No tRNA3Lys unwinding in a complex with HIV NCp7

The nucleocapsid protein NCp7 of the human immunodeficiency virus (HIV) type 1 is important for the annealing of HIV RNA and tRNA3Lys, the tRNA acting as a primer during reverse transcription of HIV RNA. A wild type NCp7 and a Cys23 mutant having a disrupted zinc finger were analyzed with far UV circular dichroism (CD). CD data analysis revealed that NCp7 has a high content of extended structures in aqueous buffer, decreasing in Cys23 NCp7 and in NCp7 in the absence of zinc. An increase in beta-turn structures is observed in NCp7 bound to tRNA3Lys. Furthermore, CD data shows that Cys23 NCp7 binds tRNA3Lys. The CD spectrum of tRNA3Lys is typical of an A-form helix and retains this structure after binding of NCp7, which demonstrates that NCp7 does not induce tRNA3Lys unwinding. CD spectra of tRNA3Lys were measured from 5 to 80 degrees C to observe CD changes resulting from tRNA3Lys melting. Molecular modeling of the complex identifies two potential tRNA anticodon binding sites in the NCp7 N-terminal region and first zinc finger. In this model, both binding sites can interact with 12 nucleotides in the anticodon domain without requiring a base specificity.