LAUSR

Significance The RAS–ERK pathway is critical for metazoan development. In development, ERK activity is regulated by a balance of phosphorylation of ERK by MEK (MAPK kinase) and dephosphorylation by DUSPs (dual specificity phosphatases). LIP-1, a DUSP6/7 family member, was previously suggested to regulate MPK-1/ERK activity by dephosphorylating MPK-1 in the Caenorhabditis elegans germline, based on LIP-1's mutant phenotype in the germline and its DUSP role in vulval development. However, our investigations demonstrate that LIP-1 does not function as an MPK-1 DUSP in the germline and likely regulates germline functions through distinct targets. Our results present a cautionary note about misinterpreting similar mutant phenotypes caused by mutations in different genes and assuming that genes function similarly in different tissues. The fidelity of a signaling pathway depends on its tight regulation in space and time. Extracellular signal-regulated kinase (ERK) controls wide-ranging cellular processes to promote organismal development and tissue homeostasis. ERK activation depends on a reversible dual phosphorylation on the TEY motif in its active site by ERK kinase (MEK) and dephosphorylation by DUSPs (dual specificity phosphatases). LIP-1, a DUSP6/7 homolog, was proposed to function as an ERK (MPK-1) DUSP in the Caenorhabditis elegans germline primarily because of its phenotype, which morphologically mimics that of a RAS/let-60 gain-of-function mutant (i.e., small oocyte phenotype). Our investigations, however, reveal that loss of lip-1 does not lead to an increase in MPK-1 activity in vivo. Instead, we show that loss of lip-1 leads to 1) a decrease in MPK-1 phosphorylation, 2) lower MPK-1 substrate phosphorylation, 3) phenocopy of mpk-1 reduction-of-function (rather than gain-of-function) allele, and 4) a failure to rescue mpk-1–dependent germline or fertility defects. Moreover, using diverse genetic mutants, we show that the small oocyte phenotype does not correlate with increased ectopic MPK-1 activity and that ectopic increase in MPK-1 phosphorylation does not necessarily result in a small oocyte phenotype. Together, these data demonstrate that LIP-1 does not function as an MPK-1 DUSP in the C. elegans germline. Our results caution against overinterpretation of the mechanistic underpinnings of orthologous phenotypes, since they may be a result of independent mechanisms, and provide a framework for characterizing the distinct molecular targets through which LIP-1 may mediate its several germline functions.