Fink Annette, Blaum Franziska, Babic Cac Marina, Ebert Stefan, Lemmermann Niels A W, Reddehase Matthias J
Institute for Virology, University Medical Center of the Johannes Gutenberg-University Mainz and Research Center for Immunotherapy (FZI), Obere Zahlbacher Strasse 67, Hochhaus am Augustusplatz, 55131, Mainz, Germany,
Med Microbiol Immunol. 2015 Jun;204(3):383-94. doi: 10.1007/s00430-015-0414-1. Epub 2015 Apr 8.
Viruses have evolved proteins that bind immunologically relevant cargo molecules at the cell surface for their downmodulation by internalization. Via a tyrosine-based sorting motif YXXΦ in their cytoplasmic tails, they link the bound cargo to the cellular adapter protein-2 (AP2), thereby sorting it into clathrin-triskelion-coated pits for accelerated endocytosis. Downmodulation of CD4 molecules by lentiviral protein NEF represents the most prominent example. Based on connecting cargo to cellular adapter molecules, such specialized viral proteins have been referred to as 'connectors' or 'adapter adapters.' Murine cytomegalovirus glycoprotein m04/gp34 binds stably to MHC class-I (MHC-I) molecules and suspiciously carries a canonical YXXΦ endocytosis motif YRRF in its cytoplasmic tail. Disconnection from AP2 by motif mutation ARRF should retain m04-MHC-I complexes at the cell surface and result in an enhanced silencing of natural killer (NK) cells, which recognize them via inhibitory receptors. We have tested this prediction with a recombinant virus in which the AP2 motif is selectively destroyed by point mutation Y248A, and compared this with the deletion of the complete protein in a Δm04 mutant. Phenotypes were antithetical in that loss of AP2-binding enhanced NK cell silencing, whereas absence of m04-MHC-I released them from silencing. We thus conclude that AP2-binding antagonizes NK cell silencing by enhancing endocytosis of the inhibitory ligand m04-MHC-I. Based on a screen for tyrosine-based endocytic motifs in cytoplasmic tail sequences, we propose here the new hypothesis that most proteins of the m02-m16 gene family serve as 'adapter adapters,' each selecting its specific cell surface cargo for clathrin-assisted internalization.
病毒已经进化出能在细胞表面结合免疫相关货物分子的蛋白质,以便通过内化作用对其进行下调。它们通过细胞质尾巴中基于酪氨酸的分选基序YXXΦ,将结合的货物与细胞衔接蛋白2(AP2)相连,从而将其分选到网格蛋白三脚蛋白复合体包被的小窝中,加速内吞作用。慢病毒蛋白NEF对CD4分子的下调是最突出的例子。基于将货物与细胞衔接分子相连,这类特殊的病毒蛋白被称为“衔接器”或“适配衔接器”。小鼠巨细胞病毒糖蛋白m04/gp34能稳定结合主要组织相容性复合体I类(MHC-I)分子,并且在其细胞质尾巴中疑似带有一个典型的YXXΦ内吞基序YRRF。通过基序突变ARRF与AP2断开连接,应该能使m04-MHC-I复合体保留在细胞表面,并导致自然杀伤(NK)细胞的沉默增强,NK细胞通过抑制性受体识别这些复合体。我们用一种重组病毒验证了这一预测,在该重组病毒中,AP2基序通过点突变Y248A被选择性破坏,并将其与Δm04突变体中完整蛋白的缺失进行比较。两者的表型相反,即AP2结合的丧失增强了NK细胞的沉默,而m04-MHC-I的缺失则使其从沉默中释放出来。因此我们得出结论,AP2结合通过增强抑制性配体m04-MHC-I的内吞作用来拮抗NK细胞的沉默。基于对细胞质尾巴序列中基于酪氨酸的内吞基序的筛选,我们在此提出一个新假说,即m02-m16基因家族的大多数蛋白都作为“适配衔接器”,各自选择其特定的细胞表面货物进行网格蛋白辅助的内化作用。