Autophagy is an intracellular degradation process that is mediated by autophagosomes. Mammalian Atg2 proteins Atg2A and Atg2B are identified and characterized as essential for autophagy. They are also present on lipid droplets and are involved in regulation of lipid droplet volume and distribution.

Reinitiation of meiosis in oocytes usually occurs as a two-step process during which release from the prophase block is followed by an arrest in metaphase of the first or second meiotic division [metaphase I (MI) or metaphase II (MII)]. The mechanism of MI arrest in meiosis is poorly understood, although it is a widely observed phenomenon in invertebrates. The blockage of fully grown starfish oocytes in prophase of meiosis I is released by the hormone 1-methyladenine. It has been believed that meiosis of starfish oocytes proceeds completely without MI or MII arrest, even when fertilization does not occur. Here we show that MI arrest of starfish oocytes occurs in the ovary after germinal vesicle breakdown. This arrest is maintained both by the Mos/MEK/MAP kinase pathway and the blockage of an increase of intracellular pH in the ovary before spawning. Immediately after spawning into seawater,activation of Na+/H+ antiporters via a heterotrimeric G protein coupling to a 1-methyladenine receptor in the oocyte leads to an intracellular pH increase that can overcome the MI arrest even in the presence of active MAP kinase.

In the starfish ovary, maturing oocytes stimulated by 1-methyladenine undergo synchronous germinal vesicle breakdown and then arrest in metaphase of the first meiotic division (metaphase I). Immediately after spawning, an increase of intracellular pH (pH i ) from ϳ7.0 to ϳ7.3 is induced by Na ؉ /H ؉ antiporter in oocytes, and meiosis reinitiation occurs. Here we show that an endogenous substrate of the proteasome, polyubiquitinated cyclin B, was stable at pH 7.0, whereas it was degraded at pH 7.3. When the MAPK pathway was blocked by MEK inhibitor U0126, degradation of polyubiquitinated cyclin B occurred even at pH 7.0 without an increase of the peptidase activity of the proteasome. These results indicate that the proteasome activity at pH 7.0 is sufficient for degradation of polyubiquitinated cyclin B and that the MAPK pathway blocks the degradation of polyubiquitinated cyclin B in the maturing oocytes in the ovary. Immediately after spawning, the increase in pH i mediated by Na ؉ /H ؉ antiporter cancels the inhibitory effects of the MAPK pathway, resulting in the degradation of polyubiquitinated cyclin B and the release of the arrest. Thus, the key step of metaphase I arrest in starfish oocytes occurs after the polyubiqutination of cyclin B but before cyclin B proteolysis by the proteasome.In many animals, oocytes are blocked twice in meiosis. Usually, the length of the first arrest at prophase I is extremely long. The release from this arrest is generally triggered by hormonal stimuli that drive the oocyte to the second arrest at metaphase I (MI) 1 in many invertebrates, including ascidians, several molluscan species, and Drosophila, or at metaphase II (MII) in vertebrates. The metaphase state is established by the activity of the complex of cyclin B and Cdc2 kinase (1-6). The metaphase/anaphase transition is induced by the ubiquitin-dependent degradation of cyclin B. The formation of ubiquitin conjugates requires the concerted activity of a series of enzymes that first activate ubiquitin (E1) and then recognize and transfer ubiquitin (E2 and E3) to proteins destined for turnover (for a review, see Ref. 7). Cyclin B is polyubiquitinated by a specific E3 called the anaphase-promoting complex/cyclosome (8) (for a review, see Ref. 9). Once such targeted proteins become polyubiquitinated, they are recognized and degraded by a particle known as the 26 S proteasome.Although the mechanisms of metaphase arrest in meiosis I (MI arrest) in invertebrate oocytes are poorly understand, MII arrest in vertebrate unfertilized eggs has been well studied. MII arrest is mediated by an activity known as cytostatic factor (CSF) (10), which stabilizes cyclin B. The expression of c-Mos, which is a MAPK kinase kinase, in one blastomere of a two-cell Xenopus embryo leads to CSF arrest (11). It has also been shown that microinjection of thiophosphorylated MAPK into one blastomere of a two-cell embryo induced metaphase arrest similar to that induced by c-Mos (12). A MAPK target, the protein kinase p90Rsk, was shown to be the sole media...