The self-assembly of the cowpea strain of Southern bean mosaic virus: formation of T=1 and T=3 nucleoprotein particles

Abstract

Virion-like particles of the cowpea strain of southern bean mosaic virus were assembled from isolated coat protein and RNA components in vitro at low ionic strength. Purified southern bean mosaic virus RNA was heterogeneous on sucrose gradients, and was separated into low (0.3-0.6×10⁶ Da) and high (1.0-1.4×10⁶ Da) molecular weight (MW) fractions. Assembly with high MW RNA resulted in T=3 particles at pH 7 and 9. Low MW RNA assembled with coat protein into T=1 particles at pH 5 and 7, and into T=3 particles at pH 9. The formation of T=3 particles at pH 9, and of T=1 particles at pH 7, required the presence of Ca²⁺ and Mg²⁺ ions. Proteolytic digestion of the basic amino-terminal arm of the coat protein in the absence of RNA indicated that the arm-RNA interaction is an early event in the assembly of an initiation complex for both types of particles. The effects of pH and divalent cations on SBMV assembly suggest that the charge configuration of carboxyl group clusters in the putative initiation complex regulates further subunit interactions and, hence, the mode (T=3 vs T=1) of assembly.