Mancias Joseph D, Goldberg Jonathan
Howard Hughes Medical Institute and the Structural Biology Program, Memorial Sloan-Kettering Cancer Center, New York, NY 10065, USA.
EMBO J. 2008 Nov 5;27(21):2918-28. doi: 10.1038/emboj.2008.208. Epub 2008 Oct 9.
Genomic analysis shows that the increased complexity of trafficking pathways in mammalian cells involves an expansion of the number of SNARE, Rab and COP proteins. Thus, the human genome encodes four forms of Sec24, the cargo selection subunit of the COPII vesicular coat, and this is proposed to increase the range of cargo accommodated by human COPII-coated vesicles. In this study, we combined X-ray crystallographic and biochemical analysis with functional assays of cargo packaging into COPII vesicles to establish molecular mechanisms for cargo discrimination by human Sec24 subunits. A conserved IxM packaging signal binds in a surface groove of Sec24c and Sec24d, but the groove is occluded in the Sec24a and Sec24b subunits. Conversely, LxxLE class transport signals and the DxE signal of VSV glycoprotein are selectively bound by Sec24a and Sec24b subunits. A comparative analysis of crystal structures of the four human Sec24 isoforms establishes the structural determinants for discrimination among these transport signals, and provides a framework to understand how an expansion of coat subunits extends the range of cargo proteins packaged into COPII-coated vesicles.
基因组分析表明，哺乳动物细胞中运输途径复杂性的增加涉及SNARE、Rab和COP蛋白数量的扩充。因此，人类基因组编码了四种形式的Sec24，即COPII囊泡衣被的货物选择亚基，据推测这增加了人类COPII包被囊泡所容纳货物的范围。在本研究中，我们将X射线晶体学和生化分析与货物包装到COPII囊泡中的功能测定相结合，以建立人类Sec24亚基区分货物的分子机制。一个保守的IxM包装信号结合在Sec24c和Sec24d的表面凹槽中，但该凹槽在Sec24a和Sec24b亚基中被封闭。相反，LxxLE类运输信号和VSV糖蛋白的DxE信号被Sec24a和Sec24b亚基选择性结合。对四种人类Sec24异构体晶体结构的比较分析确定了区分这些运输信号的结构决定因素，并提供了一个框架，以理解衣被亚基的扩充如何扩展包装到COPII包被囊泡中的货物蛋白范围。
