An investigation of acid-soluble nuclear proteins of human leucocytes in relation to fraction RP2-L, a component of neoplastic cells.

1. The claim that tumour cells contain a specific nuclear protein was investigated. The presence of this component was confirmed in Walker tumour cells by the chromatography on CM-cellulose of nuclear proteins labelled with [(14)C]lysine. This protein was studied further in a number of human leucocyte cells. 2. The labelling of leucocyte nuclear proteins with [(14)C]lysine was attempted during incubation and culture in vitro. Incorporation of the label into acid-soluble nuclear proteins was highest in normal lymphocytes cultured with phytohaemagglutinin, followed by chronic-myeloid-leukaemic leucocytes and mixed samples of normal leucocytes incubated in plasma. Little incorporation was seen in similar extracts of chronic-lymphatic or normal leucocytes. 3. Lymphocytes were the only cells that gave nuclear extracts with amino acid analysis similar to that of unfractionated histones. 4. Little of the [(14)C]lysine in nuclear extracts of incubated leucocytes proved to be of chromosomal origin. No evidence was found of an RP2-L component in the highly labelled nuclear extracts of phytohaemagglutinin-treated lymphocytes until after 6 days of culture with [(14)C]lysine. This component was soluble in saline. 5. Evidence is presented that fraction RP2-L is a non-histone protein constituent of cell nuclei whose labelling with [(14)C]lysine may be dependent on the metabolic state of the cell. Thus this component is not specific to the neoplastic state.