title: An open conformation of switch I revealed by Sar1-GDP crystal structure at low Mg2+.
abstract: Mg2+ is essential for guanosine triphosphatase activity and plays key roles in guanine nucleotide binding and preserving the structural integrity of GTP-binding proteins. To understand the structural basis for Mg2+ function during the GDP/GTP exchange process, we determined the crystal structure of Delta9-Sar1-GDP at low Mg2+ concentration at 1.8A. Two Sar1-GDP molecules in the crystal form a dimer with Mg2+ presenting only in molecule B but not in molecule A. The absence of Mg2+ induces significant conformational changes in the switch I region in molecule A that shows similarities with those of Ha-Ras bound to Sos. The current structure reveals an important regulatory role for Mg2+. We suggest that guanine nucleotide exchange factor may utilize this feature to generate an open conformation for GDP/GTP exchange. Furthermore, we propose a mechanism for COPII assembly and disassembly in which dimerization of Sar1 plays an important role.
authors.affiliation: State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, 155 Yang Qiao Xi Lu, Fuzhou 350002, People's Republic of China.
authors.forename: Yijian
authors.initials: Y
authors.lastname: Rao
authors.forename: Chuanbing
authors.initials: C
authors.lastname: Bian
authors.forename: Cai
authors.initials: C
authors.lastname: Yuan
authors.forename: Yongdong
authors.initials: Y
authors.lastname: Li
authors.forename: Liqing
authors.initials: L
authors.lastname: Chen
authors.forename: Xiaoming
authors.initials: X
authors.lastname: Ye
authors.forename: Zixiang
authors.initials: Z
authors.lastname: Huang
authors.forename: Mingdong
authors.initials: M
authors.lastname: Huang
mesh_terms: D000818:Animals; D022181:COP-Coated Vesicles / Q000378:metabolism; D006224:Cricetinae; D003412:Cricetulus; D003460:Crystallization; D018360:Crystallography, X-Ray; D019281:Dimerization; D006153:Guanosine Diphosphate / Q000737:chemistry* / Q000378:metabolism; D008274:Magnesium / Q000737:chemistry* / Q000502:physiology; D011487:Protein Conformation; D017510:Protein Folding; D011994:Recombinant Proteins / Q000737:chemistry / Q000378:metabolism; D020663:ras Guanine Nucleotide Exchange Factors / Q000737:chemistry* / Q000502:physiology; D018631:ras Proteins / Q000737:chemistry* / Q000502:physiology
chemical_list: D011994:Recombinant Proteins; D020663:ras Guanine Nucleotide Exchange Factors; D006153:Guanosine Diphosphate; D018631:ras Proteins; D008274:Magnesium