Structure of the Small Outer Capsid Protein, Soc: A Clamp for Stabilizing Capsids of T4-like Phages

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 “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 Å 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 cryo-electron 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.

Structure and genome release of Twort-like Myoviridae phage with a double-layered baseplate

“…6A). Residues 268-399 are inserted into the axial domain and form the "I domain," a protrusion on the outer surface of the capsid (40) (Fig. 6A).…”

“…S12). However, it was reported that T4 Soc proteins form trimers around quasi-threefold axes and do not interact with the I domain of T4 major capsid proteins (40). The T4 Soc protein increases the stability of the T4 capsid (40), and it is possible that phi812 cement protein has the same function, even though it has a different location within the capsid.…”

Structure of bacteriophage ϕ 29 head fibers has a supercoiled triple repeating helix-turn-helix motif

“…Although many minor capsid proteins are not essential for infection in laboratory conditions (8)(9)(10)(11), they may have provided favorable properties for the survival of phage in hostile environments. For instance, the minor capsid protein Soc of phage T4 provides enhanced capsid stability (9).…”

CryoEM and mutagenesis reveal that the smallest capsid protein cements and stabilizes Kaposi's sarcoma-associated herpesvirus capsid

“…The auxiliary role of SCP in gammaherpesvirus capsid stabilization mirrors that of cementing proteins often found in dsDNA bacteriophages, such as the Soc protein in phage T4 (42,43) and the gpD protein in phage λ (44), as proposed previously for VP26 of HSV (45). In phage λ, the 11.4-kDa gpD forms a trimer and binds to capsid surface at quasi-and icosahedral threefold axes after capsid maturation; it stabilizes the capsid structure by fastening six gpE (the MCP) subunits from three neighboring capsomers (44).…”

“…These nonessential auxiliary proteins in phages "cement" adjacent MCPs to stabilize the mature capsid against extremes in pH and temperature as well as other factors in their hostile environment. They also help the thin capsid shell to withstand the pressure exerted by the packaged DNA (42,44,46,47). In gammaherpesvirus, it is conceivable that the cross-linking by hexon SCP would also fasten the MCP subunits in hexons and thus increase the overall stability of the capsid.…”