No tRNA3/(Lys) unwinding in a complex with HIV NCp7

Abstract

The nucleocapsid protein NCp7 of the human immunodeficiency virus (HIV) type 1 is important for the annealing of HIV RNA and tRNA3/(Lys), 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 β-turn structures is observed in NCp7 bound to tRNA3/(Lys). Furthermore, CD data shows that Cys23 NCp7 binds tRNA3/(Lys). The CD spectrum of tRNA3/(Lys) is typical of an A-form helix and retains this structure after binding of NCp7, which demonstrates that NCp7 does not induce tRNA3/(Lys) unwinding. CD spectra of tRNA3/(Lys) were measured from 5 to 80 °C to observe CD changes resulting from tRNA3/(Lys) 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.