A cyclin-abundance cycle-independent p34cdc2 tyrosine phosphorylation cycle in early sea urchin embryos.

Abstract

The activity of the cell cycle control protein p34cdc2 is post‐translationally regulated in a variety of cell types. Using anti‐phosphotyrosine antibodies, we find that p34cdc2‐directed tyrosine kinase activity increases at fertilization in sea urchin eggs, leading to a gradual accumulation of phosphotyrosine on p34 during the early part of the cell cycle. Loss of phosphotyrosine from p34 accompanies entry into mitosis and phosphotyrosine reaccumulates as the embryo enters the next cell cycle. A similar pattern is seen when eggs are parthenogenetically activated with ammonium chloride. Tyrosine phosphorylation and phosphorylation/dephosphorylation cycles are suppressed when embryos are treated with the tyrosine kinase inhibitor genistein. On the other hand, a cycle persists when protein synthesis is inhibited with emetine, indicating that it is independent of the synthesis of another class of cell cycle control proteins, the cyclins. Additional experiments with the phorbol ester, phorbol myristate acetate, demonstrate that activating protein synthesis alone in unfertilized eggs does not result in stimulation of p34cdc2 tyrosine kinase activity. Our results indicate that p34 tyrosine phosphorylation cycles are triggered by the fertilization Cai transient. The first cycle is independent of the fertilization pHi signal, confirming that, in sea urchin embryos, the cycle is not tightly coupled to the cycle of cyclin abundance that is a prominent feature of the eukaryotic cell division cycle.

Intracellular and Extracellular pH and Ca Are Bound to Control Mitosis in the Early Sea Urchin Embryo via ERK and MPF Activities

“…1Ab), despite alterations of mitosis as expected [22]. Similar alterations in Ca i mitotic levels were measured in fertilized eggs treated with 20 µM Roscovitine, an inhibitor of CDK that did not modify the P-ERK level (not shown) but blocks the cell cycle before mitosis by inhibiting MPF activity [30] and nuclear envelope breakdown (NEB) (Fig. 1Ac).…”

Inactivation of MAPK in mature oocytes triggers progression into mitosis via a Ca2+-dependent pathway but without completion of S phase

“…Dephosphorylation of Cdc2-TyrP induced after inactivation of the MEK-ERK pathway is pH i -independent, as reported after fertilization (Edgecombe et al, 1991). Microtubule polymerization, chromosome condensation and NEB might not be strictly pH i -dependent events.…”

Extracellular heat shock protein 70 has novel functional effects on sea urchin eggs and coelomocytes

“…One candidate for this effect is the cell division cycle kinase, cdc2, because of its purported role in phosphorylation of nuclear lamins (Peter et al, 1991), which is required for NEB (Burke, 1990). In the resting egg, cdc2 is highly phosphorylated and inactive and is dephosphorylated during the initiation of mitosis (Edgecombe et al, 1991). Therefore, we examined the levels of phosphorylated cdc2 in unfertilized and fertilized eggs with or without treatment with Hsc70.…”

“…We examined the levels of phospho-cdc2 following fertilization as an initial test of this idea. This kinase is known to be activated by being dephosphorylated rapidly at the onset of mitosis (Edgecombe et al, 1991). Therefore, we expected that the levels of phospho-cdc2 would fall more rapidly in Hsc70-treated eggs compared with controls, so that it could phosphorylate lamin, which is necessary for NEB (Burke, 1990;Peter et al, 1991).…”