrees-3 degrees) is generated from a field of multipotent precursor cells during C. elegans vulval development. We demonstrate that the epidermal growth factor-like domain of the LIN-3 protein can induce either of two distinct vulval cell fates: a high dose of LIN-3 induces a 1 degree fate; a lower dose of LIN-3 induces a 2 degrees fate. A high dose of LIN-3 can also induce adjacent vulval precursor cells to assume 1 degree fates; thus, high levels of LIN-3 can override the lateral signaling that normally inhibits formation of adjacent 1 degree fates. We propose that the invariant pattern of vulval cell fates is generated by a graded distribution of LIN-3 that promotes different vulval fates according to local concentration and by a lateral signal that reinforces this initial bias.

The structure of a Ca(2+)-binding epidermal growth factor-like domain: its role in protein-protein interactions

Various diverse extracellular proteins possess Ca(2+)-binding epidermal growth factor (EGF)-like domains, the function of which remains uncertain. We have determined, at high resolution (1.5 A), the crystal structure of such a domain, from human clotting factor IX, as a complex with Ca2+. The Ca2+ ligands form a classic pentagonal bipyramid with six ligands contributed by one polypeptide chain and the seventh supplied by a neighboring EGF-like domain. The crystal structure identifies the role of Ca2+ in maintaining the conformation of the N-terminal region of the domain, but more importantly demonstrates that Ca2+ can directly mediate protein-protein contacts. The observed crystal packing of the domains provides a plausible model for the association of multiple tandemly linked EGF-like domains in proteins such as fibrillin-1, Notch, and protein S. This model is consistent with the known functional data and suggests a general biological role for these domains.

Notch signaling

The Notch/Lin-12/Glp-1 receptor family mediates the specification of numerous cell fates during development in Drosophila and Caenorhabditis elegans. Studies on the expression, mutant phenotypes, and developmental consequences of unregulated receptor activation have implicated these proteins in a general mechanism of local cell signaling, which includes interactions between equivalent cells and between different cell types. Genetic approaches in flies and worms have identified putative components of the signaling cascade, including a conserved family of extracellular ligands and two cellular factors that may associate with the Notch Intracellular domain. One factor, the Drosophila Suppressor of Hairless protein, is a DNA-binding protein, which suggests that Notch signaling may involve relatively direct signal transmission from the cell surface to the nucleus. Several vertebrate Notch receptors have also been discovered recently and play important roles in normal development and tumorigenesis.

EMR1, an unusual member in the family of hormone receptors with seven transmembrane segments

Proteins with seven transmembrane segments (7TM) define a superfamily of receptors (7TM receptors) sharing the same topology: an extracellular N-terminus, three extramembranous loops on either side of the plasma membrane, and a cytoplasmic C-terminal tail. Upon ligand binding, cytoplasmic portions of the activated receptor interact with heterotrimeric G-coupled proteins to induce various second messengers. A small group, recently recognized on the basis of homologous primary amino acid sequences, comprises receptors to hormones of the secretin/vasoactive intestinal peptide/glucagon family, parathyroid hormone and parathyroid hormone-related peptides, growth hormone-releasing factor, corticotropin-releasing factor, and calcitonin. A cDNA, extracted from a neuroectodermal cDNA library, was predicted to encode a new 886-amino-acid protein with three distinct domains. The C-terminal third contains the seven hydrophobic segments and characteristic residues that allow the protein to be readily aligned with the various hormone receptors in the family. Six egf-like modules, at the N-terminus of the predicted mature protein, are separated from the transmembrane segments by a serine/threonine-rich domain, a feature reminiscent of mucin-like, single-span, integral membrane glycoproteins with adhesive properties. Because of its unique characteristics, this putative egf module-containing, mucin-like hormone receptor has been named EMR1. Southern analysis of a panel of somatic cell hybrids and fluorescence in situ hybridization have assigned the EMR1 gene to human chromosome 19p13.3.

Reciprocal changes in expression of the receptor lin-12 and its ligand lag-2 prior to commitment in a C. elegans cell fate decision

During development of the C. elegans hermaphrodite gonad, two cells interact with each other, so that one chooses to become the anchor cell (AC) and the other becomes a ventral uterine precursor cell (VU). This interaction is mediated by the receptor LIN-12 and its apparent ligand LAG-2. We show that initially lin-12 and lag-2 are expressed in both cells, but prior to commitment, the expression patterns change in a reciprocal manner, so that lin-12 expression becomes restricted to the presumptive VU and lag-2 expression becomes restricted to the presumptive AC. In addition, lin-12 activity promotes expression of lin-12 and represses expression of lag-2. Furthermore, we show that positive autoregulation of lin-12 transcription in the presumptive VU is mediated by a cis-acting 5' regulatory sequence and is necessary to specify the VU fate. Our results suggest that transcriptional control is a component of the feedback mechanism involved in specifying the AC and VU fates.

The lin-3/let-23 pathway mediates inductive signalling during male spicule development in Caenorhabditis elegans

During Caenorhabditis elegans male spicule development, four pairs of precursor cells respond to multiple positional cues and establish a pattern of fates that correlates with relative anterior-posterior cell position. One of the extracellular cues is provided by the F and U cells, which promote anterior fates. We show that the genes in the lin-3/let-23 signalling pathway required for hermaphrodite vulval induction also mediate this F/U signal. Reduction-of-function mutations in lin-3, let-23, sem-5, let-60 or lin-45 disrupt the fate of anterior cells. Likewise, activation of the pathway with ubiquitously produced signal results in posterior cells inappropriately adopting the anterior fates even in the absence of F and U. We have further used this genetic pathway to begin to understand how multiple positional cues are integrated to specify cell fate. We demonstrate that lin-15 acts in spicule development as it does in vulval induction, as a negative regulator of let-23 receptor activity. A second extracellular cue, from Y.p, also acts antagonistically to the lin-3/let-23 pathway. However, this signal is apparently integrated into the lin-3/let-23 pathway at some step after lin-45 raf and is thus functionally distinct from lin-15. We have also investigated the role of lin-12 in forming the anterior/posterior pattern of fates. A lin-12 gain-of-function defect is masked by redundant positional information from F and U.

lag-2 may encode a signaling ligand for the GLP-1 and LIN-12 receptors of C. elegans

The C. elegans lag-2 gene is required for several cell-cell interactions that rely on the receptors GLP-1 and LIN-12. In this paper, we report that lag-2 encodes a putative membrane protein with sequence similarity to Drosophila Delta, a proposed ligand for the Notch receptor. Furthermore, we show that the lag-2 promoter drives expression of a reporter protein in the signaling distal tip cell (DTC) of the DTC/germline interaction. By in situ hybridization, we have found that endogenous lag-2 mRNA is present in the DTC but not the germ line. One fusion protein, called LAG-2::beta-gal(intra), rescues a lag-2 null mutant and can be detected in both DTC and germ line. Taking these results together, we propose that lag-2 may encode a signaling ligand for GLP-1/LIN-12 and that the entire LAG-2 protein may be taken up into the receiving cell during induction by GLP-1 and lateral signaling by LIN-12.

SWH1 from yeast encodes a candidate nuclear factor containing ankyrin repeats and showing homology to mammalian oxysterol-binding protein

The isolation of a gene from Saccharomyces cerevisiae, SWH1, with a coding capacity for a 135 kDa protein is reported. The deduced amino acid sequence is homologous to mammalian oxysterol-binding protein (33.6% identical residues at homologous positions) but, in addition, predicts several structural modules that are not present in the mammalian counterpart. These comprise two ankyrin repeats as an N-terminal extension, and highly acidic clusters, poly-asparagine tracts as well as domains that constitute presumptive nuclear targeting signals interspersed in the N-terminal half of the yeast protein. The gene is transcribed constitutively at a low level from a promoter lacking an obvious TATA element. Heterozygous chromosomal deletion of the gene in a diploid yeast strain has no effect on sporulation or on germination of the four spores from one tetrad nor do haploid deletion mutants display any obvious disadvantage regarding growth behaviour or mating.

An activated Notch suppresses neurogenesis and myogenesis but not gliogenesis in mammalian cells

P19 cells, a mouse embryonal carcinoma line, can be induced to differentiate into neurons. After induction, however, only a small subpopulation of cells develop as neurons, suggesting that equipotent cells adopt different cell fates. In invertebrate systems, the lin-12-Notch family of genes is thought to control the choice of cell fate. We have therefore asked whether activation of murine Notch (mNotch) regulates neuronal differentiation in P19 cells. We demonstrate that a dominant gain-of-function mutant of mNotch suppresses neurogenesis, as well as myogenesis in P19 cells. Overexpression of the full-length mNotch protein also suppresses neurogenesis. In contrast, the differentiation of glia is not affected by an activated mNotch homologue. These data indicate that mNotch may play a central role in the choice of cell fate in differentiating cells in culture and suggests that mNotch may play a similar role in the choice of fate in the developing mammalian embryo.

Structural characterization and alternate splicing of the gene encoding the preadipocyte EGF-like protein pref-1

Preadipocyte factor 1 (pref-1), a member of the EGF-like protein family, is a transmembrane protein with six tandem EGF-like repeats in the putative extracellular domain. Expression of pref-1 is abolished during the in vitro differentiation of 3T3-L1 preadipocytes to adipocytes, and constitutive expression of pref-1 in preadipocytes inhibits their differentiation [Smas, C.M., & Sul, H.S. (1993) Cell 73, 725-734]. In the present studies, we have isolated and characterized genomic clones for pref-1 and have identified multiple pref-1 transcripts generated by alternate splicing. The pref-1 gene consists of five exons and four introns spanning approximately 7.3 kb. By primer extension analysis, the transcription start site was determined to be 169 bp upstream from the translation initiation codon. We have identified functional promoter sequences by transient transfection using a 2.1 kb fragment of the pref-1 5' flanking region linked to a luciferase gene; the pref-1-luciferase fusion gene construct gave 20-fold higher promoter activity as compared to the promoterless vector. Analysis of exon-intron junctions reveals that unlike the majority of the mammalian EGF-like genes, EGF-like repeats of pref-1 are not encoded by discrete exons. Through RT-PCR and the isolation and analysis of multiple pref-1 cDNA clones, we have identified, in addition to full-length pref-1, five alternately spliced forms with various in-frame deletions of all or a part of the sixth EGF-like repeat, juxta-membrane, and predicted transmembrane domains.(ABSTRACT TRUNCATED AT 250 WORDS)

Establishment of initial asymmetry in early Caenorhabditis elegans embryos

The Caenorhabditis elegans embryo has anterior/posterior, dorsal/ventral, and left/right axes that correspond to spatially asymmetric patterns of cell differentiation. Recent studies have provided insight into how the different embryonic axes are determined and have shown that the products of the glp-1, skn-1, cap-1, and cap-2 genes appear to be distributed asymmetrically in the early embryo. These gene products should provide important tools for understanding how asymmetries are established initially in nematode embryogenesis.

Signal transduction and cell fate specification during Caenorhabditis elegans vulval development

A receptor tyrosine kinase/Ras signaling pathway controls the specification of vulval cell fates in Caenorhabditis elegans. Recently, C. elegans genes encoding proteins with similarity to mammalian Raf (lin-45), mitogen-activated protein kinase (mpk-1/sur-1), and an HNF-3 transcription factor (lin-31) have been identified and shown to act downstream of let-60 (ras) in this pathway. These genetically identified gene products bridge the gap between signal transduction at the plasma membrane and the control of cell fate specification in the nucleus.

Two cellular proteins that bind to wild-type but not mutant p53

p53 is a tumor-suppressor protein that can activate and repress transcription. Using the yeast two-hybrid system, we identified two previously uncharacterized human proteins, designated 53BP1 and 53BP2, that bind to p53. 53BP1 shows no significant homology to proteins in available databases, whereas 53BP2 contains two adjacent ankyrin repeats and a Src homology 3 domain. In vitro binding analyses indicate that both of these proteins bind to the central domain of p53 (residues 80-320) required for site-specific DNA binding. Consistent with this finding, p53 cannot bind simultaneously to 53BP1 or 53BP2 and to a DNA fragment containing a consensus p53 binding site. Unlike other cellular proteins whose binding to p53 has been characterized, both 53BP1 and 53BP2 bind to the wild-type but not to two mutant p53 proteins identified in human tumors, suggesting that binding is dependent on p53 conformation. The characteristics of these interactions argue that 53BP1 and 53BP2 are involved in some aspect of p53-mediated tumor suppression.

Translational control of maternal glp-1 mRNA establishes an asymmetry in the C. elegans embryo

In C. elegans, the glp-1 gene encodes a membrane receptor that is required for anterior cell fates in the early embryo. We report that GLP-1 protein is localized to anterior blastomeres in 2- to 28-cell embryos. By contrast, glp-1 mRNA is present in all blastomeres until the 8-cell stage. Furthermore, the glp-1 3' untranslated region can restrict translation of a reporter mRNA to anterior blastomeres. Therefore, the translation of maternal glp-1 mRNA is temporally and spatially regulated in the C. elegans embryo. The regulation of maternal glp-1 mRNA has striking parallels to the regulation of maternal hunchback mRNA in the Drosophila embryo. Thus, the establishment of embryonic asymmetry in diverse organisms may involve conserved mechanisms of maternal mRNA regulation.

The maternal genes apx-1 and glp-1 and establishment of dorsal-ventral polarity in the early C. elegans embryo

The sister blastomeres ABp and ABa are equipotent at the beginning of the 4-cell stage in C. elegans embryos, but soon become committed to different fates. We show that the glp-1 gene, a homolog of the Notch gene of Drosophila, functions in two distinct cell-cell interactions that specify the ABp and ABa fates. These interactions both require maternal expression of glp-1. We show that a second maternal gene, apx-1, functions with glp-1 only in the specification of the ABp fate and that apx-1 can encode a protein homologous to the Delta protein of Drosophila. Our results suggest how interactions mediated by glp-1 and apx-1 contribute to the establishment of the dorsal-ventral axis in the early C. elegans embryo.

2.2 Mb of contiguous nucleotide sequence from chromosome III of C. elegans

As part of our effort to sequence the 100-megabase (Mb) genome of the nematode Caenorhabditis elegans, we have completed the nucleotide sequence of a contiguous 2,181,032 base pairs in the central gene cluster of chromosome III. Analysis of the finished sequence has indicated an average density of about one gene per five kilobases; comparison with the public sequence databases reveals similarities to previously known genes for about one gene in three. In addition, the genomic sequence contains several intriguing features, including putative gene duplications and a variety of other repeats with potential evolutionary implications.

Arabidopsis homologs of the shaggy and GSK-3 protein kinases: molecular cloning and functional expression in Escherichia coli

The conservation in evolution of fundamental signal transduction modules offers a means of isolating genes likely to be involved in plant development. We have amplified by PCR Arabidopsis cDNA and genomic sequences related to the product of the shaggy/zeste-white 3 (sgg) segment polarity gene of Drosophila. This regulatory protein is functionally homologous to glycogen synthase kinase-3 in mammals (GSK-3), which regulates, among others, the DNA-binding activity of the c-jun/AP1 transcription factor. Analysis of PCR products led to the identification of five genes; for two of which, corresponding full-length cDNAs, ASK-alpha and gamma (for Arabidopsis shaggy-related protein kinase), were characterized. The encoded proteins were 70% identical to GSK-3 and sgg over the protein kinase catalytic domain and, after production in Escherichia coli, autophosphorylated mainly on threonine and serine residues, but phosphotyrosine was also detected. ASK-alpha and ASK-gamma also phosphorylated phosphatase inhibitor-2 and myelin basic protein, on threonine and serine, respectively. The high conservation of the protein kinases of GSK-3 family, and their action at the transcriptional level, suggest that the ASK proteins have important functions in higher plants.

Cell contact regulates neuroblast formation in the Caenorhabditis elegans lateral epidermis

A single line of epidermal seam cells lies along each side of the nematode C. elegans. During normal development, one of these cells, V5, produces a neuroblast that will give rise to a sensory structure, the postdeirid. If seam cells located either anterior or posterior to V5 are ablated however, this neuroblast formation is blocked. Because of this requirement for the presence of adjacent seam cells, we have asked whether V5's ability to produce a neuroblast depends on direct contact with its seam cell neighbors. We find that direct contact between seam cells is required for commitment to neuroblast production. Seam cells lose and reform their contacts with each other as they go through rounds of cell division during larval development. Signaling required for neuroblast formation occurs when the seam cells make contact after their first round of division. If this contact is prevented, no neuroblast is made; when it is delayed, the time of signaling is also delayed. The characteristics of these signals suggest that a seam cell must be part of a continuous epithelium in order to develop normally and that signaling may occur via a cell recognition/cell adhesion pathway. The effect of seam cell ablations on neuroblast formation is altered in mab-5(−) animals, suggesting that this HOM-C gene is part of the pathway by which seam cell signaling controls the decision to make a postdeirid neuroblast.

Drosophila retinal degeneration A gene encodes an eye-specific diacylglycerol kinase with cysteine-rich zinc-finger motifs and ankyrin repeats

The Drosophila visual mutant, carrying the retinal degeneration A gene (rdgA), has photoreceptor cells that degenerate within a week after eclosion. Morphological studies suggested that this mutant harbors abnormalities in membrane turnover of the photoreceptor cells. Biochemically, the rdgA mutant lacks an eye-specific and membrane-associated diacylglycerol kinase (DGK; EC 2.7.1.107) activity in a gene-dosage-dependent manner, suggesting that rdgA gene encodes a DGK. We report the molecular cloning and characterization of a DGK gene, which maps to the rdgA locus. This gene, designated as DGK2, has a single open reading frame that encodes 1454 amino acids. Like porcine DGK, DGK2 has two cysteine-rich zinc-finger motifs as well as a DGK catalytic domain. The DGK2 protein contains four ankyrin-like repeats at the C-terminal region, suggesting that DGK2 is likely anchored to the membrane or cytoskeleton. Northern blot analysis and tissue in situ hybridization to adult sections revealed that DGK2 is expressed exclusively in the adult retina and that the amount of its mRNA is reduced in some of the rdgA mutant alleles. Furthermore, in two rdgA alleles, rdgA1 and rdgA2, nonsense and missense mutations occur within their DGK2 gene, respectively. Thus, we conclude that rdgA encodes an eye-specific DGK, the absence of which leads to rhabdomere degeneration due to defective phospholipid turnover.

Further evidence for function of the Drosophila Notch protein as a transmembrane receptor

N locus mutations associated with unusual mutant phenotypes were found to alter the structure of the encoded protein. Two mutations, NCo and N60g11, eliminate much of the cytoplasmic domain. NCo can act as a null allele or as a competitive inhibitor of N+ function, whereas N60g11 produces dominant gain of function in some cell types. This difference in function can be attributed to retention of cdc10/SWI6 repeats in the Notch60g11 protein. The results suggest a role for these repeats in intracellular signaling and are consistent with action of Notch as a receptor. nd3 and l(1)NB alter extracellular epidermal growth factor-like and lin-12/Notch elements, respectively. nd3 eliminates a conserved cysteine residue, so the mutation may result in complete loss of function for a single Notch epidermal growth factor element. N60g11 and l(1)NB produce related gain-of-function phenotypes. It is proposed that l(1)NB produces an extracellular modification of the protein that stimulates aberrant intracellular signaling by the Notch cytoplasmic domain.

Suppressors of a lin-12 hypomorph define genes that interact with both lin-12 and glp-1 in Caenorhabditis elegans

The lin-12 gene of Caenorhabditis elegans is thought to encode a receptor which mediates cell-cell interactions required to specify certain cell fates. Reversion of the egg-laying defective phenotype caused by a hypomorphic lin-12 allele identified rare extragenic suppressor mutations in five genes, sel-1, sel-9, sel-10, sel-11 and sel(ar40) (sel = suppressor and/or enhancer of lin-12). Mutations in each of these sel genes suppress defects associated with reduced lin-12 activity, and enhance at least one defect associated with elevated lin-12 activity. None of the sel mutations cause any obvious phenotype in a wild-type background. Gene dosage experiments suggest that sel-1 and sel(ar40) mutations are reduction-of-function mutations, while sel-9 and sel-11 mutations are gain-of-function mutations. sel-1, sel-9, sel-11 and sel(ar40) mutations do not suppress amorphic lin-12 alleles, while sel-10 mutations are able to bypass partially the requirement for lin-12 activity in at least one cell fate decision. sel-1, sel-9, sel-10, sel-11 and sel(ar40) mutations are also able to suppress the maternal-effect lethality caused by a partial loss-of-function allele of glp-1, a gene that is both structurally and functionally related to lin-12. These sel genes may therefore function in both lin-12 and glp-1 mediated cell fate decisions.

A zebrafish homologue of the Drosophila neurogenic gene Notch and its pattern of transcription during early embryogenesis

We describe here the primary structure of a zebrafish homologue of the Drosophila neurogenic gene Notch and its pattern of mRNA accumulation during embryogenesis. The gene produces a 8.5 kb transcript encoding a putative transmembrane protein with a high degree of sequence similarity to members of the Notch family, comprising 36 EGF-like repeats, three lin-12/Notch repeats, six cdc10/SWI6 repeats, OPA repeats and a PEST sequence. Transcription of the zebrafish Notch gene is spatially and temporally regulated. A high density of transcripts, most probably of maternal origin, can already be detected in the 2-cell stage. During pregastrulation stages, RNA is present in all cells. However, following gastrulation, transcripts accumulate in specific regions of the embryo following a rapidly changing pattern. In some of these regions, cell divisions take place at the time of Notch expression, in others processes of cell differentiation. This holds true for various mesodermal derivatives, such as the prospective notochord, and for different neural primordia, such as the neural plate and the brain vesicles. This pattern of transcript accumulation suggests a role for the zebrafish Notch homologue in processes of regionalization and cell diversification.

Antineurogenic phenotypes induced by truncated Notch proteins indicate a role in signal transduction and may point to a novel function for Notch in nuclei

Loss of any one of several neurogenic genes of Drosophila results in overproduction of embryonic neuroblasts at the expense of epidermoblasts. In this paper a variety of altered Notch proteins are expressed in transgenic flies. Dominant lethal, antineurogenic phenotypes were produced by expression of three classes of mutant proteins: (1) a protein comprised of the cytoplasmic domain of Notch and devoid of sequences permitting membrane association; (2) a transmembrane protein lacking the extracellular, lin12/Notch repeats; and (3) transmembrane proteins carrying amino acid substitutions replacing one or both extracellular cysteines thought to be involved in Notch dimerization. These Notch proteins not only suppress the neural hypertrophy observed in Notch- embryos, but also generate a phenotype in which elements of the embryonic nervous system are underproduced. Action of the intracellular cdc10 repeats appears to be essential for wild-type Notch function or for the antineurogenic activity of these proteins. The activities of the dominant, gain-of-function proteins indicate that Notch functions as a signal transducing receptor during ectoderm development. Production of antineurogenic Notch proteins in embryos deficient for the other neurogenic genes allowed functional dependencies to be established. Delta, mastermind, bigbrain, and neuralized appear to function in elaboration of a signal upstream of Notch. Genes of the Enhancer of split complex act after Notch. The cytoplasmic domain of Notch contains nuclear localization sequences that function in cultured cells, and one of the Notch antineurogenic proteins, the cytoplasmic domain, accumulates in nuclei in vivo.

Intrinsic activity of the Lin-12 and Notch intracellular domains in vivo

The lin-12 gene of C. elegans and the Notch gene of D. melanogaster encode structurally related transmembrane proteins that mediate intercellular signaling. We show that truncated forms of these proteins consisting of only the intracellular domains cause cell fate transformations associated with constitutive activity in their respective organisms. This activity does not depend on endogenous gene function. Our results indicate that the intracellular domains of Lin-12 and Notch have intrinsic activity and that the principal role of the extracellular domains in the intact proteins is to regulate this activity. Our results also suggest that equivalent truncated forms of lin-12/Notch family members in vertebrates, including known oncogenes, are similarly active.

A gene for a low density lipoprotein receptor-related protein in the nematode Caenorhabditis elegans

A >23-kb gene that encodes a large integral membrane protein with a predicted structure similar to that of the low density lipoprotein (LDL) receptor-related protein (LRP) of mammals has been isolated and sequenced from the free-living nematode Caenorhabditis elegans. The 4753-amino acid predicted C. elegans product shares a nearly identical number and arrangement of amino acid sequence motifs with human LRP, and several exons of the C. elegans LRP gene correspond to exons of related parts of the human LDL receptor gene. The existence of an apparent homolog of LRP in C. elegans offers the possibility of genetic analysis of the in vivo roles of LRP and of the relationship between protein structure and function in a simple model organism.

cDNA cloning of transcription factor E4TF1 subunits with Ets and notch motifs

E4TF1 was originally identified as one of the transcription factors responsible for adenovirus E4 gene transcription. It is composed of two subunits, a DNA binding protein with a molecular mass of 60 kDa and a 53-kDa transcription-activating protein. Heterodimerization of these two subunits is essential for the protein to function as a transcription factor. In this study, we identified a new E4TF1 subunit, designated E4TF1-47, which has no DNA binding activity but can associate with E4TF1-60. We then cloned the cDNAs for each of the E4TF1 subunits. E4TF1 was purified, and the partial amino acid sequence of each subunit was determined. The predicted amino acid sequence of each cDNA clone revealed that E4TF1-60 had an ETS domain, which is a DNA binding domain common to ets-related transcription factors. E4TF1-53 had four tandemly repeated notch-ankyrin motifs. The putative cDNA of E4TF1-47 coded almost the same amino acid sequences as E4TF1-53. Three hundred and thirty-two amino acids of the N termini of E4TF1-47 and -53 were identical except for one amino acid insertion in E4TF1-53, and they differ from each other at the C terminus. These three recombinant cDNA clones were expressed in Escherichia coli, and the proteins behaved in the same manner as purified proteins in a gel retardation assay. Nucleotide and predicted amino acid sequences were highly homologous to GABP-alpha and -beta, which is further supported by the observation that GABP-specific antibody can recognize human E4TF1.

Altered epidermal growth factor-like sequences provide evidence for a role of Notch as a receptor in cell fate decisions

In Drosophila each neural precursor is chosen from a group of cells through cell interactions mediated by Notch and Delta which may function as receptor and ligand (signal), respectively, in a lateral signalling pathway. The cells of a group are equipotential and express both Notch and Delta. Hyperactive mutant Notch molecules, (Abruptex), probably have an enhanced affinity for the ligand. When adjacent to wild-type cells, cells bearing the Abruptex proteins are unable to produce the signal. It is suggested that in addition to the binding of Notch molecules on one cell to the Delta molecules of opposing cells, the Notch and Delta proteins on the surface of the same cell may interact. Binding between a cell's own Notch and Delta molecules would alter the availability of these proteins to interact with their counterparts on adjacent cells.

cDNA cloning of transcription factor E4TF1 subunits with Ets and notch motifs

E4TF1 was originally identified as one of the transcription factors responsible for adenovirus E4 gene transcription. It is composed of two subunits, a DNA binding protein with a molecular mass of 60 kDa and a 53-kDa transcription-activating protein. Heterodimerization of these two subunits is essential for the protein to function as a transcription factor. In this study, we identified a new E4TF1 subunit, designated E4TF1-47, which has no DNA binding activity but can associate with E4TF1-60. We then cloned the cDNAs for each of the E4TF1 subunits. E4TF1 was purified, and the partial amino acid sequence of each subunit was determined. The predicted amino acid sequence of each cDNA clone revealed that E4TF1-60 had an ETS domain, which is a DNA binding domain common to ets-related transcription factors. E4TF1-53 had four tandemly repeated notch-ankyrin motifs. The putative cDNA of E4TF1-47 coded almost the same amino acid sequences as E4TF1-53. Three hundred and thirty-two amino acids of the N termini of E4TF1-47 and -53 were identical except for one amino acid insertion in E4TF1-53, and they differ from each other at the C terminus. These three recombinant cDNA clones were expressed in Escherichia coli, and the proteins behaved in the same manner as purified proteins in a gel retardation assay. Nucleotide and predicted amino acid sequences were highly homologous to GABP-alpha and -beta, which is further supported by the observation that GABP-specific antibody can recognize human E4TF1.

Sct1 functions in partnership with Cdc10 in a transcription complex that activates cell cycle START and inhibits differentiation

A fission yeast cell cycle START gene has been identified, sct1. Loss of sct1 function results in cell cycle arrest at START and simultaneously in derepression of the mating pathway. sct1 therefore functions both as an essential activator of the mitotic cell cycle and as a repressor of differentiation. p72sct1 shares 36% sequence similarity with p85cdc10. p72sct1 is shown to act in partnership with p85cdc10 in a cell cycle regulatory transcription complex. A single dominant mutation within the putative DNA-binding domain of p72sct1 renders the cell independent of cdc10 function for the execution of START.

Single amino acid substitutions in EGF-like elements of Notch and Delta modify Drosophila development and affect cell adhesion in vitro

Notch locus EGF-like element mutations spl, altering eye development, and AxE2, affecting wing and sensilla development, are modified by mutations at Delta. It is shown that two allele-specific suppressors of spl involve single amino acid substitutions in the 4th (Dlsup5) and 9th (Dlsup4) EGF-like elements of the Delta protein. Cultured cells producing spl or AxE2 aggregate with cells producing wild-type Delta or Dlsup5 protein, and Dlsup5-producing cells adhere to cells producing wild-type Notch protein. However, spl,AxE2, and Dlsup5 are each defective in promoting these cell affinities, as none of the mutant proteins can compete with the corresponding wild-type proteins for formation of cell aggregates. Thus, widely separated EGF-like elements of Notch and Delta appear to participate in functional molecular interactions between the proteins. Dlsup5 does not improve adhesiveness of spl in vitro, so suppression in vivo may involve altered developmental signaling by spl-Dlsup5 complexes, rather than modified cell adhesion.

Molecular basis of loss-of-function mutations in the glp-1 gene of Caenorhabditis elegans

The glp-1 gene encodes a membrane protein required for inductive cell interactions during development of the nematode Caenorhabditis elegans. Here we report the molecular characterization of 15 loss-of-function (lf) mutations of glp-1. Two nonsense mutations appear to eliminate glp-1 activity; both truncate the glp-1 protein in its extracellular domain and have a strong loss-of-function phenotype. Twelve missense mutations and one in-frame deletion map to sites within the repeated motifs of the glp-1 protein (10 epidermal growth factor [EGF]-like and 3 LNG repeats extracellularly and 6 cdc10/SWI6, or ankyrin, repeats intracellularly). We find that all three types of repeated motifs are critical to glp-1 function, and two individual EGF-like repeats may have distinct functions. Intriguingly, all four missense mutations in one phenotypic class map to the N-terminal EGF-like repeats and all six missense mutations in a second phenotypic class reside in the intracellular cdc10/SWI6 repeats. These two clusters of mutations may identify functional domains within the glp-1 protein.

The gene lin-3 encodes an inductive signal for vulval development in C. elegans

The lin-3 gene is necessary for induction of the Caenorhabditis elegans vulva by the anchor cell. It encodes a molecule similar to epidermal growth factor and to transforming growth factor-alpha and acts through the epidermal growth factor receptor homologue let-23. Expression of lin-3 in the anchor cell stimulates vulval induction; lin-3 may encode the vulval inducing signal.

The Drosophila spitz gene encodes a putative EGF-like growth factor involved in dorsal-ventral axis formation and neurogenesis

We describe the molecular characterization of the Drosophila gene spitz (spi), which encodes a putative 26-kD, EGF-like transmembrane protein that is structurally similar to TGF-alpha. Temporal and spatial expression patterns of spi transcripts indicate that spi is expressed throughout the embryo. Examination of mutant embryos reveals that spi is involved in a number of unrelated developmental choices, for example, dorsal-ventral axis formation, glial migration, sensory organ determination, and muscle development. We propose that spi may act as a ligand for cell-specific receptors, possibly rhomboid and/or the Drosophila EGF receptor homolog.

A rat cerebellar protein containing the cdc10/SWI6 motif

Systematic analysis of soluble proteins in developing rat cerebellum by an automated two-dimensional liquid-chromatography system detected a number of proteins which increased transiently during the initial stage of postnatal development. One of the proteins, V-1, was isolated using a liquid-chromatography system, and its amino acid sequence was determined by analysis of the purified protein. The sequence showed that the V-1 protein consists of 117 amino acids with an acetylated N-terminus, and has 2.5 internal sequence repeats of 33 amino acids. Computer retrieval of the sequence indicated that the repeated sequences have a structural characteristics of the cdc10/SWI6 motif, which is found in a series of proteins, including those involved in cell-cycle control and cell-fate determination in yeast, Drosophila melanogaster and Caenorhabditis elegans. The structure of V-1, coupled with its controlled expression in early postnatal development, implies a potential role for V-1 in cerebellar morphogenesis.

The pie-1 and mex-1 genes and maternal control of blastomere identity in early C. elegans embryos

During C. elegans embryogenesis an 8-cell stage blastomere, called MS, undergoes a reproducible cleavage pattern, producing pharyngeal cells, body wall muscles, and cell deaths. We show here that maternal-effect mutations in the pie-1 and mex-1 genes cause additional 8-cell stage blastomeres to adopt a fate very similar to that of the wild-type MS blastomere. In pie-1 mutants one additional posterior blastomere adopts an MS-like fate, and in mex-1 mutants four additional anterior blastomeres adopt an MS-like fate. We propose that maternally provided pie-1(+) and mex-1(+) gene products may function in the early embryo to localize or regulate factors that determine the fate of the MS blastomere.

Direct association of pp40/I kappa B beta with rel/NF-kappa B transcription factors: role of ankyrin repeats in the inhibition of DNA binding activity

To understand the mechanism by which pp40/I kappa B beta inhibits DNA binding activity of the rel/NF-kappa B family of transcription factors, we have investigated the role of ankyrin repeats on the biological function of pp40 by deleting or mutating conserved residues. We show that (i) ankyrin repeats alone are not sufficient to manifest biological activity but require the C-terminal region of the pp40 protein; (ii) four out of the five ankyrin repeats are essential for inhibiting the DNA binding activity; (iii) pp40 mutants that do not inhibit DNA binding of rel protein also do not associate with rel; (iv) although pp40 can associate with the p65 and p50 subunits of NF-kappa B, pp40 inhibits the DNA binding activity of only the p50-p65 heterodimer and the p65 homodimer; and (v) pp40 inhibits the transcription of genes linked to kappa B site; however, mutants that do not affect DNA binding have no effect. We propose that the ankyrin repeats and the C-terminal region of pp40 form a structure that associates with the rel homology domain to inhibit DNA binding activity.

A novel endothelial cell surface receptor tyrosine kinase with extracellular epidermal growth factor homology domains

Endothelial cell surfaces play key roles in several important physiological and pathological processes such as blood clotting, angiogenic responses, and inflammation. Here we describe the cloning and characterization of tie, a novel type of human endothelial cell surface receptor tyrosine kinase. The extracellular domain of the predicted tie protein product has an exceptional multidomain structure consisting of a cluster of three epidermal growth factor homology motifs embedded between two immunoglobulinlike loops, which are followed by three fibronectin type III repeats next to the transmembrane region. Additionally, a cDNA form lacking the first of the three epidermal growth factor homology domains was isolated, suggesting that alternative splicing creates different tie-type receptors. Cells transfected with tie cDNA expression vector produce glycosylated polypeptides of 117 kDa which are reactive to antisera raised against the tie carboxy terminus. The tie gene was located in chromosomal region 1p33 to 1p34. Expression of the tie gene appeared to be restricted in some cell lines; large amounts of tie mRNA were detected in endothelial cell lines and in some myeloid leukemia cell lines with erythroid and megakaryoblastoid characteristics. In addition, mRNA in situ studies further indicated the endothelial expression of the tie gene. The tie receptor tyrosine kinase may have evolved for multiple protein-protein interactions, possibly including cell adhesion to the vascular endothelium.