Subcellular location of a soluble factor that stimulates DNA replication in permeable animal cells

The subcellular location of a soluble DNA replication-stimulating factor released from lysolecithin-permeabilized cultured mammalian cells was investigated by cytochalasin B (CB) enucleation. In both Chinese hamster ovary cells and baby hamster kidney (BHK) cells, approx. 70% of the total activity of the replication-stimulating factor was associated with the karyoplast fraction. In neither cell line did the replication-stimulating factor show any significant change in activity or in subcellular location between G1 and S period synchronized cells. The general nature and mechanism of action of this stimulatory factor were characterized in order to show that it was similar to "cytoplasmic" factors reported to stimulate DNA replication in other subcellular systems. The predominantly nuclear location of the replication-stimulating factor supports a physiological role in DNA metabolism in animal cells.