Novel WD-Repeat Protein Mip1p Facilitates Function of the Meiotic Regulator Mei2p in Fission Yeast

Abstract

In fission yeast, the onset of meiosis is triggered by activation of the RNA-binding protein Mei2p. We screened for a high-copy-number suppressor of the ectopic meiosis induced by expression of an active form of Mei2p. Consequently we isolated a truncated form of a novel gene, named mip1, from a fission yeast genomic library. The mip1 gene encoded a protein of 1,313 amino acids which carried a WD-repeat motif in the C-terminal region and was apparently conserved among eukaryotes. Mip1p was cytoplasmic, and two-hybrid and immunoprecipitation analyses demonstrated that Mip1p was bound to Mei2p in vivo. Genetic evidence indicated that wild-type Mip1p was required for the function of Mei2p to induce meiosis and that the truncated form of it (Mip1-15p) dominantly interfered with Mei2p. Mip1p appeared to be involved also in conjugation, associating with Ste11p, which is a key transcription factor for sexual development. Furthermore, Mip1p was essential for cell growth, to which neither Mei2p nor Ste11p is relevant. These results suggest that Mip1p assists functional expression of a number of proteins required for proliferation and sexual development in fission yeast.Sexual differentiation in the fission yeast Schizosaccharomyces pombe proceeds under nutrient starvation (4, 27, 28). Ste11p, which is a high-mobility-group protein, serves as a key transcription factor in this process. Upon starvation, Ste11p activates a number of genes required for mating and/or meiosis, including the mating-type genes (mat1-P and mat1-M) and mei2 (21). The mei2 gene encodes a pivotal regulator of meiosis (3,20,25). Pat1p (Ran1p) kinase (6,11,17), which is active during the mitotic cell cycle, phosphorylates Mei2p on two amino acid residues, Ser438 and Thr527, and thereby blocks the function of Mei2p (24). In diploid cells, the gene products of mat1-P and mat1-M cooperate to turn on expression of mei3, which encodes an inhibitor of Pat1p kinase and hence induces derepression of Mei2p (12,13,24,26). Thus, mei2 is activated both transcriptionally and posttranscriptionally in diploid cells under starved conditions. If a mutant Mei2p carrying alanine residues in place of Ser438 and Thr527 (Mei2-SATA) is expressed in proliferating cells, the cells cease growth immediately and enter meiosis, indicating that dephosphorylated Mei2p has the ability to switch the mitotic cell cycle to the meiotic one (24).Mei2p is an RNA-binding protein with three RNA recognition motifs that is required for both induction of premeiotic DNA synthesis and promotion of the first meiotic division (meiosis I). Cytoplasmic Mei2p performs the former function, and the RNA-binding ability of Mei2p is essential for it. To perform the latter function, Mei2p must move into the nucleus, where it forms a distinct dot structure. A specific RNA species, named meiRNA, has been shown to promote meiosis I as a cofactor that assists nuclear transport of Mei2p (25,29). However, the molecular function of Mei2p in the cytoplasm as well as that in the nucleus remains unknown.To identify ...

Fission yeast Tor1 functions as part of TORC1 to control mitotic entry through the stress MAPK pathway following nutrient stress

“…2A), we asked whether mutants in the TORC1-specific component Mip1 (raptor) played a role in mitotic control following nutrient stress. Interestingly, we found that mip1.310 and mip1-15p-FLAG mutants (Shinozaki-Yabana et al, 2000) mimicked tor1Δ in being compromised in the nutrient-stress-induced mitotic onset (Fig. 3).…”

Fission yeast Tor2 promotes cell growth and represses cell differentiation

“…However, the fact that rescue is incomplete might be linked to the finding that Tor2 overexpression also inhibits the function of Mei2, a protein required to induce meiosis in fission yeast. In this respect, it is interesting to note that Mip1, another member of the TORC1 complex and the fission yeast orthologue of KOG1 in S. cerevisiae and Raptor in mammalian cells, has also been shown to interact with Ste11 and Mei2 (Shinozaki-Yabana et al, 2000). Therefore, Tor2 seems to inhibit sexual differentiation at two levels: before mating, by inhibiting Ste11; and before meiosis, by interfering with Mei2.…”

“…The KOG1 orthologue in S. pombe, mip1+, is an essential gene. Mip1 binds the RNA-binding protein Mei2 and the transcription factor Ste11, and it is involved in nutrient-responsive sexual development (Shinozaki-Yabana et al, 2000). The LST8 orthologue in S. pombe, wat1+/pop3+, has been implicated in the control of cell morphology and microtubule integrity (Kemp et al, 1997;Ochotorena et al, 2001).…”

Psk1, an AGC kinase family member in fission yeast, is directly phosphorylated and controlled by TORC1 and functions as S6 kinase

“…Of these, Mip1 and Tor2 are essential for cell proliferation (Shinozaki-Yabana et al, 2000), implying that Mip1 plays a crucial role in TORC1 function similar to its counterparts, raptor and Kog1, in mammals and budding yeast, respectively. Pop3 is known to be included also in TORC2 and to be required for TORC2-mediated phosphorylation of Gad8 (Alvarez and Moreno, 2006;Hayashi et al, 2007;Ikeda et al, 2008).…”