ORDERED AGGREGATION OF RIBONUCLEIC-ACIDS BY THE HUMAN-IMMUNODEFICIENCY-VIRUS TYPE-1 NUCLEOCAPSID PROTEIN

The nucleocapsid protein NCp7, which is the major genomic RNA binding protein of human immunodeficiency virus type 1, plays an important rol e in several keys steps of the viral life cycle. Many of the NCp7 acti vities, notably the nucleic acid annealing and the genomic RNA wrappin g ones, are thought to be linked to a nonspecific binding of NCp7 to i fs nucleic acid targets. The mechanism of these activities is still de bated but several clues are in favor of an intermediate aggregation of nucleic acids by NCp7. To check and characterize the nucleic acid agg regating properties of NCp7, we investigated the interaction of NCp7 w ith the model RNA homopolymer, polyA, by quasielastic tight scattering and optical density measurements. The ordered growth of monodisperse large particles independently of the nucleic acid size and the almost complete covering of polyA by NCp7 strongly suggested an ordered aggre gation mechanism. The aggregate kinetics of growth in the optimum prot ein concentration range (greater than or equal to 2 mu M) were governe d by a so-called Ostwald ripening mechanism limited by transfer of NCp 7-covered polyA complexes from small to large aggregates. The aggregat ion process was strongly dependent on both Na+ at and Mg2+ concentrati ons, the optimum concentrations being in the physiological range. Simi lar conclusions held true when polyA was replaced by 16S+23S ribosomal RNA, suggesting that the NCp7 aggregating properties were only poorly dependent on the nucleic acid sequence and structure. Finally, as in the NCp7 annealing activities, the basic regions of NCp7, but not the zinc fingers, were found critical in nucleic acid aggregation. Taken t ogether, our data indicate that NCp7 is a highly efficient nucleic aci d aggregating agent and strengthen the hypothesis that aggregation may constitute a transient step in various NCp7 functions. (C) 1997 John Wiley & Sons, Inc.