An okadaic acid-sensitive phosphatase negatively controls the cyclin degradation pathway in amphibian eggs.

Abstract

Inhibition of okadaic acid-sensitive phosphatases released the cyclin degradation pathway from its inhibited state in extracts prepared from unfertilized Xenopus eggs arrested at the second meiotic metaphase. It also switched on cyclin protease activity in a permanent fashion in interphase extracts prepared from activated eggs. Even after cdc2 kinase inactivation, microinjection of okadaic acid-treated interphase extracts pushed G2-arrested recipient oocytes into the M phase, suggesting that the phosphatase inhibitor stabilizes the activity of an unidentified factor which shares in common with cdc2 kinase the maturation-promoting factor activity.Exit from the M phase of the cell cycle requires inactivation of maturation-promoting factor (MPF), a universal inducer of mitosis recently identified as a stoichiometric complex between a cdc2+-encoded protein kinase and cyclin B, a protein whose concentration oscillates during the cell cycle (12, 16). MPF inactivation requires cyclin proteolysis (21), which begins at the end of metaphase and is completed within a very short window of the cell cycle (25, 28). At least two protein kinases have been shown to be involved in the control of cyclin degradation. One is cdc2 kinase itself, which promotes the abrupt degradation of cyclin about 15 min after its addition to interphase extracts made from fertilized or activated Xenopus eggs (8). At the end of the M phase, after cyclin has been destroyed, cdc2 kinase is inactivated and so is the proteolysis machinery. The second one is c-mos protein kinase, a proto-oncogene-encoded protein kinase. In vertebrates, c-mos protein kinase acts as a cytostatic factor which prevents proteolytic degradation of cyclin and arrests unfertilized eggs at the second meiotic metaphase (24). On fertilization or parthenogenetic activation, a calcium-dependent process inactivates c-mos kinase, resulting in cyclin degradation (27). The dramatic effects of inactivation of either the cdc2 or c-mos protein kinase on cyclin degradation suggest the involvement of unidentified protein phosphatases in the control of cyclin proteolysis.Therefore, the cyclin degradation pathway may be viewed as a complex network of protein kinases and protein phosphatases controlling ultimately the activity of a cyclin protease. Here we show that okadaic acid (OA), a potent inhibitor of type 1 and type 2A phosphatases (1, 3), overcomes a c-mos kinase inhibition of the cyclin degradation pathway in calcium-free extracts prepared from metaphasearrested eggs, while it deregulates and switches on cyclin protease activity in a permanent fashion in interphase extracts prepared from activated eggs. This leads us to propose that an OA-inhibited phosphatase, most likely type 2A phosphatase, exerts a negative control on the cyclin degradation pathway. Moreover, microinjection of OA-treated extracts pushes G2-arrested recipient oocytes into the M * Corresponding author.phase, even after cdc2 kinase inactivation. This suggests that the phosphatase inhibitor stabilizes the activity of an u...

Emi2 at the Crossroads: Where CSF Meets MPF

“…Inhibition of phosphatase activity by microcystin or okadaic acid causes spontaneous, Ca 2+ -independent destruction of cyclin B in CSF extract. 33,34 When we reexamined this observation in terms of Emi2, we found that, unlike Ca 2+ -induced CSF release, the cyclin B destruction caused by phosphatase inhibition was not accompanied by Emi2 destruction, suggesting the existence of a kinase-dependent loss of Emi2 function. 20 This could be through direct inactivation of Emi2, or through the APC/C becoming refractory to Emi2's inhibitory activity.…”

The polo-like kinase Plx1 prevents premature inactivation of the APCFizzy-dependent pathway in the early Xenopus cell cycle

“…We reported previously that the cyclin degradation pathway is turned on for only one round in the absence of phospatase inhibitor, and inactivates when endogenous cyclins A and B have been destroyed. In the presence of okadaic acid or microcystin in contrast, the cyclin degradation machinery remains switched on in a permanent fashion (Lorca et al, 1991a).…”

“…An okadaic acid/microcystin-sensitive phosphatase has been shown to negatively control the cyclin degradation pathway in eggs and early embryos of both vertebrates and invertebrates (Felix et al, 1990b;Lorca et al, 1991a;Lahav-Baratz et al, 1995). Constitutive activity of this phosphatase is believed to turn o APC Fizzy activity when cyclin B-cdc2 kinase drops below a critical threshold activity.…”

Regulation of EDEN-dependent deadenylation of Aurora A/Eg2-derived mRNA via phosphorylation and dephosphorylation inXenopus laevisegg extracts

“…MPF activity depends on the stability of cyclin B . Moreover, OA is known to indirectly activate MPF in Xenopus oocytes and extracts (Goris et al, 1989;Rime et al, 1990;Jessus et al, 1991;Lorca et al, 1991). Therefore, the sensitivity of EDEN-dependent deadenylation to OA could be due, in part, to activation of MPF.…”