Kauffman M G, Kelly T J
Department of Molecular Biology and Genetics, Johns Hopkins Medical School, Baltimore, Maryland 21205.
Mol Cell Biol. 1991 May;11(5):2538-46. doi: 10.1128/mcb.11.5.2538-2546.1991.
The level of human thymidine kinase (TK) polypeptide is subject to cell cycle regulation. The enzyme is barely detectable in G1 phase but increases 10- to 20-fold by M phase. The low level of human TK in G1 phase is due primarily to the specific degradation of the protein during cell division. Substitution of heterologous promoters, removal of the introns, and deletion of all of the 3' untranslated region from the human TK gene do not affect cell cycle regulation of the enzyme. However, deletion of the carboxyl-terminal 40 amino acids or fusion of beta-galactosidase to the carboxyl terminus of human TK completely abolishes cell cycle regulation and stabilizes the protein throughout the cell cycle. These alterations do not significantly alter the specific enzymatic activity of TK. Changing the carboxyl terminus or deletion of the last 10 amino acids does not alter cell cycle regulation. These data demonstrate that residues near the carboxyl terminus of TK are essential for the cell cycle phase-specific degradation of the enzyme.
人类胸苷激酶（TK）多肽的水平受细胞周期调控。在G1期几乎检测不到该酶，但到M期时其水平会增加10到20倍。G1期人类TK水平较低主要是由于细胞分裂过程中该蛋白质的特异性降解。用异源启动子替代、去除内含子以及从人类TK基因中删除所有3'非翻译区，均不影响该酶的细胞周期调控。然而，删除羧基末端的40个氨基酸或将β-半乳糖苷酶融合到人类TK的羧基末端会完全消除细胞周期调控，并使该蛋白质在整个细胞周期中保持稳定。这些改变不会显著改变TK的比酶活性。改变羧基末端或删除最后10个氨基酸不会改变细胞周期调控。这些数据表明，TK羧基末端附近的残基对于该酶的细胞周期阶段特异性降解至关重要。
