Visual Analysis of Protein-Protein Interaction Docking Models Using COZOID Tool

Characterization of the conserved features of the NSE6 subunit of the Physcomitrium patens SMC5/6 complex

Structural maintenance of chromosomes (SMC) complexes are molecular machines ensuring chromatin organization at higher levels. They play direct roles in cohesion, condensation, replication, transcription, and DNA repair. Their cores are composed of long-armed SMC, kleisin, and kleisin-associated subunits. Additional factors, like NSE6 within SMC5/6, bind to SMC core complexes and regulate their activities. In the human HsNSE6/SLF2, we recently identified a new CANIN domain. Here we tracked down its sequence homology to lower plants, selected the bryophyte Physcomitrium patens, and analyzed PpNSE6 protein-protein interactions to explore its conservation in detail. We identified a previously unrecognized core sequence motif conserved from yeasts to humans within the NSE6 CANIN domain. This motif mediates the interaction between NSE6 and its NSE5 partner in yeasts and plants. In addition, the CANIN domain and its preceding PpNSE6 sequences bind both PpSMC5 and PpSMC6 arms. Interestingly, we mapped the PpNSE6-binding site at the PpSMC5 arm right next to the PpNSE2-binding surface. The position of NSE6 at SMC arms suggests its role in the regulation of SMC5/6 dynamics. Consistent with the regulatory role of NSE6 subunits, Ppnse6 mutant lines were viable and sensitive to the DNA-damaging drug bleomycin and lost a large portion of rDNA copies. These moss mutants also exhibited reduced growth and developmental aberrations. Altogether, our data showed the conserved function of the NSE6 subunit and architecture of the SMC5/6 complex across species.

The SAGA histone acetyltransferase module targets SMC5/6 to specific genes

BACKGROUND

Structural Maintenance of Chromosomes (SMC) complexes are molecular machines driving chromatin organization at higher levels. In eukaryotes, three SMC complexes (cohesin, condensin and SMC5/6) play key roles in cohesion, condensation, replication, transcription and DNA repair. Their physical binding to DNA requires accessible chromatin.

RESULTS

We performed a genetic screen in fission yeast to identify novel factors required for SMC5/6 binding to DNA. We identified 79 genes of which histone acetyltransferases (HATs) were the most represented. Genetic and phenotypic analyses suggested a particularly strong functional relationship between the SMC5/6 and SAGA complexes. Furthermore, several SMC5/6 subunits physically interacted with SAGA HAT module components Gcn5 and Ada2. As Gcn5-dependent acetylation facilitates the accessibility of chromatin to DNA-repair proteins, we first analysed the formation of DNA-damage-induced SMC5/6 foci in the Δgcn5 mutant. The SMC5/6 foci formed normally in Δgcn5, suggesting SAGA-independent SMC5/6 localization to DNA-damaged sites. Next, we used Nse4-FLAG chromatin-immunoprecipitation (ChIP-seq) analysis in unchallenged cells to assess SMC5/6 distribution. A significant portion of SMC5/6 accumulated within gene regions in wild-type cells, which was reduced in Δgcn5 and Δada2 mutants. The drop in SMC5/6 levels was also observed in gcn5-E191Q acetyltransferase-dead mutant.

CONCLUSION

Our data show genetic and physical interactions between SMC5/6 and SAGA complexes. The ChIP-seq analysis suggests that SAGA HAT module targets SMC5/6 to specific gene regions and facilitates their accessibility for SMC5/6 loading.

A role of the Nse4 kleisin and Nse1/Nse3 KITE subunits in the ATPase cycle of SMC5/6

The SMC (Structural Maintenance of Chromosomes) complexes are composed of SMC dimers, kleisin and kleisin-interacting (HAWK or KITE) subunits. Mutual interactions of these subunits constitute the basal architecture of the SMC complexes. In addition, binding of ATP molecules to the SMC subunits and their hydrolysis drive dynamics of these complexes. Here, we developed new systems to follow the interactions between SMC5/6 subunits and the relative stability of the complex. First, we show that the N-terminal domain of the Nse4 kleisin molecule binds to the SMC6 neck and bridges it to the SMC5 head. Second, binding of the Nse1 and Nse3 KITE proteins to the Nse4 linker increased stability of the ATP-free SMC5/6 complex. In contrast, binding of ATP to SMC5/6 containing KITE subunits significantly decreased its stability. Elongation of the Nse4 linker partially suppressed instability of the ATP-bound complex, suggesting that the binding of the KITE proteins to the Nse4 linker constrains its limited size. Our data suggest that the KITE proteins may shape the Nse4 linker to fit the ATP-free complex optimally and to facilitate opening of the complex upon ATP binding. This mechanism suggests an important role of the KITE subunits in the dynamics of the SMC5/6 complexes.

Molecular Insights into the Architecture of the Human SMC5/6 Complex

A family of Structural Maintenance of Chromosome (SMC) complexes is essential for key cellular processes ensuring proper cohesion, condensation and replication. They share a common SMC-kleisin architecture allowing them to embrace DNA. In SMC5/6, the NSE1 and NSE3 KITE and NSE4 kleisin subunits form a stable subcomplex that binds DNA and regulates essential processes. In addition, NSE5 and NSE6 subunits associate with the core SMC5/6 complex and recruit it to DNA repair sites. The architecture of the SMC5/6 complex is crucial for its proper functioning, and mutations within the human SMC5/6 subunits result in severe syndromes. Therefore, we aimed to analyze interactions within the human SMC5/6 complex and determine its detailed architecture. Firstly, we analyzed different parts of SMC5/6 by crosslinking and MS/MS analysis. Our data suggested domain arrangements of hNSE1-hNSE3 and orientation of hNSE4 within the hNSE1-hNSE3-hNSE4 subcomplex. The crosslinking and electron microscopic analysis of the SMC5/6 core complex showed its rod-like architecture with juxtaposed hSMC5-hSMC6 arms. Additionally, we observed fully or partially opened hSMC5-hSMC6 shapes with the hNSE1-hNSE3-hNSE4 trimer localized in the SMC head domains. To complete mapping of the human SMC5/6 complex architecture, we analyzed positions of hNSE5-hNSE6 at the hSMC5-hSMC6 arms. We showed that hNSE6 binding to hNSE5 and the coiled-coil arm of hSMC6 is mediated by a conserved FAM178 domain, which we therefore renamed CANIN (Coiled-coil SMC6 And NSE5 INteracting) domain. Interestingly, hNSE6 bound both hSMC5 and hSMC6 arms, suggesting that hNSE6 may lock the arms and regulate the dynamics of the human SMC5/6 complex.