Ubiquitin-protein conjugates selectively distribute during early chicken embryogenesis

Demonstration of ubiquitin thiolester formation of UBE2Q2 (UBCi), a novel ubiquitin-conjugating enzyme with implantation site-specific expression

We recently identified a differentially expressed gene in implantation stage rabbit endometrium encoding a new member of the ubiquitin-conjugating enzyme family designated UBE2Q2 (also known as UBCi). Its unusually high molecular mass, novel N-terminus extension, and highly selective pattern of mRNA expression suggest a specific function in implantation. This study analyzes its relationship to the E2 ubiquitin-conjugating enzyme superfamily, investigates its enzymatic activity, and examines its localization in implantation site endometrium. Construction of a dendrogram indicated that UBE2Q2 is homologous to the UBC2 family of enzymes, and isoforms are present in a broad range of species. In vitro enzymatic assays of ubiquitin thiolester formation demonstrated that UBE2Q2 is a functional ubiquitin-conjugating enzyme. The Km for transfer of ubiquitin thiolester from E1 to UBE2Q2 is 817 nM compared to 100 nM for other E2 paralogs; this suggests that the unique amino terminal domain of UBE2Q2 confers specific functional differences. Affinity-purified antibodies prepared with purified recombinant UBE2Q2 showed that the protein was undetectable by immunoblot analysis in endometrial lysates from estrous and Day 6(3/4) pregnant (blastocyst attachment stage) rabbits but was expressed in both mesometrial and antimesometrial implantation site endometrium of Day 8 pregnant animals. No expression was detected in adjacent interimplantion sites. Immunohistochemistry demonstrated UBE2Q2 expression exclusively in mesometrial and antimesometrial endometrial luminal epithelial cells of the Day 8 implantation chamber. Immunohistochemical localization of ubiquitin mirrored UBE2Q2 expression, with low-to-undetectable levels in implantation sites of Day 6(3/4) pregnant endometrium but high levels in luminal epithelial cells of Day 8 pregnant endometrium. This implantation site-specific expression of UBE2Q2 in luminal epithelial cells could play major roles in orchestrating differentiation events through the modification of specific protein substrates.

Induction and patterning of the primitive streak, an organizing center of gastrulation in the amniote

The primitive streak is the organizing center for amniote gastrulation. It defines the future embryonic midline and serves as a conduit of cell migration for germ layer formation. The migration patterns of endodermal and mesodermal precursors through the streak have been studied in great detail. Additional new breakthroughs recently have revealed the cell biological and molecular mechanisms that govern streak induction and patterning. These findings include (1) identification of the ontogeny and inductive signals of streak precursors, (2) the potential cellular mechanism of streak extension, and (3) the molecular and functional diversification along the anterior-posterior and mediolateral axes within the primitive streak. These findings indicate that amniote embryos initiate gastrulation by using both evolutionarily conserved and divergent mechanisms. The data also provide a foundation for understanding how the midline axis is defined and maintained during gastrulation of the amniotes.

Protein interactions within the N-end rule ubiquitin ligation pathway

Rate studies have been employed as a reporter function to probe protein-protein interactions within a biochemically defined reconstituted N-end rule ubiquitin ligation pathway. The concentration dependence for E1-catalyzed HsUbc2b/E2(14kb) transthiolation is hyperbolic and yields K(m) values of 102 +/- 13 nm and 123 +/- 19 nm for high affinity binding to rabbit and human E1/Uba1 orthologs. Competitive inhibition by the inactive substrate and product analogs HsUbc2bC88A (K(i) = 104 +/- 15 nm) and HsUbc2bC88S-ubiquitin oxyester (K(i) = 169 +/- 17 nm), respectively, indicates that the ubiquitin moiety contributes little to E1 binding. Under conditions of rate-limiting E3alpha-catalyzed conjugation to human alpha-lactalbumin, HsUbc2b-ubiquitin thiolester exhibits a K(i) of 54 +/- 18 nm and is competitively inhibited by the substrate analog HsUbc2bC88S-ubiquitin oxyester (K(i) = 66 +/- 29 nm). In contrast, the ligase product analog HsUbc2bC88A exhibits a K(i) of 440 +/- 55 nm with respect to the wild type HsUbc2b-ubiquitin thiolester, demonstrating that ubiquitin binding contributes to the ability of E3alpha to discriminate between substrate and product E2. A survey of E1 and E2 isoform distribution in selected cell lines demonstrates that Ubc2 isoforms are the predominant intracellular ubiquitin carrier protein. Intracellular levels of E1 and Ubc2 are micromolar and approximately equal based on in vitro quantitation by stoichiometric (125)I-ubiquitin thiolester formation. Comparison of intracellular E1 and Ubc2 pools with the corresponding ubiquitin pools reveals that most of the free ubiquitin in cells is present as thiolesters to the components of the conjugation pathways. The present data represent the first comprehensive analysis of protein interactions within a ubiquitin ligation pathway.

Cell death along the embryo midline regulates left-right sidedness

During embryogenesis, left-right sidedness is established by asymmetric expression of laterality genes. A recent model predicts the presence of a functional midline that divides the left side of the embryonic disc from the right side, separating left- and right-inducing signals. We show evidence that this midline is formed from a distinct population of cells within the primitive streak. Cells in the dorsal midline of the chick primitive streak display unique expression of the gastrulation markers fgf-8 and brachyury. These midline cells are fated to die, and dead cells remain in the midline during gastrulation. Inhibition of midline cell death compromises the early expression of laterality genes, such as shh and nodal and randomizes the direction of heart looping. We suggest that cell death along the primitive streak midline is a novel mechanism involved in the regulation of left-right asymmetry during early embryogenesis.

Ubiquitin-activating enzyme (E1) isoforms in lens epithelial cells: origin of translation, E2 specificity and cellular localization determined with novel site-specific antibodies

Lens development and response to peroxide stress are associated with dramatic changes in protein ubiquitination, reflecting dynamic changes in activity of the ubiquitin-activating enzyme (E1). Two isoforms of E1 (E1A and E1B) have been identified in lens cells although only one E1 mRNA, containing three potential translational start sites, has been detected. Novel, site-specific antibodies to E1 were generated and the hypothesis that the two isoforms of E1 are translated from alternative initiation codons of a single mRNA was tested. Antibodies raised against E1A-N peptide (Met(1)to Cys(23)of E1A) reacted only with E1A by immunoblot and immunoprecipitation. Antibodies raised against E1B-N peptide (Met(1)to Glu(25)of E1B or Met(41)to Glu(65)of E1A) and E1AB-C peptide (His(1030)to Arg(1058)of E1A or His(990)to Arg(1018)of E1B) reacted with both E1A and E1B. These results indicate that (1) E1A and E1B contain the same C-terminal residues; (2) E1A contains the N terminal sequence of E1B; and (3) E1B does not contain the N terminal sequence of E1A. The two isoforms of lens E1 are therefore translated from a single mRNA. Specifically, E1A is translated from the first initiation codon, and E1B translated from the second initiation codon. E1A and E1B were affinity-purified, and their ability to 'charge' ubiquitin carrier proteins (E2s) with activated ubiquitin was compared in a cell-free system. E1A and E1B were indistinguishable with respect to charging different E2s. However, E1 immunolocalization studies with human lens epithelial cells indicate that E1A and E1B are preferentially localized to the nucleus and cytosol, respectively. This observation suggests that E1A and E1B ubiquitinate different proteins and serve different functions in intact cells.

The 22q11.2 deletion: from diversity to a single gene theory

The 22q11 deletion syndromes are a group of conditions in which a characteristic spectrum of congenital cardiac defects may be associated with a wide range of noncardiological congenital anomalies. These syndromes are all linked by a deletion in the long arm of chromosome 22. Although it is a large deletion, containing many genes, recent advances have led to the belief that the etiology of the diverse abnormalities of these syndromes may be a single gene deletion. This review outlines the historical development of the various "22q deletion syndromes," including the DiGeorge, velocardiofacial, Takao, Cayler, and CATCH-22 syndromes, briefly describes the relevant cardiac embryogenesis, and then explains how a single gene deletion may encompass the full phenotypic spectrum.

Cloning and expression analysis of a novel mouse gene with sequence similarity to the Drosophila fat facets gene

The Drosophila fat facets (faf) gene is a ubiquitin-specific protease necessary for the normal development of the eye and of the syncytial stage embryo in the fly. Using a gene trap approach in embryonic stem cells we have isolated a murine gene with extensive sequence similarity to the Drosophila faf gene and called it Fam (fat facets in mouse). The putative mouse protein shows colinearity and a high degree of sequence identity to the Drosophila protein over almost its entire length of 2554 amino acids. The two enzymatic sites characteristic of ubiquitin-specific proteases are very highly conserved between mice and Drosophila and this conservation extends to yeast. Fam is expressed in a complex pattern during postimplantation development. In situ hybridisation detected Fam transcripts in the rapidly expanding cell populations of gastrulating and neurulating embryos, in post-mitotic cells of the CNS as well as in the apoptotic regions between the digits, indicating that it is not associated with a single developmental or cellular event. The strong sequence similarity to faf and the developmentally regulated expression pattern suggest that Fam and the ubiquitin pathway may play a role in determining cell fate in mammals, as has been established for Drosophila.

UFD1L, a developmentally expressed ubiquitination gene, is deleted in CATCH 22 syndrome

The CATCH 22 acronym outlines the main clinical features of 22q11.2 deletions (cardiac defects, abnormal facies, thymic hypoplasia, cleft palate and hypocalcemia), usually found in DiGeorge (DGS) and velo-cardio-facial (VCFS) syndromes. Hemizygosity of this region may also be the cause of over 100 different clinical signs. The CATCH 22 locus maps within a 1.5 Mb region, which encompasses several genes. However, no single defect in 22q11.2 hemizygous patients can be ascribed to any gene so far isolated from the critical region of deletion. We have identified a gene in the CATCH 22 critical region, whose functional features and tissue-specific expression suggest a distinct role in embryogenesis. This gene, UFD1L, encodes the human homolog of the yeast ubiquitin fusion degradation 1 protein (UFD1p), involved in the degradation of ubiquitin fusion proteins. Cloning and characterization of the murine homolog (Ufd1l) showed it to be expressed during embryogenesis in the eyes and in the linear ear primordia. These data suggest that the proteolytic pathway that recognizes ubiquitin fusion proteins for degradation is conserved in vertebrates and that the UFD1L gene hemizygosity is the cause of some of the CATCH 22-associated developmental defects.