Research Projects

During mitosis, the architecture and spatial organization of interphase chromatin undergo significant remodeling. Structural anomalies in mitotic chromosomes, such as insufficient compaction or entangled sister chromatids, can impede accurate segregation. As the cell transitions from the G2 phase to mitosis, replicated chromosomes condense into rod-shaped structures, with sister chromatids aligned side-by-side. This precise organization is essential for their equal distribution to daughter cells via the mitotic spindle. Accurate chromosome condensation is, therefore, vital for faithful chromosome segregation during anaphase. Disruption in this process often results in genomic instability, aneuploidy, and conditions such as cancer, apoptosis, or congenital defects. In vertebrates, growing evidence supports that the conserved five-subunit protein complexes, condensin I and condensin II, mediate the dramatic reorganization of chromosomes during mitotic entry. Both complexes share SMC subunits (SMC2/CAP-E and SMC4/CAP-C), while differing in their non-SMC components: condensin I includes CAP-D2, CAP-G, and CAP-H, whereas condensin II includes CAP-D3, CAP-G2, and CAP-H2. Condensin I and II regulate mitotic chromosomes in a spatiotemporally distinct manner. Condensin I is predominantly cytoplasmic during interphase, whereas condensin II resides in the nucleus, enabling early chromatin condensation before nuclear envelope breakdown. In prometaphase, nuclear envelope disintegration allows condensin I to access chromatin. The dynamic recruitment of condensins is regulated through phosphorylation and dephosphorylation, critical post-translational modifications. Protein Phosphatase 2A (PP2A) is a major serine/threonine phosphatase in the phosphoprotein phosphatase (PPP) family, catalyzing about 90% of ser/thr dephosphorylation events in eukaryotes. The PP2A holoenzyme comprises a scaffolding A subunit (PPP2R1), a catalytic C subunit (PPP2C), and a variable regulatory B subunit (e.g., PR55/PR61/PR48/PR93), which directs substrate specificity and subcellular localization. PP2A is widely recognized as a tumor suppressor, and its inactivation by small molecules, viral proteins, or natural inhibitors can drive tumorigenesis. Previous studies suggest that PP2A is involved in directing condensin II via CAPD3 subunit but not condensin I to chromosomes, through a mechanism that does not rely on its enzymatic activity. Notably, CAPD3, a subunit of condensin II, has been implicated in promoting tumorigenesis through activation of oncogenic factors such as cMYC, E2F1, and AKT in colorectal and prostate cancers. Similarly, the chromokinesin KIF4A which is involved in condensin I activation appears to be guided to chromosomes via a non-catalytic function of PP2A. Therefore, we propose to investigate the structural and molecular mechanisms underlying the interaction between PP2A and condensin II/KIF4a complexes during mitosis using single-particle cryo-EM.