PICH: A DNA Translocase Essential for Resolving Anaphase Bridged DNA

Abstract

PICH (PLK1-Interacting Checkpoint Helicase) is a recently identified member of the RAD54 subgroup of SNF2 family proteins. PICH localizes to so-called ultra-fine anaphase bridges (UFBs) in mitosis alongside a multi-protein complex of DNA repair proteins including BLM, the Bloom's syndrome gene product. Very little is known about the function of PICH or how it is recruited selectively to UFBs. Nevertheless, depletion of PICH results in genomic instability, including an elevated frequency of sister chromatid exchanges, micronuclei and loss of heterozygosity. using a combination of microfluidics, single-molecule fluorescence microscopy and optical tweezers, we have defined the properties of PICH in an in vitro model of an anaphase bridge. We show that PICH binds with a remarkably high affinity to dsDNA and displays ATP-dependent dsDNA translocase activity. The application of stretching forces to the DNA, which mimics the effects of the mitotic spindle on a UFB, enhances the binding of PICH to dsDNA, but also serves to diminish stretching-induced DNA melting. Based on our findings, we suggest that PICH plays several roles in the development and processing of UFBs: (i) to recognize and bind to dsDNA exposed by the mitotic spindle force-induced unwrapping of nucleosomes, (ii) to help expel exclusively these unwrapped nucleosomes, (iii) to stabilize stretched dsDNA, and (iv) to recruit the DNA repair machinery required for UFB resolution in anaphase.