Pervasive SUMOylation of heterochromatin and piRNA pathway proteins

The fission yeast SUMO-targeted ubiquitin ligase Slx8 functionally associates with clustered centromeres and the silent mating-type region at the nuclear periphery

The SUMO-targeted ubiquitin ligase (STUbL) family is involved in multiple cellular processes via a wide range of mechanisms to maintain genome stability. One of the evolutionarily conserved functions of STUbL is to promote changes in the nuclear positioning of DNA lesions, targeting them to the nuclear periphery. In Schizossacharomyces pombe, the STUbL Slx8 is a regulator of SUMOylated proteins and promotes replication stress tolerance by counteracting the toxicity of SUMO conjugates. In order to study the dynamic dialectic between ubiquitinylation and SUMOylation in the nuclear space of the S. pombe genome, we analyzed Slx8 localization. Unexpectedly, we did not detect replication stress-induced Slx8 foci. However, we discovered that Slx8 forms a single nuclear focus, enriched at the nuclear periphery, which marks both clustered centromeres at the spindle pole body and the silent mating-type region. The formation of this single Slx8 focus requires the E3 SUMO ligase Pli1, poly-SUMOylation and the histone methyl transferase Clr4 that is responsible for the heterochromatin histone mark H3-K9 methylation. Finally, we established that Slx8 promotes centromere clustering and gene silencing at heterochromatin domains. Altogether, our data highlight evolutionarily conserved and functional relationships between STUbL and heterochromatin domains to promote gene silencing and nuclear organization.

Transposon and Transgene Tribulations in Mosquitoes: A Perspective of piRNA Proportions

Mosquitoes, like Drosophila, are dipterans, the order of "true flies" characterized by a single set of two wings. Drosophila are prime model organisms for biomedical research, while mosquito researchers struggle to establish robust molecular biology in these that are arguably the most dangerous vectors of human pathogens. Both insects utilize the RNA interference (RNAi) pathway to generate small RNAs to silence transposons and viruses, yet details are emerging that several RNAi features are unique to each insect family, such as how culicine mosquitoes have evolved extreme genomic feature differences connected to their unique RNAi features. A major technical difference in the molecular genetic studies of these insects is that generating stable transgenic animals are routine in Drosophila but still variable in stability in mosquitoes, despite genomic DNA-editing advances. By comparing and contrasting the differences in the RNAi pathways of Drosophila and mosquitoes, in this review we propose a hypothesis that transgene DNAs are possibly more intensely targeted by mosquito RNAi pathways and chromatin regulatory pathways than in Drosophila. We review the latest findings on mosquito RNAi pathways, which are still much less well understood than in Drosophila, and we speculate that deeper study into how mosquitoes modulate transposons and viruses with Piwi-interacting RNAs (piRNAs) will yield clues to improving transgene DNA expression stability in transgenic mosquitoes.

Direct interaction of Su(var)2-10 via the SIM-binding site of the Piwi protein is required for transposon silencing in Drosophila melanogaster

Nuclear Piwi/Piwi-interacting RNA complexes mediate co-transcriptional silencing of transposable elements by inducing local heterochromatin formation. In Drosophila, sumoylation plays an essential role in the assembly of the silencing complex; however, the molecular mechanism by which the sumoylation machinery is recruited to the transposon loci is poorly understood. Here, we show that the Drosophila E3 SUMO-ligase Su(var)2-10 directly binds to the Piwi protein. This interaction is mediated by the SUMO-interacting motif-like (SIM-like) structure in the C-terminal domain of Su(var)2-10. We demonstrated that the SIM-like structure binds to a special region found in the MID domain of the Piwi protein, the structure of which is highly similar to the SIM-binding pocket of SUMO proteins. Abrogation of the Su(var)2-10-binding surface of the Piwi protein resulted in transposon derepression in the ovary of adult flies. Based on our results, we propose a model in which the Piwi protein initiates local sumoylation in the silencing complex by recruiting Su(var)2-10 to the transposon loci.