Metabolic behaviour of nonhistone chromosomal proteins in proliferating and in resting fibroblasts.

The metabolism of nonhistone chromosomal proteins was studied in two lines of cells showing a different degree of contact inhibition: human diploid fibroblasts, which are easily contact-inhibited, and Chinese hamster fibroblasts, which had been made to stop proliferating by fasting. By following the 3H414C ratio of [3H]tryptophan-labelled nonhistone chromosomal proteins and [14C]thymidine-labelled DNA in chase experiments three main groups of these proteins could be detected with respect to their metabolic behaviour: (a) a metabolically stable group which is acid-insoluble and represents the bulk of nonhistone chromosomal proteins in proliferating cells; this group is conserved when the cells enter a resting phase; (b) a metabolically labile group which is acid-soluble and is observed as a minor fraction in proliferating cells; (c) a metabolically labile group which is acid-insoluble and accumulates in resting cells; this fraction is much larger in contact-inhibited cells. Stimulation of cell proliferation by trypsinization decreases the amount of nonhistone chromosomal proteins in resting cells to the basic level observed in proliferating cells.