Dulev Stanimir, de Renty Christelle, Mehta Rajvi, Minkov Ivan, Schwob Etienne, Strunnikov Alexander
National Institutes of Health, National Institute of Child Health and Human Development, Bethesda, Maryland 20892, USA.
Proc Natl Acad Sci U S A. 2009 Aug 25;106(34):14466-71. doi: 10.1073/pnas.0900190106. Epub 2009 Aug 7.
The CDC14 family of multifunctional evolutionarily conserved phosphatases includes major regulators of mitosis in eukaryotes and of DNA damage response in humans. The CDC14 function is also crucial for accurate chromosome segregation, which is exemplified by its absolute requirement in yeast for the anaphase segregation of nucleolar organizers; however the nature of this essential pathway is not understood. Upon investigation of the rDNA nondisjunction phenomenon, it was found that cdc14 mutants fail to complete replication of this locus. Moreover, other late-replicating genomic regions (10% of the genome) are also underreplicated in cdc14 mutants undergoing anaphase. This selective genome-wide replication defect is due to dosage insufficiency of replication factors in the nucleus, which stems from two defects, both contingent on the reduced CDC14 function in the preceding mitosis. First, a constitutive nuclear import defect results in a drastic dosage decrease for those replication proteins that are regulated by nuclear transport. Particularly, essential RPA subunits display both lower mRNA and protein levels, as well as abnormal cytoplasmic localization. Second, the reduced transcription of MBF and SBF-controlled genes in G1 leads to the reduction in protein levels of many proteins involved in DNA replication. The failure to complete replication of late replicons is the primary reason for chromosome nondisjunction upon CDC14 dysfunction. As the genome-wide slow-down of DNA replication does not trigger checkpoints [Lengronne A, Schwob E (2002) Mol Cell 9:1067-1078], CDC14 mutations pose an overwhelming challenge to genome stability, both generating chromosome damage and undermining the checkpoint control mechanisms.
CDC14家族是一类多功能的、在进化上保守的磷酸酶，包括真核生物有丝分裂的主要调节因子以及人类DNA损伤反应的调节因子。CDC14的功能对于准确的染色体分离也至关重要，这在酵母中核仁组织区后期分离对其的绝对需求中得到了体现；然而，这条基本途径的本质尚不清楚。在对核糖体DNA（rDNA）不分离现象进行研究时，发现cdc14突变体无法完成该位点的复制。此外，在进行后期的cdc14突变体中，其他晚复制的基因组区域（占基因组的10%）也复制不足。这种全基因组范围内的选择性复制缺陷是由于细胞核中复制因子的剂量不足，这源于两个缺陷，且都取决于前一个有丝分裂中CDC14功能的降低。首先，持续性的核输入缺陷导致那些受核转运调节的复制蛋白的剂量急剧下降。特别是，必需的复制蛋白A（RPA）亚基的mRNA和蛋白质水平都较低，并且存在异常的细胞质定位。其次，G1期MBF和SBF控制基因的转录减少导致许多参与DNA复制的蛋白质水平降低。晚复制子无法完成复制是CDC14功能障碍时染色体不分离的主要原因。由于全基因组范围内DNA复制的减慢不会触发检查点[Lengronne A, Schwob E（2002年）《分子细胞》9:1067 - 1078]，CDC14突变对基因组稳定性构成了巨大挑战，既会产生染色体损伤，又会破坏检查点控制机制。
