Callis J, Bedinger P
Section of Molecular and Cellular Biology, University of California, Davis 95616.
Proc Natl Acad Sci U S A. 1994 Jun 21;91(13):6074-7. doi: 10.1073/pnas.91.13.6074.
Eukaryotic cells typically contain 0.2-1.0% of their total protein as the highly conserved protein ubiquitin, which exists both free and covalently attached to cellular proteins. The attachment of ubiquitin to cellular proteins occurs posttranslationally by a three-enzyme pathway and results in a peptide linkage of the C terminus of ubiquitin either to a lysyl epsilon-amino group of a substrate protein or to a lysyl epsilon-amino group of a previously linked ubiquitin molecule. The multiple conjugation of ubiquitin to substrate proteins via ubiquitin-ubiquitin linkages is thought to be necessary, but not sufficient, for recognition and degradation by a ubiquitin-dependent protease. In higher plant cells the steady-state level of ubiquitinated proteins is generally constant and can be readily detected in all somatic tissues. In contrast, we have found that a developmentally regulated loss of free ubiquitin and ubiquitinated proteins occurs during maize (Zea mays L.) pollen maturation. This dramatic loss of ubiquitin correlates temporally with commitment to the gametophytic developmental program. Northern blot analysis indicates that the loss of ubiquitin is not due to low levels of ubiquitin mRNA, suggesting that a posttranscriptional regulatory mechanism is responsible.
真核细胞通常含有其总蛋白的0.2 - 1.0%作为高度保守的泛素蛋白，该蛋白既以游离形式存在，也以共价连接到细胞蛋白上的形式存在。泛素与细胞蛋白的连接在翻译后通过一种三酶途径发生，导致泛素C末端与底物蛋白的赖氨酸ε-氨基或与先前连接的泛素分子的赖氨酸ε-氨基形成肽键。泛素通过泛素-泛素连接与底物蛋白的多次缀合被认为对于泛素依赖性蛋白酶的识别和降解是必要的，但不是充分的。在高等植物细胞中，泛素化蛋白的稳态水平通常是恒定的，并且在所有体细胞组织中都能很容易地检测到。相比之下，我们发现玉米（Zea mays L.）花粉成熟过程中会发生游离泛素和泛素化蛋白的发育调控性损失。这种泛素的显著损失在时间上与配子体发育程序的启动相关。Northern印迹分析表明泛素的损失不是由于泛素mRNA水平低，这表明转录后调控机制起了作用。
