Cdc6 protein causes premature entry into S phase in a mammalian cell-free system

We exploit an improved mammalian cell-free DNA replication system to analys e quiescence and Cdc6 function. Quiescent 3T3 nuclei cannot initiate replic ation in S phase cytosol from HeLa or 3T3 cells. Following release from qui escence, nuclei become competent to initiate semiconservative DNA replicati on in S phase cytosol, but not in G(0) phase cytosol. Immunoblots show that quiescent cells lack Cdc6 and that minichromosome maintenance (MCM) protei ns are not associated with chromatin. Competence of G(1) phase nuclei to re plicate in vitro coincides with maximum Cdc6 accumulation and MCM protein b inding to chromatin in vivo. Addition of recombinant Cdc6 to permeabilized, but not intact, G(1) nuclei causes up to 82% of the nuclei to initiate and accelerates G(1) progression, making nuclei competent to replicate prematu rely.