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的周转状态。蛋白质分解负责基本的细胞功能，如关键酶和调节蛋白水平的调节和异常蛋白的去除。据报道，对于单个蛋白质的半衰期，n端氨基酸是一个重要因素。为了研究n端氨基酸对细胞蛋白分解的调控作用，我们以不同的异-1细胞色素c突变体为模型系统，研究了新合成蛋白中n端加工的规律，并分离了相关的酶。结果表明，引发剂蛋氨酸（Met）在1,29 A或更小的甘氨酸半径的倒数第二残基上被完全去除，而那些新出现的氨基酸（至少甘氨酸、丝氨酸和丙氨酸）根据某些结构特征被乙酰化。此外，在保留的n端蛋氨酸残基中，具有Met-Asp-和Met-Glu序列的蛋白质也被乙酰化。从这些结果可以估计，至少有以下三种酶在蛋白质的n端加工中偶联。第一种酶是甲硫氨酸氨基肽酶（methionine aminopeptidase， MAP），其作用是去除引发剂蛋氨酸。第二种酶和第三种酶是n -乙酰基转移酶，其特异性不同，如上文所述（NAT1：以甘氨酸、丙氨酸和丝氨酸为n端氨基酸；nat2：以蛋氨酸后接酸性残基）。目前正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.