Mitogen-induced preferential synthesis of proteins during the G0 to S phase transition in human lymphocytes

We have followed the induction of protein synthesis in mitogen-activated human peripheral blood mononuclear cells during the transition from quiescence, or G0, through the prereplicative phase and into first S phase. Doses of mitogens optimal for proliferative response preferentially enhance the synthesis of a subset of intracellular proteins during the approximately 24-h lag interval. The mitogenic lectin phytohemagglutinin (PHA) and OKT3, a mitogenic monoclonal antibody to the CD3 component of the T cell antigen receptor, preferentially enhance bands of the same molecular weight in one-dimensional SDS-PAGE. The proteins are low detergent soluble (0.1% Triton X-100) "cytoplasmic" cellular components and some have been identified as single spots on two-dimensional gels. Bands of 51 and 66 kDa are induced early in lag phase (4 h after stimulation) but are transiently synthesized, decreasing later in lag phase. The majority of the mitogen-induced proteins, 39, 51, 55, 60, 73, and 95 kDa are enhanced by mid lag phase (12 h after stimulation). With the exception of the 55-kDa band, five of these proteins are clearly enhanced in T cells purified after mitogen stimulation. The same five bands show sustained synthesis in actively cycling cells 42-48 h after stimulation and are major synthesized proteins, and corresponding bands are synthesized in a transformed T cell line, MOLT-4. Two of the proteins in this group that are most prominently synthesized during the lag interval have been previously identified as the heat shock proteins, HSP 90 (95-kDa band) and HSC 70 (73-kDa band). We speculate that this group of five proteins, including HSP 90 and HSC 70, may be coordinately expressed in actively replicating T cells and may have some common structural or functional role in sustaining the replicative state.