A new locus for autosomal recessive hereditary spastic paraplegia maps to chromosome 16q24.3.

Hereditary spastic paraplegia is a genetically and phenotypically heterogeneous disorder. Both pure and complicated forms have been described, with autosomal dominant, autosomal recessive, and X-linked inheritance. Various loci (SPG1-SPG6) associated with this disorder have been mapped. Here, we report linkage analysis of a large consanguineous family affected with autosomal recessive spastic paraplegia with age at onset of 25-42 years. Linkage analysis of this family excluded all previously described spastic paraplegia loci. A genomewide linkage analysis showed evidence of linkage to chromosome 16q24.3, with markers D16S413 (maximum LOD score 3.37 at recombination fraction [theta] of .00) and D16S303 (maximum LOD score 3.74 at straight theta=.00). Multipoint analysis localized the disease gene in the most telomeric region, with a LOD score of 4.2. These data indicate the presence of a new locus linked to pure recessive spastic paraplegia, on chromosome 16q24.3, within a candidate region of 6 cM.     

Crystal structure of the human ubiquitin conjugating enzyme complex, hMms2-hUbc13

The ubiquitin conjugating enzyme complex Mms2-Ubc13 plays a key role in post-replicative DNA repair in yeast and the NF-kappaB signal transduction pathway in humans. This complex assembles novel polyubiquitin chains onto yet uncharacterized protein targets. Here we report the crystal structure of a complex between hMms2 (Uev1) and hUbc13 at 1.85 A resolution and a structure of free hMms2 at 1.9 A resolution. These structures reveal that the hMms2 monomer undergoes a localized conformational change upon interaction with hUbc13. The nature of the interface provides a physical basis for the preference of Mms2 for Ubc13 as a partner over a variety of other structurally similar ubiquitin-conjugating enzymes. The structure of the hMms2-hUbc13 complex provides the conceptual foundation for understanding the mechanism of Lys 63 multiubiquitin chain assembly and for its interactions with the RING finger proteins Rad5 and Traf6.     

A novel locus for disseminated superficial actinic porokeratosis maps to chromosome 16q24.1-24.3

Disseminated superficial actinic porokeratosis (DSAP) is an uncommon autosomal dominant keratinization disorder with genetic heterogeneity characterized by multiple superficial keratotic lesions surrounded by a slightly raised keratotic border. Thus far, there have been three susceptible loci determined for DSAP and one locus for disseminated superficial porokeratosis (DSP), i.e. 12q23.2-24.1, 15q25.1-26.1, 1p31.3-p31.1 and 18p11.3. Moreover, the locus for porokeratosis palmaris plantaris et disseminata (PPPD) was mapped to 12q24.1-24.2, which overlapped with the first DSAP locus. Following the exclusion of these known loci in a four-generation Chinese DSAP family, we performed a genome-wide linkage analysis and identified a new locus on chromosome 16q24.1-24.3. The maximum two-point LOD score of 3.73 was obtained with the marker D16S3074 at a recombination fraction θ of 0.00. Haplotype analysis defined the critical 17.4-cM region for DSAP between D16S3091 and D16S413. This is regarded to be the forth locus for DSAP (DSAP4). ATP2C1 was sequenced as a candidate gene, however, no mutation was found. Further investigation for the genetic basis of DSAP is under way.     

Determinants of functionality in the ubiquitin conjugating enzyme family

The E2 enzymes are key enzymes in the ubiquitin and ubiquitin-like protein ligation pathways. To understand the functionality of the different E2 enzymes, we analyzed 190 protein sequences and 211 structures and electrostatic potentials. Key findings include: The ScUbc1 orthologs are defined by a C-terminal UBA domain. An N-terminal sequence motif that is highly conserved in all E2s except for Cdc34 orthologs is important for the stabilization of the L7 loop and is likely to be involved in E1 binding. ScUbc11p has a different electrostatic potential from E2-Cp and other proteins with which it has high sequence similarity but different functionality. All the E2s known to ubiquitinate histones have a negative potential. The members of the NCUBE family have a positive electrostatic potential, although its form is different from that of the SUMO conjugating E2s. The specificities of only the ScUbc4/Ubc5 and ScUbc1p orthologs are reflected in their L4 and L7 loops.     

A locus for migraine without aura maps on chromosome 14q21.2-q22.3

Migraine is a common and disabling neurological disease of unknown origin characterized by a remarkable clinical variability. It shows strong familial aggregation, suggesting that genetic factors are involved in its pathogenesis. Different approaches have been used to elucidate this hereditary component, but a unique transmission model and causative gene(s) have not yet been identified. We report clinical and molecular data from a large Italian pedigree in which migraine without aura (MO) segregates as an autosomal dominant trait. After exclusion of any association between MO and the known familial hemiplegic migraine and migraine with aura loci, we performed a genomewide linkage analysis using 482 polymorphic microsatellite markers. We obtained significant evidence of linkage between the MO phenotype and the marker D14S978 on 14q22.1 (maximum two-point LOD score of 3.70, at a recombination fraction of 0.01). Multipoint parametric analysis (maximum LOD score of 5.25 between markers D14S976 and D14S978) and haplotype construction showed strong evidence of linkage in a region of 10 cM flanked by markers D14S1027 and D14S980 on chromosome 14q21.2-q22.3. These results indicate the first evidence of a genetic locus associated with MO on chromosome 14.     

A second locus for Rieger syndrome maps to chromosome 13q14.

Rieger syndrome is a genetically and phenotypically heterogeneous disorder typically characterized by malformations of the eyes, teeth, and umbilicus. The syndrome is inherited as an autosomal dominant trait and exhibits significant variable expressivity. One locus associated with this disorder has been mapped to 4q25. Using a large four-generation pedigree, we have identified a second locus for Rieger syndrome located on chromosome 13q14.     

A human ubiquitin conjugating enzyme (E2)-HECT E3 ligase structure-function screen

Here we describe a systematic structure-function analysis of the human ubiquitin (Ub) E2 conjugating proteins, consisting of the determination of 15 new high-resolution three-dimensional structures of E2 catalytic domains, and autoubiquitylation assays for 26 Ub-loading E2s screened against a panel of nine different HECT (homologous to E6-AP carboxyl terminus) E3 ligase domains. Integration of our structural and biochemical data revealed several E2 surface properties associated with Ub chain building activity; (1) net positive or neutral E2 charge, (2) an "acidic trough" located near the catalytic Cys, surrounded by an extensive basic region, and (3) similarity to the previously described HECT binding signature in UBE2L3 (UbcH7). Mass spectrometry was used to characterize the autoubiquitylation products of a number of functional E2-HECT pairs, and demonstrated that HECT domains from different subfamilies catalyze the formation of very different types of Ub chains, largely independent of the E2 in the reaction. Our data set represents the first comprehensive analysis of E2-HECT E3 interactions, and thus provides a framework for better understanding the molecular mechanisms of ubiquitylation.     

A new locus for autosomal dominant amelogenesis imperfecta on chromosome 8q24.3

Amelogenesis imperfecta (AI) is a collective term used to describe phenotypically diverse forms of defective tooth enamel development. AI has been reported to exhibit a variety of inheritance patterns, and several loci have been identified that are associated with AI. We have performed a genome-wide scan in a large Brazilian family segregating an autosomal dominant form of AI and mapped a novel locus to 8q24.3. A maximum multipoint LOD score of 7.5 was obtained at marker D8S2334 (146,101,309 bp). The disease locus lies in a 1.9 cM (2.1 Mb) region according to the Rutgers Combined Linkage-Physical map, between a VNTR marker (at 143,988,705 bp) and the telomere (146,274,826 bp). Ten candidate genes were identified based on gene ontology and microarray-facilitated gene selection using the expression of murine orthologues in dental tissue, and examined for the presence of a mutation. However, no causative mutation was identified.     

Mechanism of polyubiquitin chain recognition by the human ubiquitin conjugating enzyme Ube2g2

Ube2g2 is a human ubiquitin conjugating (E2) enzyme involved in the endoplasmic reticulum-associated degradation pathway, which is responsible for the identification and degradation of unfolded and misfolded proteins in the endoplasmic reticulum compartment. The Ube2g2-specific role is the assembly of Lys-48-linked polyubiquitin chains, which constitutes a signal for proteasomal degradation when attached to a substrate protein. NMR chemical shift perturbation and paramagnetic relaxation enhancement approaches were employed to characterize the binding interaction between Ube2g2 and ubiquitin, Lys-48-linked diubiquitin, and Lys-63-linked diubiquitin. Results demonstrate that ubiquitin binds to Ube2g2 with an affinity of 90 μM in two different orientations that are rotated by 180° in models generated by the RosettaDock modeling suite. The binding of Ube2g2 to Lys-48- and Lys-63-linked diubiquitin is primarily driven by interactions with individual ubiquitin subunits, with a clear preference for the subunit containing the free Lys-48 or Lys-63 side chain (i.e. the distal subunit). This preference is particularly striking in the case of Lys-48-linked diubiquitin, which exhibits an ～3-fold difference in affinities between the two ubiquitin subunits. This difference can be attributed to the partial steric occlusion of the subunit whose Lys-48 side chain is involved in the isopeptide linkage. As such, these results suggest that Lys-48-linked polyubiquitin chains may be designed to bind certain proteins like Ube2g2 such that the terminal ubiquitin subunit carrying the reactive Lys-48 side chain can be positioned properly for chain elongation regardless of chain length.     

Mapping of the alpha-1,6-fucosyltransferase gene, FUT8, to human chromosome 14q24.3.

Alpha-1,6-Fucosyltransferase (alpha1,6FucT) is involved in the biosynthesis of asparagine-linked glycoprotein oligosaccharides. In this study, we isolated a genomic clone for the human alpha1,6FucT gene (FUT8) and mapped it by fluorescence in situ hybridization to chromosome 14q24.3. This study suggests a distinct localization of FUT8 from genes for other human fucosyltransferases reported to date.     

A mouse Y Chromosome pseudogene is related to human ubiquitin activating enzyme E1

A 2041 bp DNA fragment isolated from the Sxr (sex reversed) region of the mouse Y Chromosome (Chr) was sequenced and characterized. The sequence, pY8/b, contains four exons that are highly similar to 525 contiguous bases from the cDNA of human ubiquitin activating enzyme El. Two of the exons contain stop codons, indicating that pY8/b is not part of a functional gene. Sequences related to pY8/b were amplified from the Y Chr of the inbred mouse strain, C57BL/6J. These sequences may be portions of the recently discovered functional equivalent of pY8/b. Despite a high degree of similarity with the human El gene, the functional equivalent of pY8/b is not the mouse El gene, because unlike El, the functional equivalent of pY8/b is expressed in a tissue-specific manner. These data are discussed with respect to theory on the evolution of the mammalian Y Chr, and in particular, to the prediction that functional genes on the Y Chr have a male-specific function.     

Identification of the structural and functional human homolog of the yeast ubiquitin conjugating enzyme UBC9.

Ubiquitin conjugating enzymes (UBCs) are a family of proteins directly involved in ubiquitination of proteins. Ubiquitination is known to be involved in control of a variety of cellular processes, including cell proliferation, through the targeting of key regulatory proteins for degradation. The ubc9 gene of the yeast Saccharomyces cerevisiae (Scubc9) is an essential gene which is required for cell cycle progression and is involved in the degradation of S phase and M phase cyclins. We have identified a human homolog of Scubc9 (termed hubc9) using the two hybrid screen for proteins that interact with the human papillomavirus type 16 E1 replication protein. The hubc9 encoded protein shares a very high degree of amino acid sequence similarity with ScUBC9 and with the homologous hus5+ gene product of Schizosaccharomyces pombe. Genetic complementation experiments in a S.cerevisiae ubc9ts mutant reveal that hUBC9 can substitute for the function of ScUBC9 required for cell cycle progression.     

The AKT1 proto-oncogene maps to human chromosome 14, band q32

The human AKT1 gene is the proto-oncogene of the viral oncogene v-akt. The AKT1 gene has been localized to human chromosome 14, band q32, proximal to the heavy-chain immunoglobulin locus (IGHM), by analysis of human-hamster somatic cell hybrids and by in situ hybridization. Chromosome rearrangements of this band which occur in T-lymphoid malignancies and Hodgkin's disease may affect the AKT1 gene.     

Yeast RAD6 encoded ubiquitin conjugating enzyme mediates protein degradation dependent on the N-end-recognizing E3 enzyme.

The RAD6 gene of Saccharomyces cerevisiae encodes a 20 kd ubiquitin conjugating (E2) enzyme that is required for DNA repair, DNA damage-induced mutagenesis, and sporulation. Here, we demonstrate a novel activity of RAD6 protein--its ability to mediate protein degradation dependent on the N-end-recognizing ubiquitin protein ligase (E3). In reaction mixtures containing E1, E3 and the ubiquitin specific protease from rabbit reticulocytes, RAD6 is as effective as mammalian E214k in E3 dependent ubiquitin--protein conjugate formation and subsequent protein degradation. The ubiquitin conjugating activity of RAD6 is required for these reactions as indicated by the ineffectiveness of the rad6 Ala88 and rad6 Val88 mutant proteins, which lack the ability to form a thioester adduct with ubiquitin and therefore do not conjugate ubiquitin to substrates. We also show that the highly acidic carboxyl-terminus of RAD6 is dispensable for the interaction with E3, and that purified S. cerevisiae E2(30k), product of the UBC1 gene, does not function with E3. These findings demonstrate a specific interaction between RAD6 and E3, and highlight the strong conservation of the ubiquitin conjugating system in eukaryotes. We suggest a function for RAD6 mediated E3 dependent protein degradation in sporulation, and discuss the possible role of this activity during vegetative growth.