Appropriate cell number and organ size in a multicellular organism are determined by coordinated cell growth, proliferation, and apoptosis. Disruption of these processes can cause cancer. Recent studies have identified the Large tumor suppressor (Lats)/Warts (Wts) protein kinase as a key component of a pathway that controls the coordination between cell proliferation and apoptosis. Here we describe growth inhibitory functions for a Mob superfamily protein, termed Mats (Mob as tumor suppressor), in Drosophila. Loss of Mats function results in increased cell proliferation, defective apoptosis, and induction of tissue overgrowth. We show that mats and wts function in a common pathway. Mats physically associates with Wts to stimulate the catalytic activity of the Wts kinase. A human Mats ortholog (Mats1) can rescue the lethality associated with loss of Mats function in Drosophila. As Mats1 is mutated in human tumors, Mats-mediated growth inhibition and tumor suppression is likely conserved in humans.

Insulators are protein-bound DNA elements that are thought to play a role in chromatin organization and the regulation of gene expression by mediating intra-and interchromosomal interactions. Suppressor of Hair-wing [Su(Hw)] and Drosophila CTCF (dCTCF) insulators are found at distinct loci throughout the Drosophila melanogaster genome and function by recruiting an additional protein, Centrosomal Protein 190 (CP190). We performed chromatin immunoprecipitation (ChIP) and microarray analysis (ChIP-chip) experiments with wholegenome tiling arrays to compare Su(Hw), dCTCF, boundary element-associated factor (BEAF), and CP190 localization on DNA in two different cell lines and found evidence that BEAF is a third subclass of CP190-containing insulators. The DNA-binding proteins Su(Hw), dCTCF, and BEAF show unique distribution patterns with respect to the location and expression level of genes, suggesting diverse roles for these three subclasses of insulators in genome organization. Notably, cell line-specific localization sites for all three DNA-binding proteins as well as CP190 indicate multiple levels at which insulators can be regulated to affect gene expression. These findings suggest a model in which insulator subclasses may have distinct functions that together organize the genome in a cell type-specific manner, resulting in differential regulation of gene expression.[Keywords: Insulators; chromatin organization; BEAF; CTCF; gypsy] Supplemental material is available at http://www.genesdev.org.

Several multiprotein DNA complexes capable of insulator activity have been identified in Drosophila melanogaster, yet only CTCF, a highly conserved zinc finger protein, and the transcription factor TFIIIC have been shown to function in mammals. CTCF is involved in diverse nuclear activities, and recent studies suggest that the proteins with which it associates and the DNA sequences that it targets may underlie these various roles. Here we show that the Drosophila homolog of CTCF (dCTCF) aligns in the genome with other Drosophila insulator proteins such as Suppressor of Hairy wing [SU(HW)] and Boundary Element Associated Factor of 32 kDa (BEAF-32) at the borders of H3K27me3 domains, which are also enriched for associated insulator proteins and additional cofactors. RNAi depletion of dCTCF and combinatorial knockdown of gene expression for other Drosophila insulator proteins leads to a reduction in H3K27me3 levels within repressed domains, suggesting that insulators are important for the maintenance of appropriate repressive chromatin structure in Polycomb (Pc) domains. These results shed new insights into the roles of insulators in chromatin domain organization and support recent models suggesting that insulators underlie interactions important for Pc-mediated repression. We reveal an important relationship between dCTCF and other Drosophila insulator proteins and speculate that vertebrate CTCF may also align with other nuclear proteins to accomplish similar functions.

SUMMARY Insulators are multi-protein-DNA complexes thought to affect gene expression by mediating inter- and intra-chromosomal interactions. Drosophila insulators contain specific DNA binding proteins plus common components, such as CP190, that facilitate these interactions. Here we examine changes in the distribution of Drosophila insulator proteins during the heat-shock and ecdysone responses. We find that CP190 recruitment to insulator sites is the main regulatable step in controlling insulator function during heat shock. In contrast, both CP190 and DNA binding protein recruitment are regulated during the ecdysone response. CP190 is necessary to stabilize specific chromatin loops and for proper activation of transcription of genes regulated by this hormone. These findings suggest that cells may regulate recruitment of insulator proteins to the DNA in order to activate insulator activity at specific sites and create distinct patterns of nuclear organization that are necessary to achieve proper gene expression in response to different stimuli.

Insulin binds to a receptor on the cell surface, thereby triggering a biological response within the target cell. Mutations in the insulin receptor gene can render the cell resistant to the biological action of insulin. We have studied a family in which two sisters have a genetic form of insulin‐resistant diabetes mellitus. The technique of homozygosity mapping has been used to demonstrate that the mutation causing diabetes in this consanguineous family is genetically linked to the insulin receptor gene. The two insulin‐resistant sisters are homozygous for a mutation encoding substitution of valine for phenylalanine at position 382 in the alpha‐subunit of the insulin receptor. Transfection of mutant insulin receptor cDNA into NIH3T3 cells demonstrated that the Val382 mutation impaired post‐translational processing and retarded transport of the insulin receptor to the plasma membrane. Thus, the mutation causes insulin resistance by decreasing the number of insulin receptors on the surface of the patients' cells.