Aggregation of polyA–HIV-1 nucleocapsid protein NCp7 complexes and properties of the aggregates

We have previously shown that in solution, depending on solute and solution conditions, an ordered aggregation of polyA–NCp7 complexes takes place. The kinetic behaviour of the polyA(homoribopolynucleotide)–NCp7(human immunodeficiency virus-I nucleocapsid protein) aggregating system was studied at physiological NaCl concentration (150mM) using quasielastic light scattering measurements. The kinetic curves were fitted to a power law equation. For the aggregate size growth with time at initial concentrations of NCp7=0.3μM and nucleotide-to-protein molar ratios of 17 and 50 we obtained for power exponents of 0.15 and 0.23, respectively. On the basis of the aggregate size at which maxima are observed on the light scattering versus time curves we suggested a method for determining the mechanism of aggregation. Thus we found that in the process of polya–NCp7 aggregation, the growth proceeds by fusion of commensurable in size aggregates. The aggregate size dependence on NCp7 concentration showed critical behaviour. The calculated parameters of the power law (rate of growth n and a, related to the aggregate properties) showed in the critical region (0.2–0.3μM NCp7) a marked change. Electro-optic measurements (electric birefringence and electric light scattering) gave no evidence for the existence of optically anisotropic and/or anisodiametric suspended aggregates under the experimental conditions accessible for the electro-optic instruments used. Quasielastic light scattering and electron microscopy studies showed that at all stages of aggregate growth, under a broad range of experimental conditions, the aggregates were spherical with a unimodal, relatively narrow size distribution. We consider that combining the method suggested by us for studying the mechanism of aggregation with electro-optic and electron microscopy studies might be helpful for modelling the HIV-1 nucleocapsid (composed of genomic RNA–NCp7 complexes) assembly as well as transfectious particles for gene transfer (vectors) or non-infectious particles as candidate vaccines.