PHOSPHORESCENCE AND OPTICALLY DETECTED MAGNETIC-RESONANCE INVESTIGATION OF THE BINDING OF THE NUCLEOCAPSID PROTEIN OF THE HUMAN-IMMUNODEFICIENCY-VIRUS TYPE-1 AND RELATED PEPTIDES TO RNA
Wc. Lam et al., PHOSPHORESCENCE AND OPTICALLY DETECTED MAGNETIC-RESONANCE INVESTIGATION OF THE BINDING OF THE NUCLEOCAPSID PROTEIN OF THE HUMAN-IMMUNODEFICIENCY-VIRUS TYPE-1 AND RELATED PEPTIDES TO RNA, Biochemistry, 33(35), 1994, pp. 10693-10700
The RNA and DNA complexes of nucleocapsid protein p7.Zn (NCp7.Zn) of t
he human immunodeficiency virus type 1 (HIV-1) are studied by phosphor
escence and optically detected magnetic resonance (ODMR). The single t
ryptophan, Trp37, which is located on the C-terminal zinc finger domai
n is used as an intrinsic probe. Reductions in the triplet state zero-
field splitting (zfs) D parameter of Trp37 upon complex formation with
poly(I) and poly(U) are observed. These results, in conjunction with
the phosphorescence red-shifts and triplet state lifetime reductions t
hat are observed, suggest the presence of aromatic stacking interactio
ns between NCp7.Zn and the bases of the RNA polymers. An alteration of
the intersystem crossing pattern upon complex formation, in addition
to the above mentioned spectroscopic shifts, also is consistent with p
reviously observed tryptophans that undergo stacking interactions with
DNA bases [Zang, L.-H., Maki, A. H., Murphy, J. B., and Chase, J. W.
(1987) Biophys. J. 52, 867-872. Tsao, D. H. H., Casas-Finet, J. R., Ma
ki, A. H., and Chase, J. W. (1989) Biophys. J. 55, 927-936]. These con
clusions support those from a recent ODMR study [Lam, W.-C., Maki, A.
H., Casas-Finet, J. R., Erickson, J. W., Sowder, R. C., II, and Hender
son, L. E. (1993) FEBS Lett. 328, 45-48] of NCp7.Zn binding to 5-mercu
rated polyuridylic acid [poly(5-HgU)] in which stacking interactions b
etween the RNA and NCp7.Zn are inferred from the observation of an ext
ernal heavy atom effect induced on Trp37. The extent of the spectrosco
pic effects observed varies with different RNA complexes; the phosphor
escence red-shifts, for instance, correlate with the affinities of NCp
7.Zn for various RNA bases as measured by fluorescence quenching exper
iments [Casas-Finet, J. R., Sowder, R. C., II, Sakaguchi, K., Appella,
E., Henderson, L. E., and Erickson, J. W. (1993) Biophys. J. 64, A178
]. The complexes of an 18mer synthetic second zinc finger peptide of N
Cp7 with RNA polymers gave results similar to NCp7.Zn, indicating that
tryptophan in either the wild type protein or in the synthetic peptid
e experience similar environments. However, spectroscopic effects of s
maller magnitude are observed in the synthetic second zinc finger pept
ide complexes, relative to those in the NCp7.Zn complexes, suggesting
that the two zinc fingers in NCp7.Zn may act in concert to bind RNA, A
synthetic carboxymethylated second zinc finger peptide in which a zin
c finger structure cannot be formed also is studied. The triplet state
properties observed for the uncomplexed synthetic carboxymethylated s
econd zinc finger peptide are similar to those of the noncarboxymethyl
ated synthetic second zinc finger peptide, suggesting that the tryptop
hans in the two fingers have similar environments in the uncomplexed f
orm. When either poly(I) or poly(U) is added to the synthetic carboxym
ethylated second zinc finger peptide, practically no spectroscopic eff
ects are observed, indicating weak or no interaction between Trp37 and
the RNAs under experimental conditions similar to those used for NCp7
.Zn and the synthetic second zinc finger peptide binding.