Maintenance of Interphase Chromosome Compaction and Homolog Pairing in Drosophila Is Regulated by the Condensin Cap-H2 and Its Partner Mrg15

Abstract

Dynamic regulation of chromosome structure and organization is critical for fundamental cellular processes such as gene expression and chromosome segregation. Condensins are conserved chromosome-associated proteins that regulate a variety of chromosome dynamics, including axial shortening, lateral compaction, and homolog pairing. However, how the in vivo activities of condensins are regulated and how functional interactors target condensins to chromatin are not well understood. To better understand how Drosophila melanogaster condensin is regulated, we performed a yeast two-hybrid screen and identified the chromo-barrel domain protein Mrg15 to interact with the Cap-H2 condensin subunit. Genetic interactions demonstrate that Mrg15 function is required for Cap-H2-mediated unpairing of polytene chromosomes in ovarian nurse cells and salivary gland cells. In diploid tissues, transvection assays demonstrate that Mrg15 inhibits transvection at Ubx and cooperates with Cap-H2 to antagonize transvection at yellow. In cultured cells, we show that levels of chromatin-bound Cap-H2 protein are partially dependent on Mrg15 and that Cap-H2-mediated homolog unpairing is suppressed by RNA interference depletion of Mrg15. Thus, maintenance of interphase chromosome compaction and homolog pairing status requires both Mrg15 and Cap-H2. We propose a model where the Mrg15 and Cap-H2 protein–protein interaction may serve to recruit Cap-H2 to chromatin and facilitates compaction of interphase chromatin.

Polyteny: still a giant player in chromosome research

“…Mutations in the condensin CapH2 are viable but lead to ovarian nurse cell chromosomes that remain polytene (Hartl et al 2008). Conversely, overexpression of CapH2 is able to create separation between chromatids in both salivary glands measured by cytology and in diploid cells measured by transvection (Smith et al 2013; Bauer et al 2012). What causes the chromosomes to separate?…”

Structural Basis for Multi-specificity of MRG Domains

“…Going beyond its role in transcription regulation, MRG15 also associates with a BRCA complex that is involved in homologous recombination-mediated DNA repair and a complex that promotes alternative RNA splicing (Hayakawa et al, 2010; Luco et al, 2010; Sy et al, 2009). Additionally, MRG15 has been implicated in interactions with condensin to regulate chromosome condensation during interphase (Smith et al, 2013). In line with its varied roles, MRG15 deletion in mouse causes embryonic lethality with profound defects in cell proliferation, differentiation, and organ development and impaired DNA-damage response (Garcia et al, 2007; Tominaga et al, 2005).…”

Condensin II Subunit dCAP-D3 Restricts Retrotransposon Mobilization in Drosophila Somatic Cells

“…The differential placement and number of double strand breaks in the C. elegans Condensin mutants were hypothesized to be caused by the changes in axial chromatin structure since axis lengths did not change in response to varying numbers of double strand breaks between mutants. Loss of Drosophila Condensin II subunits also lead to axial expansion [14], [55], [56]. Interestingly, the mdg1-1403 locus appears expanded in the dCAP-D3 mutants (Figure 7, Figure S7), and it is possible that this local expansion and change in chromatin structure could be the cause of the repositioning of double stand breaks shown in Figure 4.…”

“…However, the authors of this study do acknowledge the possibility that the Drosophila Condensin II complex may only form on chromatin, and therefore may not be picked up by their assays. Given that 1) dCAP-D3 and dCAP-H2 have been shown to be physical members of Condensin II in other organisms, 2) that the phenotypes which result from loss of expression of these subunits in Drosophila are almost identical and 3) that dCAP-H2 overexpression phenotypes have been shown to be dependent on dCAP-D3 [16], [56], we will continue to label them as such until an extensive analysis of dCAP-D3 interaction partners involving multiple tissues, retention of chromatin dependent interactions, and testing of specific dCap-D3 mutants has been performed.…”