MISS depeletion (bottom) causes defects in meiosis II spindles.

In meiosis II, vertebrate oocytes undergo a cell cycle arrest that is maintained until fertilization, but it has been difficult to identify the factors responsible for this process. By screening a novel cDNA library, Lefebvre et al., page 603, have discovered and characterized a protein that specifically regulates meiosis II spindle integrity during arrest. The work helps to explain how the cell distinguishes between meiosis I and II, and the new library should be useful in discovering additional genes involved in early development.

Immature oocytes do not initiate an arrest until they reach meiosis II, but the kinases known to trigger this process are expressed through both meiotic divisions, so other factors must determine how and when the arrest occurs. The authors generated a cDNA library from mouse immature oocytes, representing transcripts that are specifically relevant to meiotic maturation. Two-hybrid screening identified a MAP kinase-interacting and spindle-stabilizing protein (MISS). MISS is unstable in meiosis I, but becomes stable in meiosis II, when it localizes in dots on the spindles. Interfering with endogenous MISS RNA causes severe spindle defects only in meiosis II. Its specific stabilization during meiosis II suggests that MISS provides a link between MAP kinase activity and meiotic arrest. â–ª