AN intact nuclear membrane restricts DNA replication to only one round
in each cell cycle, apparently by excluding an essential replication-
licensing factor throughout interphase(1-5). A family of related yeast
replication proteins, MCM2, 3 and 5 (also called, after cell-division
cycle, CDC46), resemble licensing factor, entering the nucleus only d
uring mitosis(6-8). We have cloned a Xenopus homologue of MCM3 (XMCM3)
and raised antibodies against expressed protein, Immunodepletion of X
enopus egg extracts removes a complex of MCM2, 3 and 5 homologues and
inhibits replication of Xenopus sperm nuclei or permeable G2 HeLa nucl
ei. However, G1 HeLa nuclei still replicate efficiently. Mock-depleted
extracts replicate all three templates. XMCM3 accumulates in nuclei b
efore replication but anti-XMCM3 staining decreases during replication
. These results can explain why replicated nuclei are unable to reinit
iate replication in a single cell cycle.