Active MPK-1 ERK (dpMPK-1) displays a spatially restricted and dynamic pattern in the C. elegans hermaphrodite germline. It is inactive in the distal germline and first activated in pachytene germ cells. Once active, MPK-1 executes various cell biological processes such as pachytene cell membrane organization via activating or inactivating substrates upon phosphorylation. Absence of
mpk-1 signaling results in severe loss of cell membranes from pachytene germ cells. We find that a null allele of
nos-3, a member of the NANOS family of translational repressors, partially suppresses the severe loss of membranes from the
mpk-1 null germ cells. NOS-3 is likely a direct target of MPK-1 as it is a robust in vitro substrate of activated ERK2 and in vivo MPK-1 dependent phospho-NOS-3 accumulates in pachytene stage germ cells overlapping with dpMPK-1. Together, this suggests that MPK-1 mediated phosphorylation likely inactivates NOS-3 during pachytene to promote membrane organization. Previous studies show that during sex determination NOS-3 represses FEM-3 translation. FEM-3 in turn functions as part of an E3 ubiquitin ligase complex to degrade nuclear TRA-1/Gli which acts as a transcriptional repressor. The sex determination function of this pathway likely acts in the distal germline. However, in pachytene germ cells, where MPK-1 mediated phosphorylation likely inactivates NOS-3, the resulting high FEM-3 levels should lead to decreased TRA-1. Consistent with this hypothesis, we find a higher accumulation of TRA-1 in distal germ line nuclei, but low/no accumulation where phosphorylated NOS-3 and dpMPK-1 accumulate. Also, we find that absence of
nos-3 results in lower accumulation of TRA-1 throughout presumably because of high FEM-3. Examination of
fem-3 null germlines reveals that indeed TRA-1 accumulates at a higher level in pachytene. Most interestingly, loss of
mpk-1 activity also results in higher level of TRA-1 suggesting a model where control of NOS-3 activity by MPK-1 dependent phosphorylation spatially regulates the NOS-3/FEM-3/TRA-1 cascade for two distinct biological outputs: female germ cell fate and membrane organization during oogenesis. In distal germline, where there is no active MPK-1, NOS-3 mediated repression of FEM-3 translation leads to high TRA-1 activity likely assisting in specification of the female germ cell fate. However, in pachytene, active MPK-1 through phosphorylation of NOS-3 switches pathway activity leading to inactivation of TRA-1, which presumably allows transcription of genes that function in membrane organization during oogenesis.