Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session Q2: The Physics of Self-Assembled Protein Cages |
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Sponsoring Units: DBP DMP Chair: Bogdan Dragnea, Indiana University Room: Morial Convention Center LaLouisiane C |
Wednesday, March 12, 2008 11:15AM - 11:51AM |
Q2.00001: Packaging of Polyelectrolytes in Viral Capsids: The Interplay Between Polymer Length and Capsid Size Invited Speaker: Each particle of the Cowpea Chlorotic Mottle Virus (CCMV) has a very small ``parts list,'' consisting of two components: a molecule of single-stranded RNA and a 190-residue protein that makes up the 28-nm diameter icosahedral capsid. When purified viral RNA and capsid protein are mixed in solution at an appropriate pH and ionic strength, infectious wild-type viruses form spontaneously. Virus-like particles (VLPs) are formed when the protein self assembles around other anionic polymers such as poly(styrene sulfonate) (PSS). Under different pH and ionic strength conditions the capsid protein can assemble by itself into empty capsids, multishell structures, tubes and sheets. To explore the effect on virion size of the competition between the preferred curvature of the protein and the size of the packaged cargo we have examined the formation of VLPs around PSS polymers with molecular weights ranging from 400 kDa to 3.4 MDa. Two distinct sizes are observed -- 22 nm for the lower molecular weights, jumping to 27 nm at 2 MDa. While under given conditions the size of PSS in solution is directly determined by its molecular weight, the self-complementarity of RNA makes its solution structure dependent on the nucleotide \textit{sequence} as well. We have therefore employed Small-Angle X-ray Scattering and Fluorescence Correlation Spectroscopy to examine the sizes of viral and non-viral RNAs of identical lengths. A model for the assembly that includes both the self-interactions of the polyelectrolyte and the capsid proteins and the interactions between them provides insight into the experimental results. [Preview Abstract] |
Wednesday, March 12, 2008 11:51AM - 12:27PM |
Q2.00002: Diversity in virus assembly: biology makes things complicated Invited Speaker: Icosahedral viruses have an elegance of geometry that implies a general path of assembly. However, structure alone provides insufficient information.~ Cowpea Chlorotic Mottle Virus (CCMV), an important system for studying virus assembly, consists of 90 coat protein (CP) homodimers condensed around an RNA genome. The crystal structure (Speir et al, 1995) reveals that assembly causes burial of hydrophobic surface and formation of $\beta$ hexamers, the intertwining of N-termini of the CPs surrounding a quasi-sixfold. This structural view leads to reasonable and erroneous predictions: (i) CCMV capsids are extremely stable, and (ii) $\beta$ hexamer formation is critical to assembly.~ Experimentally, we have found that capsids are based on a network of extremely weak (4-5 kT) pairwise interactions and that pentamer formation is the critical step in assembly kinetics. Because of the fragility of CP-Cp interaction, we can redirect assembly to generate and dissociate tubular nanostructures. The dynamic behavior of CCMV reflects the requirements and peculiarities of an evolved biological system; it does not necessarily reflect the behavior predicted from a more static picture of the virus. [Preview Abstract] |
Wednesday, March 12, 2008 12:27PM - 1:03PM |
Q2.00003: Dynamic Models for Templated Viral Capsid Assembly Invited Speaker: The replication of many viruses with single-stranded genomes requires the simultaneous assembly of an ordered protein shell, or capsid, and encapsidation of the genome. In this talk, I will present coarse-grained computational and theoretical models that describe the assembly of viral capsid proteins around interior cores, such as polymers and rigid spheres. These models are motivated by two recently developed experimental model systems in which viral proteins dynamically encapsidate inorganic nanoparticles and polyelectrolytes. Model predictions suggest that some forms of cooperative interactions between subunits and cores can dramatically enhance rates and robustness of assembly, as compared to the spontaneous assembly of subunits into empty capsids. For large core-subunit interactions, subunits adsorb onto a core en masse in a disordered manner, and then undergo a cooperative rearrangement into an ordered capsid structure. These assembly pathways are unlike any seen for empty capsids formation. While model predictions suggest that cooperative interactions between disparate assembling components can overcome some limitations of spontaneous assembly, the complexity of multicomponent assembly introduces new forms of kinetic traps that can frustrate assembly, and hence introduces new limitations. These findings have implications for a mechanism in which viruses use interactions between proteins and genomic molecules to promote and control assembly, and thereby control the replication process. [Preview Abstract] |
Wednesday, March 12, 2008 1:03PM - 1:39PM |
Q2.00004: Self-Assembly of the HIV Virus Invited Speaker: The talk will discuss the application of the continuum theory of elastic shells to understand the different morphologies of Retroviral capsids. Minor differences in molecular structure between different capsid proteins produce large changes in capsid morphology. Continuum elasticity theory can account for the capsid shape ``phase-diagram.'' The conical shape of the capsid of the HIV virus is the result of assembly ``constraints'' in the form of the enclosing lipid bilayer and the osmotic pressure of the encapsidated genome molecules. [Preview Abstract] |
Wednesday, March 12, 2008 1:39PM - 2:15PM |
Q2.00005: Menagerie of Viruses: Diverse Chemical Sequences or Simple Electrostatics? Invited Speaker: The genome packing in hundreds of viruses is investigated by analyzing the chemical sequences of the genomes and the corresponding capsid proteins, in combination with experimental facts on the structures of the packaged genomes. Based on statistical mechanics arguments and computer simulations, we have derived a universal model, based simply on non-specific electrostatic interactions. Our model is able to predict the essential aspects of genome packing in diversely different viruses, such as the genome size and its density distribution. Our result is in contrast to the long-held view that specific interactions between the sequenced amino acid residues and the nucleotides of the genome control the genome packing. Implications of this finding in the evolution and biotechnology will be discussed. [Preview Abstract] |
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