Stromal interaction molecule 1 (STIM1), the ER calcium sensor, couples to Orai1, the calcium channel, and mediates store-operated calcium entry (SOCE). STIM1 localizes to the ER membrane. Following Ca²+ store depletion, STIM1 forms puncta that localize to the cortical ER and binds Orai1 to allow Ca²+ influx. The egg's competency to activate at fertilization is dependent on its ability to generate a fertilization-specific Ca2+ transient. This is achieved through remodeling of its calcium signaling machinery during oocyte maturation. Oocyte maturation is driven by kinase cascade including MOS- mitogen-activated protein kinase (MAPK) - maturation promoting factor (MPF). Coordinately, SOCE inactivates during maturation. This inactivation is due to internalizing of Orai1 and the inability of STIM1 to form puncta. Here we show that MPF phosphorylates STIM1. Through molecular and pharmacological manipulation we are able to differentially activate the kinases along the kinase cascade that drives oocyte maturation. STIM1 phosphorylation in these studies was monitored through a mobility shift on SDS-PAGE. When MPF is activated a mobility shift is observed. However, this is not the case when only Mos or the MAPK cascade were active. Our next step is to determine the role of MPF phosphorylation of STIM1, because previous studies have argued that phosphorylation does not inhibit STIM1 clustering during meiosis. Our data argue that MPF is the primary kinase that phosphorylates STIM1 during meiosis.


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