Project 2

The role of cytoskeletal proteins in genome integrity

The emerging role of actin and myosin in the 3D organization of the genome suggests a general role for these cytoskeletal proteins as gate keepers of genome stability and, therefore, integrity. Compatible with this hypothesis, we have recently reported that NM1 associates with the mammalian genome and that loss of NM1 compromises cell cycle progression. We also reported that NM1 association with the chromatin is regulated by the glycogen synthase kinase (GSK) 3β, a downstream effector of several wnt signaling pathways. In the absence of GSK3β, NM1 is polyubiquitinated and degraded via the proteasome route (Figure 3).

Figure 3. GSK3β phosphorylation of the NM1 C-terminal tail at G1 promotes chromatin association and activation of RNA polymerase-transcription. (I) In the presence of GSK3β, NM1 is phosphorylated and binds to rDNA chromatin. This phosphorylation event triggers a domino effect that leads to stabilization of the actomyosin complex and B-WICH multi-protein assembly on the rDNA. This mechanism leads to recruitment of PCAF, maintains the levels of H3K9 acetylation and activates transcription. (II) When GSK3β does not phosphorylate NM1, NM1 becomes polyubiquitinated by UBR5 and degraded by the proteasome. Consequently, the WICH complex is not assembled on the chromatin. At G1 NM1 degradation leads to suppression of pol I transcription and alterations in cell cycle progression (Adapted from Sarshad et al., 2013)

We are currently pursuing the hypothesis that actin and NM1 contribute to the architecture of the cell nucleus during the DNA damage response. Ongoing projects aim at understanding whether NM1 promotes genome stability by guarding against the formation of persistent R-loops that are believed to negatively regulate the activation of p53 signaling. We follow a global genome wide approach, in combination with high content phenotypic profiling and superresolution microscopy.