Role of the Processing at Amino-Terminus on the Regulation of Half-life of Protein

很大一部分细胞蛋白质处于动态周转状态。蛋白质分解负责细胞的基本功能，如关键酶和调节蛋白水平的调节以及异常蛋白质的去除。对于单个蛋白质的半衰期来说，N-末端氨基酸是一个重要的因素。为了研究N-末端氨基酸在调节细胞蛋白质分解中的作用，我们以N-末端改变的各种iso-1-细胞色素c突变体为模型系统，研究了在新合成的蛋白质中观察到的N-末端加工规律，并分离了相关的酶。结果表明，引发剂蛋氨酸(Met)从倒数第二个残基上完全去掉了，其甲基化半径在1，29 A或更小，而那些新出现的氨基酸至少是甘氨酸，丝氨酸和丙氨酸是乙酰化的，取决于某些结构特征。此外，在保留的N-末端蛋氨酸残基中，同时具有Met-Asp-和Met-Glu序列的蛋白质也被乙酰化。从这些结果可以推测，在蛋白质的N-末端加工过程中，至少有以下三种酶是偶联的。第一种酶是蛋氨酸氨基肽酶(MAP)，作用于去除引发剂蛋氨酸，第二种和第三种是N-乙酰基转移酶，其特性如上所述(NAT1：甘氨酸、丙氨酸和丝氨酸为N-末端氨基酸；Nat2：甲硫氨酸和酸性残基)。目前正在以酿酒酵母为原料进行这三种酶的分离。

A large part of cellular proteins are in a dynamic state of turnover. Protein breakdown is responsible for essential cellular functions such as the modulation of the levels of key enzymes and regulatory proteins and removal of abnormal proteins. For the half-lives of individual proteins it has been reported that N-terminal amino acid is an important factor.To study for the role of N-terminal amino acid on regulation for breakdown of cellular proteins, we have investigated to elucidate the rules of N-terminal processing observed in newly synthesized proteins by using various iso-1-cytochrome c mutants altered at their N-terminal region as a model system, and to isolate the related enzymes.The results suggest that initiator methionine (Met) is completely removed from penultimate residue having radii of glynation on 1, 29 A or less, and that of those newly, appeared amino acids at least glycine, serine and alanine are acetylated depending on some structural characterization. Furthermore, of the retained N-terminal methionine residues, the proteins having both Met-Asp- and Met-Glu sequences are also acetylated. From these results it has been thus estimated that at least the following three enzymes are conjugated in N-terminal processing of proteins. The first enzyme is a methionine aminopeptidase (MAP), which acts on removal of initiator methionine, and the second and the third are N-acetyltransferases with different specificities as suggested above (NAT1 : for glycine, alanine and serine as the N-terminal amino acid ; nat2 : for methionine followed by acidic residues). The isolation of these three enzymes are now undergoing using saccharomyces cerevisiae as the materials.