Oocyte maturation and<b style='mso-bidi-font-weight:normal'> fertilization are very dramatic transitions in which dormant oocytes are converted into rapidly dividing, differentiating embryos.<span style="mso-spacerun: yes"> </span>Although this transition is clearly important for successful initiation of embryogenesis, its mechanism is poorly understood.<span style="mso-spacerun: yes"> </span>We believe that OMA-1 and OMA-2 play important roles in the regulation of the egg-to-embryo transition and that timely degradation of OMA-1/2 is critical for this transition.<span style="mso-spacerun: yes"> </span><i style='mso-bidi-font-style: normal'>
oma-1 and <i style='mso-bidi-font-style:normal'>
oma-2 are close homologues that are redundantly required for oocyte maturation as well as embryogenesis.<span style="mso-spacerun: yes"> </span>They encode CCCH Tis-11 like zinc finger proteins that are predicted to regulate RNA targets. OMA-1/2 are expressed in oocytes and are rapidly degraded in newly fertilized embryos.<span style="mso-spacerun: yes"> </span>This timely degradation appears to be important for proper embryogenesis since the<i style='mso-bidi-font-style:normal'>
oma-1 (<i style='mso-bidi-font-style:normal'>
zu405) gain-of-function mutation, which causes a delay in OMA-1 degradation, results in embryonic lethality.<span style="mso-spacerun: yes"> </span>Although our preliminary results suggest that OMA-1/2 are degraded through the ubiquitin proteasome pathway, the precise mechanism remains unclear.<span style="mso-spacerun: yes"> </span>It has been shown previously that degradation of OMA-1, and several other maternal proteins, requires the MBK-2 kinase, a protein that also coordinates the egg-to-embryo transition.We show that OMA-1 is a direct target of MBK-2.<span style="mso-spacerun: yes"> </span>We show that MBK-2 phosphorylates OMA-1 <i style='mso-bidi-font-style:normal'><span class=msoChangeProp style='mso-prop-change:"Scott Robertson" 20050323T1735'><span class=msoChangeProp style='mso-prop-change:"Scott Robertson" 20050323T1735'>in vitro</span></span> predominantly on T239, immediately next to the amino acid mutated in the <i style='mso-bidi-font-style:normal'>
oma-1(
zu405) gain-of-function mutant.<span style="mso-spacerun: yes"> </span>The amino acid change of the <i style='mso-bidi-font-style:normal'>
zu405 mutation prevents phosphorylation of OMA-1 at T239.<span style="mso-spacerun: yes"> </span>Using an antibody recognizing OMA-1 phosphorylated at T239, we show that OMA-1 is phosphorylated <i style='mso-bidi-font-style:normal'><span class=msoChangeProp style='mso-prop-change:"Scott Robertson" 20050323T1735'><span class=msoChangeProp style='mso-prop-change:"Scott Robertson" 20050323T1735'>in vivo</span></span> at T239 during the very short developmental time window immediately after MBK-2 activation and until OMA-1 degradation.<span style="mso-spacerun: yes"> </span>The <i style='mso-bidi-font-style: normal'><span class=msoChangeProp style='mso-prop-change:"Scott Robertson" 20050323T1735'><span class=msoChangeProp style='mso-prop-change:"Scott Robertson" 20050323T1735'>in vivo</span></span> phosphorylation of OMA-1 at T239 depends on MBK-2 kinase activity and is not observed in <span style='mso-bidi-font-style: italic'>the</span><i style='mso-bidi-font-style:normal'>
zu405 mutant embryos.<span style="mso-spacerun: yes"> </span>In addition, we show that an OMA-1 GFP reporter in which T239 is mutated to alanine fails to degrade properly, demonstrating that MBK-2 phosphorylation of OMA-1 at T239 promotes its degradation.In conclusion, our data suggest that OMA-1 is phosphorylated by MBK-2 at T239 shortly before its degradation and that this phosphorylation promotes OMA-1 degradation at the egg-to-embryo transition.