Interactions within a single chromosome and between chromosomes are dynamically regulated throughout the cell cycle to control such processes as transcription, DNA repair, and chromosome segregation. Cohesin is a protein complex that clamps around the chromosomes to hold them together. PDS5 proteins both promote the binding and release of cohesin from chromosomes. Interactions between PDS5 and WAPL enhance the release of cohesin from chromosomes, and interactions between PDS5 and CDCA5 (also known as sororin) stabilize cohesin clamping of chromosomes. To understand this dual role, Ouyang et al. crystallized human PDS5B bound to a peptide from WAPL. In vitro binding assays identified two conserved motifs, a Tyr-Ser-Arg (YSR) motif and an Phe-Gly-Phe (FGF) motif, in both WAPL and CDCA5, and these motifs were functionally important for the interaction between these proteins and PDS5B. Whereas the YSR motif was important for both in vitro binding and interactions between the proteins and PDS5B in cells; the FGF motif was unimportant for in vitro binding but important for the interaction between PDS5B and WAPL or CDCA5 in cells. Mutation of either motif impaired the ability of WAPL to rescue mitotic chromosome defects in WAPL-knockdown cells; thus both motifs are functionally important. The crystal structure of PDS5B with the WAPL YSR-containing peptide revealed a conformation of repeated HEAT domains that formed a “plier”-like structure with the WAPL peptide bound at the end of the “handle,” a “jaw” at the C-terminal end important for cohesin binding, and an inositol hexakisphosphate (IP6) bound at a bend in the jaw. Mutation of residues predicted from the structure to contact the WAPL peptide disrupted both WAPL peptide and CDCA5 peptide binding in vitro, and expression of the mutants in cells confirmed the functional importance of the interaction. Mutation of the IP6-binding residues compromised protein stability or production when the mutants were expressed in cells. However, when expressed at similar amounts as that of transfected wild-type PDS5B by increasing the amount of plasmid, these mutants failed to coimmunoprecipitate components of cohesin or WAPL or CDCA5. Thus, IP6 appears to be a necessary cofactor that enables the proper folding and function of PDS5 proteins. This study provides insight into the regulation of chromosome dynamics by PDS5 proteins and suggests that signals that alter IP6 abundance may regulate this process.
Z. Ouyang, G. Zheng, D. R. Tomchick, X. Luo, H. Yu, Structural basis and IP6 requirement for Pds5-dependent cohesin dynamics. Mol. Cell 62, 248–259 (2016). [PubMed]