Protein acetylation is important in regulating DNA-templated processes specifically and protein-protein interactions more generally in eukaryotes. The geminivirus movement protein NSP is essential for virus movement, shuttling the viral DNA genome between the nucleus and the cytoplasm. We have identified a novel Arabidopsis protein, AtNSI, that interacts with NSP. AtNSI is highly conserved among widely divergent plants. Biochemical studies show that its interaction with NSP is direct and that AtNSI acetylates histones, but not NSP, in vitro. Rather, AtNSI specifically acetylates the viral coat protein. AtNSI is a nuclear protein but does not act as a transcriptional coactivator in vitro, which distinguishes it from known eukaryotic histone acetyltransferases. Its overexpression enhances the efficiency of infection by Cabbage leaf curl virus. These findings suggest a role for protein acetylation in coordinating replication of the viral DNA genome with its export from the nucleus.