The M2-type isoenzyme of pyruvate kinase phosphorylates prothymosin alpha in proliferating lymphocytes.

摘要

Prothymosin alpha (ProTalpha) is a multifunctional protein that, in mammalian cells, is involved in nuclear metabolism through its interaction with histones and that also has a cytosolic role as an apoptotic inhibitor. ProTalpha is phosphorylated by a protein kinase (ProTalphaK), the activity of which is dependent on phosphorylation. ProTalpha phosphorylation also correlates with cell proliferation. Mass spectrometric analysis of ProTalphaK purified from human tumor lymphocytes (NC37 cells) enabled us to identify this enzyme as the M2-type isoenzyme of pyruvate kinase. A study on the relationship between ProTalphaK activity and pyruvate kinase isoforms in NC37 cells and in other cell types confirmed that the M2 isoform is the enzyme responsible for ProTalphaK activity in proliferating cells. Yet, about 10% of the cellular pool of the M2 isoform shows specific affinity for ProTalpha and is responsible for ProTalphaK activity. This pool of M2 protein possesses no observable pyruvate kinase activity and changes its responses to various effectors of pyruvate kinase activity; however, these responses to PK effectors are maintained by the main cellular fraction containing the M2 isoform. Acquisition of ProTalphaK activity by M2 seems to be due to the phosphorylation of serine and threonine residues, which, besides being essential for its catalytic activity, induces a trimeric association of ProTalphaK. This association can be shifted to a tetrameric form by fructose 1, 6-bisphosphate, which results in a decrease in ProTalphaK activity.