RNA-assisted nuclear transport of the meiotic regulator Mei2p in fission yeast.

Fission yeast Mei2p is an RNA-binding protein required for both premeiotic DNA synthesis and meiosis I. Mei2p binds to a polyadenylated RNA molecule, meiRNA, loss of which blocks meiosis I. Mei2p forms a dot in meiotic prophase nuclei. Here, we show that meiRNA is required for the nuclear localization of Mei2p and is detectable in the dot. However, Mei2p carrying a nuclear localization signal can produce a nuclear dot and promote meiosis I in the absence of meiRNA. Mei2p expressed in cultured mammalian cells stays in the cytoplasm, but it accumulates in the nucleolus if meiRNA is coexpressed. These results indicate that meiRNA contributes to the promotion of meiosis I exclusively as a cofactor that assists nuclear transport of Mei2p.