Liberating cohesin from cohesion

History of DNA Helicases

Since the discovery of the DNA double helix, there has been a fascination in understanding the molecular mechanisms and cellular processes that account for: (i) the transmission of genetic information from one generation to the next and (ii) the remarkable stability of the genome. Nucleic acid biologists have endeavored to unravel the mysteries of DNA not only to understand the processes of DNA replication, repair, recombination, and transcription but to also characterize the underlying basis of genetic diseases characterized by chromosomal instability. Perhaps unexpectedly at first, DNA helicases have arisen as a key class of enzymes to study in this latter capacity. From the first discovery of ATP-dependent DNA unwinding enzymes in the mid 1970's to the burgeoning of helicase-dependent pathways found to be prevalent in all kingdoms of life, the story of scientific discovery in helicase research is rich and informative. Over four decades after their discovery, we take this opportunity to provide a history of DNA helicases. No doubt, many chapters are left to be written. Nonetheless, at this juncture we are privileged to share our perspective on the DNA helicase field - where it has been, its current state, and where it is headed.

Distinct roles of cohesin acetyltransferases Esco1 and Esco2 in porcine oocyte meiosis I

In mammalian cells, cohesin acetyltransferases Esco1 and Esco2 acetylate cohesin subunit Smc3 to establish chromosome cohesion, ensuring the accurate chromosome segregation. However, we have previously documented that both Esco1 and Esco2 have unique substrates and roles in mouse oocyte meiosis I to orchestrate the meiotic progression, but whether these functions are conserved among species is still not determined. Here, we used porcine oocytes as a model to illustrate that Esco1 and Esco2 exerted conserved functions during oocyte meiosis. We observed that Esco1 and Esco2 exhibited different localization patterns in porcine oocytes. Esco1 was localized to the spindle apparatus while Esco2 was distributed on the chromosomes. Depletion of Esco1 by siRNA microinjection caused the meiotic arrest by showing the reduced frequency of first polar body extrusion and defective spindle/chromosome structure. In addition, Esco1 bound to α-tubulin and was required for its acetylation level to maintain the microtubule dynamics. By contrast, depletion of Esco2 by siRNA microinjection resulted in the accelerated meiotic progression by displaying the precocious polar body extrusion and inactivation of spindle assembly checkpoint. Notably, Esco2 was shown to be associated with histone H4 for the acetylation of H4K16 to modulate the kinetochore function. Collectively, our data reveal that Esco1 and Esco2 perform distinct and conserved functions in oocytes to drive the meiotic progression beyond their canonical roles in the cohesion establishment.

The Smc5/6 Complex: New and Old Functions of the Enigmatic Long-Distance Relative

Smc5 and Smc6, together with the kleisin Nse4, form the heart of the enigmatic and poorly understood Smc5/6 complex, which is frequently viewed as a cousin of cohesin and condensin with functions in DNA repair. As novel functions for cohesin and condensin complexes in the organization of long-range chromatin architecture have recently emerged, new unsuspected roles for Smc5/6 have also surfaced. Here, I aim to provide a comprehensive overview of our current knowledge of the Smc5/6 complex, including its long-established function in genome stability, its multiple roles in DNA repair, and its recently discovered connection to the transcription inhibition of hepatitis B virus genomes. In addition, I summarize new research that is beginning to tease out the molecular details of Smc5/6 structure and function, knowledge that will illuminate the nuclear activities of Smc5/6 in the stability and dynamics of eukaryotic genomes.