Ye X S, Fincher R R, Tang A, O'Donnell K, Osmani S A
Weis Center for Research, Geisinger Clinic, Danville, PA 17822-2617, USA.
EMBO J. 1996 Jul 15;15(14):3599-610.
We demonstrate that there are at least two S-phase checkpoint mechanisms controlling mitosis in Aspergillus. The first responds to the rate of DNA replication and inhibits mitosis via tyrosine phosphorylation of p34cdc2. Cells unable to tyrosine phosphorylate p34cdc2 are therefore viable but are unable to tolerate low levels of hydroxyurea and prematurely enter lethal mitosis when S-phase is slowed. However, if the NIMA mitosis-promoting kinase is inactivated then non-tyrosine-phosphorylated p34cdc2 cannot promote cells prematurely into mitosis. Lack of tyrosine-phosphorylated p34cdc2 also cannot promote mitosis, or lethality, if DNA replication is arrested, demonstrating the presence of a second S-phase checkpoint mechanism over mitotic initiation which we show involves the function of BIME. In order to overcome the S-phase arrest checkpoint over mitosis it is necessary both to prevent tyrosine phosphorylation of p34cdc2 and also to inactivate BIME. Lack of tyrosine phosphorylation of p34cdc2 allows precocious expression of NIMA during S-phase arrest, and lack of BIME then allows activation of this prematurely expressed NIMA by phosphorylation. The mitosis-promoting NIMA kinase is thus a target for S-phase checkpoint controls.
我们证明，在曲霉菌中至少存在两种控制有丝分裂的S期检查点机制。第一种机制对DNA复制速率作出反应，并通过p34cdc2的酪氨酸磷酸化来抑制有丝分裂。因此，无法对p34cdc2进行酪氨酸磷酸化的细胞是存活的，但无法耐受低水平的羟基脲，并且当S期减慢时会过早进入致死性有丝分裂。然而，如果NIMA有丝分裂促进激酶失活，那么未进行酪氨酸磷酸化的p34cdc2就无法促使细胞过早进入有丝分裂。如果DNA复制停滞，缺乏酪氨酸磷酸化的p34cdc2也无法促进有丝分裂或导致细胞死亡，这表明存在第二种关于有丝分裂起始的S期检查点机制，我们发现这涉及到BIME的功能。为了克服有丝分裂过程中的S期停滞检查点，既要防止p34cdc2的酪氨酸磷酸化，也要使BIME失活。p34cdc2缺乏酪氨酸磷酸化会导致在S期停滞期间NIMA过早表达，而缺乏BIME则会使这种过早表达的NIMA通过磷酸化被激活。因此，有丝分裂促进性NIMA激酶是S期检查点控制的一个靶点。
