Protein phosphorylation, a crucial posttranslational modification step controlling many diverse cellular functions, is dependent
on the opposing actions of protein kinases and protein phosphatases. The enzyme protein phosphatase 2A is 1 of 4 major
protein phosphatases identified in the cytosol of eukaryotic cells which are responsible for the dephosphorylation of serine and
threonine residues in proteins. Although all 4 protein phosphatases, PP1, PP2A, PP2B, and PP2C, have overlapping substrate
specificities in vitro, they can be distinguished by the use of inhibitor proteins and by their dependence on metal ions. PP1 is
inhibited by nanomolar concentrations of 2 thermostable proteins, inhibitor 1 and inhibitor 2, whereas the type 2 phosphatases
are unaffected by these inhibitors. The type 2 phosphatases can be distinguished by how their activity is regulated: PP2A
activity is independent of metal ions, PP2B is activated by Ca(2+)/calmodulin, and PP2C is activated by Mg(2+).
Lu Q, et al 2002 reported the regulation of Spindle Formation by Active Mitogen-Activated
Protein Kinase and Protein Phosphatase 2A During Mouse
Oocyte Meiosis.
Mitogen-activated protein kinase (MAPK) and protein phosphatase 2A (PP2A) regulate
oocyte meiosis, yet little is known regarding their mechanisms of action. This study
addressed the functional importance of active MAPK and PP2A in regulating oocyte
meiosis. Experiments were conducted to identify MAPK activation, PP2A activity,
intracellular enzyme trafficking, and ultrastructural associations during meiosis.
Questions of requisite kinase and/or phosphatase activity and chromatin condensation,
microtubule polymerization, and spindle formation were addressed. At the protein level,
MAPK and PP2A were present in constant amounts throughout the first meiotic
division. Both MAPK and PP2A were activated following germinal vesicle breakdown
(GVBD) in conjunction with metaphase I development. Immunocytochemical studies
confirmed the absence of active MAPK in germinal vesicle-intact (GVI) and GVBD
oocytes. At metaphase I and during the metaphase I/metaphase II transition, activated
MAPK colocalized with microtubules, poles, and plates of meiotic spindles. Protein
phosphatase 2A was dispersed evenly throughout the GVI oocyte cytoplasm.
Throughout the metaphase I/metaphase II transition, PP2A colocalized with
microtubules of meiotic spindles. Both active MAPK and PP2A associated with in
vitro-polymerized microtubules, suggesting that active MAPK and PP2A locally
regulate spindle formation. Inhibition of MAPK activation resulted in compromised
microtubule polymerization, no spindle formation, and loosely condensed
chromosomes. Treatment with okadaic acid (OA) or calyculin-A (CL-A), which
inhibits oocyte cytoplasmic PP2A, caused an absence of microtubule polymerization
and spindles, even though MAPK activity was increased under these treatment
conditions. Thus, active MAPK is required, but is not sufficient, for normal meiotic
spindle formation and chromosome condensation. In addition, the oocyte
OA/CL-A-sensitive PP, presumably PP2A, is essential for microtubule polymerization
and meiotic spindle formation.
Gene whose expression is detected by cDNA array hybridization: GDP/GTP exchangers, GTPase stimulators and inhibitors, apoptosis Rozenn Dalbis-Tran and Pascal Mermilloda