The proteasome is an essential mediator of the activation of pre-MPF during starfish oocyte maturation.

Starfish oocyte maturation was blocked by the addition of 100 microM MG115, a potent proteasome inhibitor, whereas no inhibition was observed by membrane permeable cysteine protease inhibitor, E-64-d. The inhibition by MG115 was diminished by adding at a time corresponding to the half time required for germinal vesicle breakdown. Potent inhibition of germinal vesicle breakdown was also observed by microinjection of anti-proteasome-a-subunit antibodies. The antibody-injected oocytes failed to activate pre-maturation promoting factor (pre-MPF), since the dephosphorylation of phospho-Tyr15 in cdc2 kinase was not observed even in the presence of 1-methyadenine, a maturation-inducing hormone. These results indicate that the proteasome triggers the activation of pre-MPF via the dephosphorylation of cdc2 kinase in the signal transduction pathway in response to the hormonal stimulus during starfish oocyte maturation.