Amino-terminal processing of intracellular proteins and its biological functions

蛋白质的氨基末端加工是非常常见的事件。在真核生物中，蛋白质的翻译起始于甲硫氨酸（Met），并且超过一半的甲硫氨酸残基被切割。随后，60-80%的细胞内蛋白质被乙酰化封闭氨基末端。在这个项目中，我试图阐明与这种氨基末端加工相关的酶系统，并进一步估计其生物学功能。作为第一个试验，我发现至少有三种酶参与了酵母，酿酒酵母的事件。一种是甲硫氨酸氨肽酶（MAP），其在新生多肽链完成之前特异性地从某些蛋白质中去除Met^i，另一种是具有不同特异性的两种N-乙酰转移酶，Nat 1和Nat 2。Nat 1作用于具有甘氨酸、丙氨酸、丝氨酸、苏氨酸、脯氨酸和缬氨酸的蛋白质的乙酰化，而Nat 2仅作用于Met-Asp/-Glu/-Asn序列的氨基末端乙酰化。进一步的研究已经阐明了每种酶的一些分子特性。对于氨基末端加工的生物学功能，认为该事件倾向于将氨基末端残基转化为细胞内泛素依赖的蛋白质降解系统中呈现的N-末端规则中的稳定残基。

Amino-terminal processing of proteins is a very common event. In eukaryotes, translation of proteins is initiated with methionine (Met^i), and more than half of the methionine residue jis cleaved. Subsequently, 60-80% of the intracellular proteins are amino-terminally blocked by acetylation. In this project, I have tried to elucidate the enzyme(s) system related to such amino-terminal processing and further to estimate its biological functions. As the first trial, I have found at least three enzymes are engaged in the event in yeast, Saccharomyces cerevisiae. One is a methionine aminopeptidase (MAP), which specifically removes Met^i from certain proteins before the completion of nascent polypeptide chains, and the others are two N-acetyltransfeases with diffrent specificity, Nat1 and Nat2. Nat1 acts on acetylation for proteins having glycine, alanine, serine, threonine, proline and valine, whereas Nat2 acts only on amino-terminal acetylation for Met-Asp/-Glu/-Asn sequence. Further studies have clarified some molecular properties for each enzymes. For the biological functions of amino-terminal processing, it is suggested that the event tends to convert amino-terminal residues to stabilizing residues in N-end rule presented on the ubiquitin dependent protein degradation sysytem in cell.