Martin CASTELNOVO - Ecole Normale Supérieure de Lyon, Lyon, France

Viral self-assembly: elasticity and capsid shapes

A wide range of capsid morphologies is observed in viral kingdom. Some of them are very regular, with icosahedral symmetries producing rather compact viruses, while others seem irregular and are associated to elongated shapes. In this talk, we will show that the rationale for these observed  behaviors can be deduced by analyzing the viral assembly pathway at the light of the elasticity of thin shells. More precisely, it is the strong coupling between pentamer "defects" and the spontaneous curvature of the shell that determines the global morphology of the viral capsid. In the case of HIV-1, our model strongly suggests that the peculiar conical shape of the capsid core could be explained by external seeding of pentamers by the interaction of the shell with the genome and/or the enveloppe in a structure whose ground state of growth is an infinite cylinder.



Pascale BOULANGER - Institute for Integrative Biology of the Cell (I2BC), Orsay, France

Assembly and maturation of Bacteriophage T5 capsid

Capsids of dsDNA bacteriophages are extraordinarily robust macromolecular assemblies capable of withstanding the strong internal pressure generated by the tightly packed DNA they contain. Their assembly and maturation are regulated stepwise processes, exhibiting conserved features in all tailed bacteriophages. An empty procapsid is initially assembled and then packaged with DNA. This procapsid is built from several hundred copies of a major coat protein arranged as pentamers at the vertices and hexamers on the faces of the icosahedron. A dodecameric portal protein occupies a unique vertex and forms a gate through which DNA is translocated. The procapsid undergoes expansion during packaging, leading to an increase in the internal volume, which allows for accommodation of the full-length genome. This shell remodeling results from a structural rearrangement of the head protein subunits that yields the highly stable mature capsid. The expansion of the large capsid of bacteriophage T5 (T = 13) was investigated in vitro by using the intermediate prohead II form, which is competent for packaging the 121 kbp dsDNA genome. Prohead II morphology and dimensions were characterized by cryo-electron microscopy and small angle X-ray scattering. Decreasing the pH or the ionic strength triggers expansion of prohead II, converting them into capsids isomorphous to the mature virion particles. The transition was further investigated using Time-Resolved SAXS experiments following a fast pH-jump. Data sets were analysed using Singular Value Decomposition (SVD). Only two components were detected. Therefore, prohead II expansion appears to be a highly cooperative process lacking any detectable intermediate. This two-state reorganization of the capsid lattice leads to a remarkable stabilization of the particle, with no need for reinforcement by inter-subunit crosslinking or additional cementing proteins. After the termination of DNA packaging, accessory proteins often decorate the surface of the DNA-filled capsid. The expanded T5 capsid can be decorated in vitro with the purified protein pb10, which binds with a very high affinity to the center of each hexamer of the major head protein. These features make T5 a very attractive system for investigating the maturation of dsDNA bacteriophage capsids and for providing functionalized nanoshells.

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