Raijmakers Reinout, Zendman Albert J W, Egberts Wilma Vree, Vossenaar Erik R, Raats Jos, Soede-Huijbregts Claudia, Rutjes Floris P J T, van Veelen Peter A, Drijfhout Jan W, Pruijn Ger J M
Department of Biomolecular Chemistry, Nijmegen Center for Molecular Life Sciences, Institute for Molecules and Materials, Radboud University Nijmegen, Nijmegen, The Netherlands.
J Mol Biol. 2007 Apr 6;367(4):1118-29. doi: 10.1016/j.jmb.2007.01.054. Epub 2007 Jan 25.
Peptidylarginine deiminase (PAD) enzymes catalyze the conversion of arginine residues in proteins to citrulline residues. Citrulline is a non-standard amino acid that is not incorporated in proteins during translation, but can be generated post-translationally by the PAD enzymes. Although the existence of citrulline residues in proteins has been known for a long time, only a few proteins have been reported to contain this amino acid under normal conditions. These include the nuclear histones, which also contain a wide variety of other post-translational modifications, as for instance methylation of arginine residues. It has been suggested that citrullination and methylation of arginine residues are competing processes and that PAD enzymes might "reverse" the methylation of arginine residues by converting monomethylated arginine into citrulline. However, conflicting data have been reported on the capacity of PADs to citrullinate monomethylated peptidylarginine. Using synthetic peptides that contain either arginine or methylated arginine residues, we show that the human PAD2, PAD3 and PAD4 enzymes and PAD enzyme present in several mouse tissues in vitro can only convert non-methylated peptidylarginine into peptidylcitrulline and that hPAD6 does not show any deiminating activity at all. A comparison of bovine histones either treated or untreated with PAD by amino acid analysis also supported the interference of deimination by arginine methylation. Taken together, these data indicate that it is unlikely that methyl groups at the guanidino position of peptidylarginine can be removed by peptidylarginine deiminases, which has important implications for the recently reported role of these enzymes in gene regulation.
肽基精氨酸脱亚氨酶（PAD）催化蛋白质中的精氨酸残基转化为瓜氨酸残基。瓜氨酸是一种非标准氨基酸，在翻译过程中不掺入蛋白质，但可在翻译后由PAD酶生成。虽然蛋白质中瓜氨酸残基的存在早已为人所知，但据报道，在正常条件下只有少数蛋白质含有这种氨基酸。这些蛋白质包括核组蛋白，其还含有多种其他翻译后修饰，例如精氨酸残基的甲基化。有人提出，精氨酸残基的瓜氨酸化和甲基化是相互竞争的过程，并且PAD酶可能通过将单甲基化精氨酸转化为瓜氨酸来“逆转”精氨酸残基的甲基化。然而，关于PAD使单甲基化肽基精氨酸瓜氨酸化的能力，已报道了相互矛盾的数据。使用含有精氨酸或甲基化精氨酸残基的合成肽，我们表明，人PAD2、PAD3和PAD4酶以及存在于几种小鼠组织中的PAD酶在体外只能将未甲基化的肽基精氨酸转化为肽基瓜氨酸，而hPAD6根本没有显示出任何脱亚胺活性。通过氨基酸分析对经PAD处理或未处理的牛组蛋白进行比较，也支持了精氨酸甲基化对脱亚胺作用的干扰。综上所述，这些数据表明，肽基精氨酸脱亚氨酶不太可能去除肽基精氨酸胍基位置的甲基，这对这些酶最近报道的在基因调控中的作用具有重要意义。
