Early Nuclear Events in the Induction of Lymphocyte Proliferation by Mitogens

Abstract

Abstract The mitogen concanavalin A (Con A) initiates a series of alterations of chromatin proteins early after addition to cultures of purified equine lymphocytes. Within 15 min of addition, Con A induces cellular non-histone proteins to bind to the chromatin and stimulates the phosphorylation of specific nuclear acidic proteins. The rate of phosphorylation of phenol-soluble nuclear acidic proteins increases as much as 4-fold within 2 hours of Con A stimulation. Phosphorylation is maximal 8 hours after Con A addition. The amount of phenol-soluble non-histone protein associated with the DNA increases as much as 4-fold within 6 hours of Con A stimulation. Con A also stimulates the synthesis of nuclear acidic proteins. Cycloheximide inhibits nuclear acidic protein synthesis by more than 90% but does not appreciably diminish the Con A-induced increase in the amount of these proteins entering the nucleus. Prelabeling of lymphocytes with [14C]leucine followed by treatment with Con A in unlabeled medium results in increased labeling of nuclear acidic proteins, indicating that presynthesized proteins are induced to bind to the chromatin in the presence of Con A. Experiments performed with lymphocytes fractionated by nonaqueous procedures indicate that proteins associating with the chromatin in response to Con A are largely of cytoplasmic origin. These results suggest that both a flux of specific proteins from the cytoplasm to the nucleus and an increase in phosphorylation of nuclear acidic proteins precede the extensive gene activation promoted by mitogens.