The endoplasmic reticulum (ER) is the cell organelle where secretory and membrane proteins are synthesized and folded. Correctly folded proteins exit the ER and are transported to the Golgi and other destinations within the cell, but proteins that fail to fold properly-misfolded proteins-are retained in the ER and their accumulation may constitute a form of stress to the cell-ER stress. Several signaling pathways, collectively known as unfolded protein response (UPR), have evolved to detect the accumulation of misfolded proteins in the ER and activate a cellular response that attempts to maintain homeostasis and a normal flux of proteins in the ER. In certain severe situations of ER stress, however, the protective mechanisms activated by the UPR are not sufficient to restore normal ER function and cells die by apoptosis. Most research on the UPR used yeast or mammalian model systems and only recently Drosophila has emerged as a system to study the molecular and cellular mechanisms of the UPR. Here, we review recent advances in Drosophila UPR research, in the broad context of mammalian and yeast literature.

Chromosome-specific gene regulation is known thus far only as a mechanism to equalize the transcriptional activity of the single male X chromosome with that of the two female X chromosomes. In Drosophila melanogaster, a complex including the five MaleSpecific Lethal (MSL) proteins, ''paints'' the male X chromosome, mediating its hypertranscription. Here, with the molecular cloning of Painting of fourth (Pof ), we describe a previously uncharacterized gene encoding a chromosome-specific protein in Drosophila. Unlike the MSL proteins, POF paints an autosome, the fourth chromosome of Drosophila melanogaster. Chromosome translocation analysis shows that the binding depends on an initiation site in the proximal region of chromosome 4 and spreads in cis to involve the entire chromosome. The spreading depends on sequences or structures specific to chromosome 4 and cannot extend to parts of other chromosomes translocated to the fourth. Spreading can also occur in trans to a paired homologue that lacks the initiation region. In the related species Drosophila busckii, POF paints the entire X chromosome exclusively in males, suggesting relationships between the fourth chromosome and the X and between POF complexes and dosage-compensation complexes.

Methylation of the vertebrate genomes at cytosines is known to be accomplished by the combined actions of proteins encoded by different cytosine-5 DNA methyltransferases or DNA MTases. These proteins include the de novo DNA MTases Dnmt3a and Dnmt3b, the maintenance DNA MTase Dnmt1, and their isomers (1-4). There are also other DNA MTase-like proteins expressed in the eukaryotic cells, but they are without well documented DNA methylation activities. These include the Dnmt3L (5) and Dnmt2 proteins.The Dnmt2 proteins are relatively shorter than Dnmt3a, Dnmt3b, or Dnmt1, and structurally they are similar to the bacteria dcm enzyme (6). The eukaryotic Dnmt2 protein family consists of the yeast pmt1 (7), the mammalian Dnmt2 including mouse mDnmt2 and human hDnmt2 (8, 9), and Drosophila dDnmt2 (10, 11). Of the Dnmt2 proteins known, pmt1 has been demonstrated to be enzymatically inactive due to amino acid change at a potential catalytic site (12). Sequence analysis showed that mDnmt2, hDnmt2, and dDnmt2 all contain the conserved DNA MTase motifs (8 -11). DNA methylation analysis of ES cells with homozygous knock-out of the mouse mD-NMT2 genes suggested that mDnmt2 protein might also be an inactive DNA MTase (8). Finally, there has been no report on the DNA methylation activity of the Drosophila dDnmt2 until very recently. By overexpression of dDnmt2 in Drosophila S2 cells and subsequent analysis of the S2 cell genome with the sodium bisulfite sequencing approach, it was shown that specific regions were anomalously methylated in comparison to S2 cells without overexpression of the dDnmt2 protein (13).To avoid potential side effects resulting from use of the long term-selected S2 cell culture in the above study, we have now examined the genome of transgenic Drosophila flies stably overexpressing dDnmt2. Interestingly, specific genomic regions of the transgenic flies were also found to be anomalously hypermethylated. To complement the fly analysis, we further carried out DNA transfection experiments to transiently express fly dDnmt2 or mouse mDnmt2 in S2 cells. As shown below, dDnmt2 as well as mDnmt2 are capable of methylating cytosines of a cotransfected plasmid. The conservation of the enzymatically active Dnmt2 proteins from mammals to the flies suggests that this DNA MTase subfamily likely carries out important and to-be-identified function(s). EXPERIMENTAL PROCEDURESPlasmid Constructs-For transgenic fly work, the dDNMT2 cDNA was released from pGEM(1)-dDNMT2 with PacI-NcoI and blunt ended. This dDNMT2 fragment, which contains bp 6964 -7024 linked to 7074 -8051 of the dDNMT2 gene, was cloned at the EcoRI site of the polylinker of pUAST (14), and the resulting plasmid, pUAST-dDNMT2, was used for germ line transformation. For transient transfection, pAC5.1/V5-HisA vector (Invitrogen) containing Drosophila actin promoter was digested with KpnI, blunt ended, and redigested with EcoRI. pSG424 plasmid (15) was cut with BglII, blunt ended, and the GAL4 DNA binding domain-containing fragment was released by EcoRI digestion. Th...