Barrier-to-autointegration factor (BAF) is a host cell protein that plays a
crucial role in retroviral integration. Preintegration complexes (PICs) st
ripped of BAF lose their normal integration activity, which can be restored
by incubation with purified BAF. BAF bridges double-stranded DNA both intr
a- and intermolecularly in a non-sequence-specific manner, leading to the f
ormation of a nucleoprotein network. BAF also binds to the nuclear protein
lamina-associated polypeptide 2 (LAP2), and is localized with chromatin dur
ing interphase and mitosis. The crystal structure of homodimeric human BAF
has been determined to 1.9 Angstrom resolution. The fold of the BAF monomer
resembles that of the second domain of RuvA. This comparison revealed the
presence of the helix-hairpin-helix (HhH) nonspecific DNA binding motif wit
hin BAF. A novel feature of BAF's HhH motif is the occupation of the metal
binding site by the epsilon-amino group of Lys 6, providing an alternative
means of sequestering positive charge. Mutational analysis corroborates the
HhH motif's prominent role in DNA binding and argues against a previously
proposed helix-turn-helix (HTH) binding site located in another region of t
he monomer. A model of BAF bridging DNA via the HhH motif is proposed.