The role of protein phosphatase 4 in regulating microtubule severing in the Caenorhabditis elegans embryo

MEl-1, the catalytic subunit of the Caenorhabditis elegans "kalanin" microtubule-severing complex, is required for meiotic spindle formation. However, MEI-1 must be inactivated after the completion of meiosis to allow formation of the first mitotic spindle. Recent work demonstrated that post-meiotic MEI-1 undergoes ubiquitin-dependent degradation mediated by two independent pathways. Here we describe another level of MEI-1 regulation involving the protein phosphatase 4 (PP4) complex. The PP4R1 regulatory subunit protein phosphatase four regulatory subunit 1 (ppfr-1) was identified in an RNA interference (RNAi) screen for suppressors of a mei-1(gf) allele that is refractory to post-meiotic degradation. RNAi to the PP4 catalytic subunit PPH-4.1 or to the a4 regulatory PPFR-4 also suppressed lethality of ectopic MEI-1. These results suggest that PP4(+) activates MEI-1, and therefore loss of PP4 decreases ectopic MEI-1 (gf) activity. PPH-4.1 and MEI-1 co-immunoprecipitate with one another, indicating that the PP4 complex likely regulates MEI-1 activity directly rather than through an intermediate. The ppfr-1 mutant has subtle meiotic defects indicating that PPFR-1 also regulates MEI-1 during meiosis. MBK-2 is the only kinase known to phosphorylate MEI-1 and triggers post-meiotic MEI-1 degradation. However, genetic interactions between PP4 and mbk-2 were not consistent with an antagonistic relationship between the phosphatase and kinase. Additionally, reducing PP4 in mei-1(gf) did not change the level or localization of post-meiotic MEI-1. Thus, by making use of a genetic background where MEI-1 is ectopically expressed, we have uncovered a third mechanism of MEI-1 regulation, one based on phosphorylation but independent of degradation. The redundant regulatory pathways likely contribute in different ways to the rapid and precise post-meiotic inactivation of MEI-1 microtubulc-severing activity.