APOBEC3G（A3G）是胞嘧啶脱氨基化酶家族APOBEC3成员，含N-端A3G-CD1结构域和C-端A3G-CD2结构域。A3G-CD1介导HIV病毒壳体化。A3G-CD2负责催化病毒单链DNA胞嘧啶脱氨基化，诱导G→A超突变，使HIV基因组不能执行其功能，抑制病毒复制。A3G-CD2催化胞嘧啶脱氨基化一般发生在病毒DNA负链含5’-CCC-3’序列的3’端胞嘧啶上，通过由3’→5’跳跃或滑动来持续发挥该功能。自由态A3G-CD2结构已有报道，但催化脱氨基化反应较快，难以捕获A3G与DNA瞬态复合体；故A3G识别5’-CCC-3’并进行脱氨基化反应的机制没有文献报道。本项目计划在活性位点碱基胞嘧啶芳环5-位引入供电子基团，阻碍脱氨基化发生，得到A3G-CD2与DNA稳定复合物，通过NMR技术解析其结构，阐述A3G-CD2催化胞嘧啶脱氨基化分子机制, 开展A3G抗HIV活性机制研究。

APOBEC3G (A3G) is a member of the family of deoxycytidine deaminases APOBEC, which can convert dC residues in single-stranded DNA (ssDNA) to dU, and act as DNA mutators. APOBEC3G contains two active domains, A3G-CD1 at its N-terminus and A3G-CD2 at its C-terminus. A3G-CD1 mediates the interactions of A3G with Vif protein of the virus. A3G-CD2 catalyzes cytidine deamination, induces G-to-A hypermutation in double-stranded DNA, impairs biological function of HIV-1 virus genes, and suppresses virus replication. The deamination site of A3G-CD2 locates at the cytidine at 3’-terminus of the hot spot 5’-CCC-3’ in the antisense ssDNA of virus gene, and is performed by jumping or sliding through 3’->5’ direction. The NMR or X-ray structures of A3G-CD2 in its free state have been reported, however, due to the fast deamination, it’s difficult to capture the stable complex of A3G-CD2 with ssDNA, thus how A3G-CD2 specifically recognizes the hot spot 5’-CCC-3’ in ssDNA and performs deamination reactions on cytidine is still unclear. In the project, we plan to do chemical modification on cytidine by inducing a electron donor group at 5-position of its aromatic ring, to hinder the deamination reaction, and to get the stable complex of A3G-CD2 with ssDNA containing hot motif. Then we will try to determine the NMR solution structure of A3G-CD2 in complex with the ssDNA, to explain the molecular basis of how A3G-CD2 performs its deamination reaction on virus ssDNA, to present the structural basis of A3G’s anti-virus activities.