Analysis of interactions between the germline RNA helicases (GLHs) and their regulators KGB-1 and CSN-5 in Caenorhabditis elegans
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The Caenorhabditis elegans germline RNA helicases (GLHs) are constitutive components of P granules, non-membranous aggregates of protein and RNA that segregate with the nematode germline. The GLHs are critical for fertility. The novel MAP kinase, KGB-1, interacts with the GLH proteins, and the null kgb-1(um3) strain results in sterile worms at high temperatures; the germlines of these worms contain endomitotic replicating oocytes (EMO). We find that in kgb-1(um3) adults, while GLH-4 levels are similar to wild type, levels of GLH-1 are increased up to seven fold and the morphology of P granules is grossly affected. Binding of KGB-1 to GLH-1 requires a MAP kinase docking site. KGB-1 can phosphorylate GLH-1 using in vitro kinase assays, and GLH-1 is degraded by the proteasome in a KGB-1-dependent manner. In addition, KGB-1 physically associates with CSN-5 (COP9 signalosome subunit 5), another GLH binding partner. RNA interference (RNAi) of csn-5 results in sterile worms with under-proliferated germlines, mirroring the combined glh-1 and glh-4 RNAi phenotype. In contrast, elimination of csn-5 in the kgb-1(um3) background results in significantly more fertile worms than in non-injected kgb-1 worms. Based on these biochemical and genetic interactions, we propose KGB-1 and CSN-5 may oppositely regulate GLH-1, with KGB-1 degrading and CSN-5 protecting GLH-1. This cooperative system could maintain proper GLH-1 levels during normal germline development, as well as prevent excess GLH accumulation during the stressful conditions of high temperature and aging.