The
mei-1 gene product is required for assembly and/or maintenance of the meiotic spindle.
mei-2 acts genetically as an activator of
mei-1. Recently,
mei-2 was identified by the RNA-mediated interference (RNAi) approach. We chose a clone within the genetically defined
mei-2 region because it contained a predicted gene with very weak similarity to a synaptonemal complex (SC) protein. Initially, we injected RNA transcribed from a genomic region that contained a portion of this putative SC gene. This gave no phenotype. However, a second clone (a 4.8kb XbaI genomic fragment derived from cosmid F57B10) did give the
mei-2(
ct102) phenotype. This clone contained the SC gene and another small gene (genefinder prediction), transcribed in the opposite direction. A 1.5kb XbaI-SpeI subclone that just contained the other gene, gave the
mei-2 RNAi phenotype (the SC gene turned out to be a red herring, but it thankfully led us to the right gene!). Chris Shelton and Bruce Bowerman independently found
mei-2 in a large-scale survey for maternal-effect RNAi-induced phenotypes (Thanks to Chris and Bruce for communicating their data). RT-PCR -derived cDNA for
mei-2 was used to establish the gene structure, shown in Figure 1.
mei-2 is trans-spliced to SL1 and encodes 280 amino acids. All four
mei-2 mutations have been sequenced (Fig 1.). Each of the alleles contain G/C-A/T transitions, resulting in missense mutations.
sb31 has an additional 35bp deletion that removes a splice donor sequence within the third intron. Based on this molecular data and complementation data, we believe
sb31 represents the null phenotype of
mei-2. Figure 1. The molecular structure of
mei-2 and the location of mutations. Northern analysis of
mei-2 (probed against gravid N2, L1, L2/L3, L4,
glp-1(lf),
fem-1(lf),
fem-3(gf) indicates strong expression in both female and male germlines. This is somewhat surprising, given that
mei-2 mutations exhibit strict maternal-effect lethality. BLAST searches with the predicted MEI-2 sequence suggest it is a pioneer protein. However, another C. elegans gene does have high similarity and we are currently testing to see if this is
mel-45, a gene that genetically interacts with
mei-2 and
mei-1. MEI-1 protein shares sequence similarity to katanin
p60 subunit, a sea urchin ATPase implicated in microtubule severing activity at the centrosome. Although it is unlikely that MEI-1 is an orthologue (MEI-1 is localized to the meiotic spindle, not the centrosome), it is possible that MEI-1's role is to produce very short meiotic spindles by severing microtubules. Surprisingly, MEI-2 sequence has a short region of similarity (albeit at a low level) to katanin's
p80 subunit. This subunit localizes the complex to the centrosome in sea urchin. Perhaps MEI-2 localizes MEI-1 to the meiotic spindle in an analogous fashion. Antibody production is currently underway to determine the localization pattern of MEI-2.