Cohesin-interacting protein WAPL-1 regulates meiotic chromosome structure and cohesion by antagonizing specific cohesin complexes

Most of the genetic material of plant and animal cells is stored in structures called chromosomes. Nearly all the cells in the body contain two copies of each chromosome, one inherited from the mother and the other from the father, but sex cells – such as egg and sperm – contain just one copy of each. If eggs or sperm contain the wrong number of copies of a chromosome, genetic disorders such as Down syndrome can occur. New sex cells form in a process called meiosis, which begins with a cell that contains two copies of each chromosome duplicating each of these copies. The duplicated copies are known as sister chromatids, and are held together by a ring-like protein complex called cohesin. In addition to tethering sister chromatids, cohesin affects the ‘higher-order’ organization of chromosome structure and promotes the recruitment of other proteins that are essential for different aspects of chromosome behavior during meiosis. Therefore, regulating cohesin binding during meiosis is key to ensuring that sex cells contain the correct number of chromosomes. Cohesin is ultimately removed from chromosomes in two steps during the consecutive cell divisions at the end of meiosis, resulting in the formation of sex cells containing a single copy of each chromosome. However, whether cohesin is actively removed from chromosomes during early meiosis, when chromosomes undergo dramatic structural changes, is not known. Using a combination of microscopy and genetic techniques to study the developing egg cells of the worm Caenorhabditis elegans, Crawley et al. investigated how a protein called WAPL-1 affects cohesin binding to chromosomes during early meiosis. This revealed that WAPL-1’s effects depend on the identity of a particular subunit of the cohesin complex. If this subunit is a protein called COH-3 or COH-4, WAPL-1 reduces the ability of cohesin to bind to chromosomes during the early stages of meiosis. However, WAPL-1 does not affect cohesin complexes that instead feature a protein called REC-8 as this subunit. By preventing excessive binding of COH-3 and COH-4 cohesin, WAPL-1 regulates chromosome structure and sister chromatid cohesion during early meiosis. Crawley et al. further observed that during the stage preceding the first meiotic division, cohesin is removed from chromosomes by a mechanism that does not involve WAPL-1. The next challenge is to work out why cohesin containing the REC-8 protein is protected from being released by WAPL-1. Whether defects in this protection can trigger the premature separation of sister chromatids is also an important question to answer.