Multishell Structures of Virus Coat Proteins

Abstract

Under conditions of low ionic strength and a pH ranging between about 3.7 and 5.0, solutions of purified coat proteins of cowpea chlorotic mottle virus (CCMV) form spherical multishell structures in the absence of viral RNA. The outer surfaces of the shells in these structures are negatively charged, whereas the inner surfaces are positively charged due to a disordered cationic N-terminal domain of the capsid protein, the arginine-rich RNA-binding motif that protrudes into the interior. We show that the main forces stabilizing these multishells are counterion release combined with a lower charge density in the RNA-binding motif region of the outer shells due to their larger radii of curvature, arguing that these compensate for the outer shells not being able to adopt the smaller, optimal, radius of curvature of the inner shell. This explains why the structures are only stable at low ionic strengths at pHs for which the outer surface is negatively charged and why the larger outer shells are not observed separately in solution. We show how to calculate the free energy of shells of nonoptimal radius of curvature from the elastic properties of the native shell. The spacing between shells is determined mainly by the entropic elasticity of the RNA-binding motifs. Although we focus on CCMV multishells, we also predict the solution conditions under which multishells formed by CCMV coat protein mutants with a lower RNA-binding motif charge are stable, and we examine other viruses as well. We conclude that at a given surface charge density, the boundaries separating regions of stable multishells with different numbers of shells shift to lower ionic strengths upon either increasing the length of the RNAbinding motif, increasing the stiffness of the shells, or decreasing the charge per RNA-binding motif.

Role of Electrostatics in the Assembly Pathway of a Single-Stranded RNA Virus

“…3a, red curve) and shifts the sedimentation peak from fraction 8 (naked CCMV*) to fraction 9 (60 CP 2 */CCMV). This shift, in combination with analytical ultracentrifugation measurements (data not shown), confirms previous observations that excess CP 2 * binds the exterior surface of CCMV (29), as expected from the electrostatic attraction between the ARMs of the CP 2 * and the negative surface charge density of CCMV (30). An identical binding assay carried out under acidic conditions (Fig.…”

“…In the latter cases, the excess surface-bound CP 2 seems to be more disordered and harder to visualize. This may be due to the fact that the smaller TA2 core particles support the growth of multishells with curvatures closer to that of the preferred TA3 capsid, whereas the larger TA3 cores are less likely to favor the generation of ordered multishells due to the smaller curvatures they would have to adopt (30).…”

In vitro assembly of the Rous Sarcoma Virus capsid protein into hexamer tubes at physiological temperature

“…At suitable pH and ionic strength, the capsid protein of cowpea chlorotic mosaic virus (CCMV) forms multi-shelled capsids 93, 94 , likely instigated by the opposing charge carried by the inner and outer surfaces of the capsid shells. Mutated forms of the major bacteriophage T4 prohead component, gp22, result in formation of multi-layered proheads 95 , and the protein will assemble into multi-layered tubes when other critical prohead components are inactivated or absent 96 .…”

Electrostatic interactions in biological DNA-related systems

“…[410][411][412][413] All these facts underline the importance of ES forces for DNA/RNA packaging and capsid self-assembly, producing a growing number of theoretical studies on this hot topic in the last years. [414][415][416][417][418][419] A number of theoretical [420][421][422] and computational [423][424][425][426] models exist for the capsid self-assembly. Some theories focus on the competition of attractive hydrophobic and repulsive ES interactions 427 acting on a homogeneous capsid surface.…”