Y. Okuno et al., Stability, chromatin association and functional activity of mammalian pre-replication complex proteins during the cell cycle, EMBO J, 20(15), 2001, pp. 4263-4277
We have examined the behavior of pre-replication complex (pre-RC) proteins
in relation to key cell cycle transitions in Chinese Hamster Ovary (CHO) ce
lls. ORC1, ORC4 and Cdc6 were stable (T-1/2 >2 h) and associated with a chr
omatin-containing fraction throughout the cell cycle. Green fluorescent pro
tein-tagged ORC1 associated with chromatin throughout mitosis in living cel
ls and co-localized with ORC4 in metaphase spreads. Association of Mcm prot
eins with chromatin took place during telophase, similar to 30 min after th
e destruction of geminin and cyclins A and B, and was coincident with the l
icensing of chromatin to replicate in geminin-supplemented Xenopus egg extr
acts. Neither Mcm recruitment nor licensing required protein synthesis thro
ughout mitosis. Moreover, licensing could be uncoupled from origin specific
ation in geminin-supplemented extracts; site-specific initiation within the
dihydrofolate reductase locus required nuclei from cells that had passed t
hrough the origin decision point (ODP). These results demonstrate that mamm
alian pre-RC assembly takes place during telophase, mediated by post-transl
ational modifications of pre-existing proteins, and is not sufficient to se
lect specific origin sites. A subsequent, as yet undefined, step selects wh
ich pre-RCs will function as replication origins.