IDENTIFICATION OF A DNA-BINDING DOMAIN IN SIMIAN VIRUS-40 CAPSID PROTEINS VP2 AND VP3

We have identified both biochemically and genetically a protein domain within the simian virus 40 virion protein Vp3, and within Vp2 since i ts carboxyl two-thirds are identical to the full-length Vp3, that bind s DNA in a sequence nonspecific manner. Both the Vp2 and Vp3 (Vp2/3) c omponents of SV40 and mutant SV40(202T) bound either SV40 or pBR322 DN A equally well. Wild type and mutant Vp2/3 proteins, expressed as fusi on proteins with glutathione S-transferase (GST), were tested for thei r ability to bind DNA. GST-Vp3 bound DNA at physiological salt concent rations with an apparent K(d) of 2.5 x 10(-8) M and also bound RNA wit h 4-fold higher affinity. Over 90% of the nucleic acid binding, and al l of the activity, was lost upon removal of the carboxyl-terminal 13 a nd 35 residues, respectively. The DNA binding domain was shown to be d istinct and separable from the Vp2/3 nuclear transport signal since mu tations within the nuclear transport signal that reduce or abolish nuc lear localization of Vp2/3 had no effect on the DNA binding activity o f mutant Vp2/3 fusion proteins. The carboxyl-terminal 40 residues of V p2/3 in the form of a beta-galactosidase fusion protein, F6, are suffi cient for DNA binding and may cause compaction of the DNA. The signifi cance of this DNA binding and possible compaction are discussed in rel ation to the assembly of virion particles.