Fluorescence and nucleic acid binding properties of the human T-cell leukemia virus-type 1 nucleocapsid protein

We used intrinsic tryptophan fluorescence to study the nucleocapsid protein from human T-cell leukemia virus-type one, HTLV-1 p15, an 85-amino-acid pr otein with two Trp-containing zinc-finger motifs. Fluorescence spectra sugg ested an interaction between the two zinc fingers and another interaction i nvolving the C-terminal tail and the zinc fingers. Titrations with nucleic acid revealed similar, sub-micromolar affinity for poly(dT) and poly(U) in 1 mM sodium phosphate, pH 7. Double-stranded DNA bound an order of magnitud e weaker, suggesting helix-destabilizing activity. Base preference of p15 w as T approximate to U>I approximate to C approximate to G>A; affinity spann ed about one order of magnitude. HTLV-1 p15 bound weaker and with less vari ation than reported values for either human or simian immunodeficiency viru s homologues. The low affinity of p15 for nonspecific nucleic acids disting uishes it from other nucleocapsid proteins, and may suggest its involvement in additional steps of the virus life cycle other than RNA packaging. (C) 2000 Elsevier Science B.V. All rights reserved.