The RAS to MAPK (mitogen-activated protein kinase) signaling cascade acts repeatedly during C. elegans development, including the development of the germ line. In the C. elegans hermaphrodite gonad, an activated, di-phosphorylated MAPK (MPK-1) is first detectable at the mid-pachytene region of the gonad, although the ligand that stimulates the activation of the pathway is not yet identified. The spatial activation pattern of MAPK, along with genetic analysis, suggests that it is required for progression through the pachytene stage of meiosis as well as later for oocyte maturation (Church, et al, 1995). Activated, phosphorylated MPK-1 rapidly disappears as germ cells pass the pachytene stage and enter the diplotene/diakinesis phase of meiotic prophase I, and returns only in the most proximal oocytes. The dual-specificity phosphatase LIP-1 is responsible for this inactivation of MAPK (Hajnal, et al, 2002). Although genetic evidence indicates that LIP-1 acts as an antagonist in the oogensis of C. elegans, how LIP-1 is regulated in the middle and late meiotic stage remains elusive. We have been using a functional proteomic approach to identify the phosphorylation substrates of the MAP kinase that mediate meiotic progression of germ cell, focusing on transcription factors, as they are typically a major phosphorylation target of MAPK in the nucleus. Our biochemical studies indicate that one of the phosphorylation targets of the MAP kinase is DPL-1. The DPL-1/EFL-1 transcription factor complex activates their responsive genes in the mid-pachytene zone of germline, similar to the region of activated MPK-1. Moreover, previous studies have indicated that DPL-1/EFL-1 negatively regulates MAPK in the proximal germ line (Page, et al, 2001). We present evidence that this regulatory effect is executed through promoting the expression of
lip-1. First,
dpl-1 and
lip-1 mutants have a similar phenotype, including excess oocytes in diakinesis stage and endomitotic oocytes. Loss of
lip-1 activity enhances the reduction-of-function mutants in
dpl-1. Second, the
lip-1 promoter has two E2F consensus motifs, implying that it could be bound by the DPL-1/EFL-1 complex. By employing RT-PCR and antibody staining, we further demonstrate that both
lip-1 transcript and its corresponding protein are down-regulated in various
dpl-1 mutant alleles. The sophisticate regulatory relationships between MAPK and the DPL-1/EFL-1 complex are under investigations. .