title: Control of DNA replication by the nucleus/cytoplasm ratio in Xenopus.
abstract: The nucleus/cytoplasm (N/C) ratio controls S phase dynamics in many biological systems, most notably the abrupt remodeling of the cell cycle that occurs at the midblastula transition in early Xenopus laevis embryos. After an initial series of rapid cleavage cycles consisting only of S and M phases, a critical N/C ratio is reached, which causes a sharp increase in the length of S phase as the cell cycle is reconfigured to resemble somatic cell cycles. How the N/C ratio determines the length of S phase has been a longstanding problem in developmental biology. Using Xenopus egg extracts, we show that DNA replication at high N/C ratio is restricted by one or more limiting substances. We report here that the protein phosphatase PP2A, in conjunction with its B55α regulatory subunit, becomes limiting for replication origin firing at high N/C ratio, and this in turn leads to reduced origin activation and an increase in the time required to complete S phase. Increasing the levels of PP2A catalytic subunit or B55α experimentally restores rapid DNA synthesis at high N/C ratio. Inversely, reduction of PP2A or B55α levels sharply extends S phase even in low N/C extracts. These results identify PP2A-B55α as a link between DNA replication and N/C ratio in egg extracts and suggest a mechanism that may influence the onset of the midblastula transition in vivo.
authors.affiliation: From the Molecular and Computational Biology Section, Department of Biological Sciences, University of Southern California, Los Angeles, California 90089 and.
authors.forename: Christopher M
authors.initials: CM
authors.lastname: Murphy
authors.forename: W Matthew
authors.initials: WM
authors.lastname: Michael
mesh_terms: D000818:Animals; D064007:Ataxia Telangiectasia Mutated Proteins / Q000378:metabolism; D002457:Cell Extracts; D002467:Cell Nucleus / Q000235:genetics / Q000378:metabolism; D000071877:Checkpoint Kinase 1; D003593:Cytoplasm / Q000235:genetics / Q000378:metabolism; D004261:DNA Replication; D005260:Female; D015151:Immunoblotting; D008297:Male; D010063:Ovum / Q000166:cytology / Q000378:metabolism; D010766:Phosphorylation; D011494:Protein Kinases / Q000378:metabolism; D054648:Protein Phosphatase 2 / Q000235:genetics / Q000378:metabolism*; D021122:Protein Subunits / Q000378:metabolism; D016196:S Phase; D015398:Signal Transduction; D029867:Xenopus Proteins / Q000235:genetics / Q000378:metabolism*; D014982:Xenopus laevis / Q000235:genetics / Q000378:metabolism*
chemical_list: D002457:Cell Extracts; D021122:Protein Subunits; D029867:Xenopus Proteins; D011494:Protein Kinases; C576898:Atr protein, Xenopus; D064007:Ataxia Telangiectasia Mutated Proteins; D000071877:Checkpoint Kinase 1; D054648:Protein Phosphatase 2; C584831:ppp2r2a protein, Xenopus laevis
keywords: B55α; Cell Cycle; Checkpoint Control; Checkpoint Kinase; DNA Replication; Midblastula Transition; Nucleus/Cytoplasm Ratio; PP2A; Xenopus