Reevaluation of the role of LIP-1 as an ERK/MPK-1 dual specificity phosphatase in the <i>C. elegans</i> germline.

Abstract

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 <i>Caenorhabditis elegans</i> germline primarily because of its phenotype, which morphologically mimics that of a <i>RAS</i>/<i>let-60</i> gain-of-function mutant (i.e., small oocyte phenotype). Our investigations, however, reveal that loss of <i>lip-1</i> does not lead to an increase in MPK-1 activity in vivo. Instead, we show that loss of <i>lip-1</i> leads to 1) a decrease in MPK-1 phosphorylation, 2) lower MPK-1 substrate phosphorylation, 3) phenocopy of <i>mpk-1</i> reduction-of-function (rather than gain-of-function) allele, and 4) a failure to rescue <i>mpk-1</i>-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 <i>C. elegans</i> 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.