PMID:19835886

Citation	Qin, L, Fokine, A, O'Donnell, E, Rao, VB and Rossmann, MG (2010) Structure of the small outer capsid protein, Soc: a clamp for stabilizing capsids of T4-like phages. J. Mol. Biol. 395:728-41
Abstract	Many viruses need to stabilize their capsid structure against DNA pressure and for survival in hostile environments. The 9-kDa outer capsid protein (Soc) of bacteriophage T4, which stabilizes the virus, attaches to the capsid during the final stage of maturation. There are 870 Soc molecules that act as a "glue" between neighboring hexameric capsomers, forming a "cage" that stabilizes the T4 capsid against extremes of pH and temperature. Here we report a 1.9 A resolution crystal structure of Soc from the bacteriophage RB69, a close relative of T4. The RB69 crystal structure and a homology model of T4 Soc were fitted into the cryoelectron microscopy reconstruction of the T4 capsid. This established the region of Soc that interacts with the major capsid protein and suggested a mechanism, verified by extensive mutational and biochemical studies, for stabilization of the capsid in which the Soc trimers act as clamps between neighboring capsomers. The results demonstrate the factors involved in stabilizing not only the capsids of T4-like bacteriophages but also many other virus capsids.
Links	PubMed PMC2813359 Online version:10.1016/j.jmb.2009.10.007
Keywords	Amino Acid Sequence; Bacteriophage T4/chemistry; Bacteriophage T4/genetics; Bacteriophage T4/physiology; Bacteriophage T4/ultrastructure; Binding Sites/genetics; Capsid Proteins/chemistry; Capsid Proteins/genetics; Capsid Proteins/ultrastructure; Cryoelectron Microscopy; Crystallography, X-Ray; Evolution, Molecular; Models, Molecular; Molecular Sequence Data; Mutation; Protein Multimerization; Protein Stability; Protein Structure, Quaternary; Recombinant Proteins/chemistry; Recombinant Proteins/genetics; Sequence Homology, Amino Acid; Virus Assembly
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Annotations

Gene product	Qualifier	GO Term	Evidence Code	with/from	Aspect	Extension	Notes	Status
BPR69:Q7Y5B1		GO:0039624: viral outer capsid	ECO:0000255:	PMID:10998631	C		Figure 6a shows how the structure of the RB69 soc protein fits into a cryo EM model of the T4 capsid through trimeric interactions, where the major capsid protein gp23 of T4 has a 94% sequence identity to the major capsid protein of RB69. Fitting was done through the use of the EMFIT program, originally developed by Michael Rossmann (2000) (PMID:10998631). The stereo diagrams of the protein structure was derived from the RESOLVE program shown in Figure 2b using the amino acid sequence shown in Figure 2a. Figure 7b shows how six trimers of the soc (shown in red) assemble around each major capsid protein gp23 hexamer (each shown in blue, black, green, or magenta), acting to stabilize the connection between each capsid protein, a key characteristic of an outer capsid protein. A larger view that shows the stabilizing cage of the whole phage capsid can be seen in Figure 7a, with the red representing the soc proteins.	complete CACAO 12539

References

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