The mutations in PFV-1 IN do not alter its karyophilic property . (A) Schematic representation of foamy virus IN showing conserved motifs and residues between retroviral INs (IN-WT). Critical amino acid residues were mutated as indicated: M5 was mutated within the HH-CC zinc finger domain. In the M8 virus, Asp160 in the invariant conserved catalytic triad, was changed to a glycine residue. Such a mutation has been shown to impair PFV-1 replication , likely due to a defective catalytic activity of the protein, as reported in HIV . Another mutation was introduced at Ile106 in the M9 mutant, since this mutation had been described to abolish the in vitro integration activity of the protein [24, 27]. (B) Confocal microscopy analysis of WT PFV-1 IN and of mutants M5, M8, M9 IN. HeLa cells were transfected with plasmids expressing the WT or mutant IN, fused to the Flag epitope. After 36 hours, cells were fixed, permeabilized, and stained with anti-Flag-antibodies. Series of optical sections at 0.7-μm intervals were recorded. One representative medial section of the immunofluorescence staining is shown.