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. Th e kinetic behaviour of the polyA(homoribopolynucleotide)-NCp7(human immunod eficiency virus-I nucleocapsid protein) aggregating system was studied at p hysiological NaCl concentration (150 mM) using quasielastic light scatterin g measurements. The kinetic curves were fitted to a power law equation. For the aggregate size growth with time at initial concentrations of NCp7 = 0. 3 mu M 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 aggrega te size at which maxima are observed on the light scattering versus time cu rves we suggested a method for determining the mechanism of aggregation. Th us we found that in the process of polya-NCp7 aggregation, the growth proce eds by fusion of commensurable in size aggregates. The aggregate size depen dence on NCp7 concentration showed critical behaviour. The calculated param eters of the power law (rate of growth n and a, related to the aggregate pr operties) showed in the critical region (0.2-0.3 mu M NCp7) a marked change . Electro-optic measurements (electric birefringence and electric light sca ttering) gave no evidence for the existence of optically anisotropic and/or anisodiametric suspended aggregates under the experimental conditions acce ssible for the electro-optic instruments used. Quasielastic light scatterin g and electron microscopy studies showed that at all stages of aggregate gr owth, under a broad range of experimental conditions, the aggregates were s pherical with a unimodal, relatively narrow size distribution. We consider that combining the method suggested by us for studying the mechanism of agg regation with electro-optic and electron microscopy studies might be helpfu l for modelling the HIV-1 nucleocapsid (composed of genomic RNA-NCp7 comple xes) assembly as well as transfectious particles for gene transfer (vectors ) or non-infectious particles as candidate vaccines. (C) 1999 Elsevier Scie nce B.V. All rights reserved.