WBSCR14, a gene mapping to the Williams--Beuren syndrome deleted region, is a new member of the Mlx transcription factor network.

Williams-Beuren syndrome (WBS) is a developmental disorder associated with haploinsufficiency of multiple genes at 7q11.23. Here, we report the functional characterization of WBS critical region gene 14 (WBSCR14), a gene contained in the WBS commonly deleted region. It encodes a basic-helix--loop--helix leucine zipper (bHLHZip) transcription factor of the Myc/Max/Mad superfamily. WBSCR14 is expressed in multiple tissues, including regions of the brain and the intestinal tract. WBSCR14 forms heterodimers with the bHLHZip protein Mlx to bind the DNA sequence CACGTG. Like Max, Mlx has no intrinsic transcriptional activity, but its association with Mad1, Mad4, Mnt or WBSCR14 can repress E-box-dependent transcription. Preliminary results suggest a possible role of WBSCR14 in growth control. Our data support the view that the Max-like bHLHZip protein, Mlx, is a key element of a transcription factor network. We thus suggest that WBSCR14 may contribute to some aspects of the WBS pathology.

Cairo S ，Merla G ，Urbinati F ，Ballabio A ，Reymond A ... -  《HUMAN MOLECULAR GENETICS》

WBSCR14, a putative transcription factor gene deleted in Williams-Beuren syndrome: complete characterisation of the human gene and the mouse ortholog.

Williams-Beuren syndrome (WBS) is a neurodevelopmental disorder affecting several systems caused by a heterozygous deletion in the chromosomal region 7q11.23. A common interval that includes up to 17 genes reported so far is deleted in the great majority of patients. Elastin haploinsufficiency is responsible for the cardiovascular features, but the specific contribution of other deleted genes to the WBS phenotype remains unknown. We have fully characterised a gene commonly deleted in WBS, WBSCR14, previously reported in a truncated form as WS-bHLH. The WBSCR14 cDNA encodes an 852amino acid protein with a basic helix-loop-helix-leucine-zipper motif (bHLHZip) and a bipartite nuclear localisation signal (BNLS), suggesting a function as a transcription factor. WBSCR14 is expressed as a 4.2kb transcript predominantly in adult liver and at late stages of foetal development. The WBSCR14 locus encompasses 33 kb of genomic DNA with 17 exons. Two intragenic polymorphic dinucleotide repeats have been identified and used to verify hemizygosity in WBS patients. We have also cloned the mouse ortholog and mapped its locus to mouse chromosome 5, in a region of conserved synteny with human 7q11.23. Given that other bHLHZip proteins are dosage sensitive and based on the putative function of WBSCR14 as a transcription factor, hemizygosity at this locus could be involved in some features of WBS.

de Luis O ，Valero MC ，Jurado LA 《EUROPEAN JOURNAL OF HUMAN GENETICS》

The subcellular localization of the ChoRE-binding protein, encoded by the Williams-Beuren syndrome critical region gene 14, is regulated by 14-3-3.

The Williams-Beuren syndrome (WBS) is a contiguous gene syndrome caused by chromosomal rearrangements at chromosome band 7q11.23. Several endocrine phenotypes, in particular impaired glucose tolerance and silent diabetes, have been described for this clinically complex disorder. The WBSCR14 gene, one of the genes mapping to the WBS critical region, encodes a member of the basic-helix-loop-helix leucine zipper family of transcription factors, which dimerizes with the Max-like protein, Mlx. This heterodimeric complex binds and activates, in a glucose-dependent manner, carbohydrate response element (ChoRE) motifs in the promoter of lipogenic enzymes. We identified five novel WBSCR14-interacting proteins, four 14-3-3 isotypes and NIF3L1, which form a single polypeptide complex in mammalian cells. Phosphatase treatment abrogates the association between WBSCR14 and 14-3-3, as shown previously for multiple 14-3-3 interactors. WBSCR14 is exported actively from the nucleus through a CRM1-dependent mechanism. This translocation is contingent upon the ability to bind 14-3-3. Through this mechanism the 14-3-3 isotypes directly affect the WBSCR14:Mlx complexes, which activate the transcription of lipogenic genes.

Merla G ，Howald C ，Antonarakis SE ，Reymond A ... -  《HUMAN MOLECULAR GENETICS》

MondoA, a novel basic helix-loop-helix-leucine zipper transcriptional activator that constitutes a positive branch of a max-like network.

Max is a common dimerization partner for a family of transcription factors (Myc, Mad [or Mxi]), and Mnt [or Rox] proteins) that regulate cell growth, proliferation, and apoptosis. We recently characterized a novel Max-like protein, Mlx, which interacts with Mad1 and Mad4. Here we describe the cloning and functional characterization of a new family of basic helix-loop-helix-leucine zipper heterodimeric partners for Mlx termed the Mondo family. MondoA forms homodimers weakly and does not interact with Max or members of the Myc or Mad families. MondoA and Mlx associate in vivo, and surprisingly, they are localized primarily to the cytoplasm of cultured mammalian cells. Treatment of cells with the nuclear export inhibitor leptomycin B results in the nuclear accumulation of MondoA and Mlx, demonstrating that they shuttle between the cytoplasmic and nuclear compartments rather than having exclusively cytoplasmic localization. MondoA preferentially forms heterodimers with Mlx, and this heterocomplex can bind to, and activate transcription from, CACGTG E-boxes when targeted to the nucleus via a heterologous nuclear localization signal. The amino termini of the Mondo proteins are highly conserved among family members and contain separable and autonomous cytoplasmic localization and transcription activation domains. Therefore, Mlx can mediate transcriptional repression in conjunction with the Mad family and can mediate transcriptional activation via the Mondo family. We propose that Mlx, like Max, functions as the center of a transcription factor network.

Billin AN ，Eilers AL ，Coulter KL ，Logan JS ，Ayer DE ... -  《-》

Mlx, a novel Max-like BHLHZip protein that interacts with the Max network of transcription factors.

Mad:Max heterodimers oppose the growth-promoting action of Myc:Max heterodimers by recruiting the mSin3-histone deacetylase (mSin3. HDAC) complex to DNA and functioning as potent transcriptional repressors. There are four known members of the Mad family that are indistinguishable in their abilities to interact with Max, bind DNA, repress transcription, and block Myc + Ras co-transformation. To investigate functional differences between Mad family proteins, we have identified additional proteins that interact with this family. Here we present the identification and characterization of the novel basic-helix-loop-helix zipper protein Mlx (Max-like protein x), which is structurally and functionally related to Max. The similarities between Mlx and Max include 1) broad expression in many tissues, 2) long protein half-life, and 3) formation of heterodimers with Mad family proteins that are capable of specific CACGTG binding. We show that transcriptional repression by Mad1:Mlx heterodimers is dependent on dimerization, DNA binding, and recruitment of the mSin3A.HDAC corepressor complex. In contrast with Max, Mlx interacts only with Mad1 and Mad4. Together, these findings suggest that Mlx may act to diversify Mad family function by its restricted association with a subset of the Mad family of transcriptional repressors.

Billin AN ，Eilers AL ，Queva C ，Ayer DE ... -  《JOURNAL OF BIOLOGICAL CHEMISTRY》