Bacteriophage Capsid

Elena Orlova
Helen White
Yuriy Chaban
Paulo Tavares (CNRS, Gif sur Yvette, France)

Capsid structure of Bacteriophage SPP1 Assembly

The structure of bacteriophage SPP1 capsid was determined at sub-nanometer resolution by cryo-electron microscopy and single-particle analysis. The icosahedral capsid is composed of the major capsid protein gp13 and the auxiliary protein gp12 that are organized in a T=7 lattice. DNA is arranged in layers with a distance of ~24.5 Å. Gp12 forms spikes that are anchored at the centre of gp13 hexamers. In a gp12-deficient mutant the centres of hexamers are closed by loops of gp13 coming together to protect the SPP1 genome from the outside environment. The HK97-like fold was used to build a pseudo-atomic model of gp13. Its structural organization remains unchanged upon tail binding and following DNA release. Gp13 exhibits enhanced thermostability in the DNA-filled capsid. A remarkable convergence between thermostability of the capsid and of the other virion components was found, revealing that the overall architecture of the SPP1 infectious particle co-evolved towards high robustness.

Segmentation of the asymmetric unit in the FP capsid and modelling of the pseudo-atomic structure of the gp13 capsid protein. A - The asymmetric unit of the 8.8 Å resolution map has been segmented to reveal seven capsid protein subunits. The unit contains an entire hexamer and one pentamer subunit. Six subunits that form the hexameric capsomer of the phage shell are coloured in rainbow colours. The capsid protein that participates in forming the pentamers is coloured in salmon, the remaining subunits of the pentamer are shown in grey. Gp12 at the centre of the hexamer is depicted in orange. B - A pseudo-atomic model fitted by Flex-EM into the asymmetric unit of the FP capsid. The colours correspond to colours in A. C – View along the 3-fold axis. D - View along the 2-fold axis. Intra-capsomer contacts near two-fold axes could possibly be formed by the E-loop of a gp13 subunit and the P-domain of an adjacent subunit within the same hexamer as found in the phage T7 capsid. Unassigned densities are encircled in purple (from White HE, Sherman MB, Brasilès S, Jacquet E, Seavers P, Tavares P, Orlova EV. Capsid structure and its Stability at the Late Stages of Bacteriophage SPP1 Assembly. (2012) J Virol. Apr 18[Epub ahead of print]).

EM group web page Elena Orlova