Maturation of phage T7 involves structural modification of both shell and inner core components

Abstract

The double-stranded DNA bacteriophages are good model systems to understand basic biological processes such as the macromolecular interactions that take place during the virus assembly and maturation, or the behavior of molecular motors that function during the DNA packaging process. Using cryoelectron microscopy and single-particle methodology, we have determined the structures of two phage T7 assemblies produced during its morphogenetic process, the DNA-free prohead and the mature virion. The first structure reveals a complex assembly in the interior of the capsid, which involves the scaffolding, and the core complex, which plays an important role in DNA packaging and is located in one of the phage vertices. The reconstruction of the mature virion reveals important changes in the shell, now much larger and thinner, the disappearance of the scaffolding structure, and important rearrangements of the core complex, which now protrudes the shell and interacts with the tail. Some of these changes must originate by the pressure exerted by the DNA in the interior of the head.

Structural investigations of a Podoviridae streptococcus phage C1, implications for the mechanism of viral entry

“…The rings of DNA are hexagonally packed and arranged as a spool wound around the central axis. The spool is very well ordered, but there is no internal body inside the C1 capsid unlike in many other phages, including ε15, T7, N4, K1-E, or P-SSP7 (16,18,26,28,29). In comparison, only two to three layers of ordered DNA are visible in the reconstruction of the prolate φ29 head (39).…”

“…Cryo-EM reconstruction showed that the C1 capsid has a diameter of 500 Å and a T ¼ 4 icosahedral arrangement, making it one of the smallest of tailed phages. The majority of structurally studied tailed phages have a larger capsid with a T ¼ 7 or higher icosahedral arrangement (P22, T7, HK97, ε15, K1-E, N4) (12,16,(26)(27)(28)(29). Bacteriophage φ29 forms a prolate head with T ¼ 3 and Q ¼ 5 triangulation numbers (14,30).…”

The conformation of double-stranded DNA inside bacteriophages depends on capsid size and shape

“…2D studies on the icosahedral phage T7 with a large protein core revealed that DNA forms a set of rings around the protein core and supported the coaxial spooling model (Cerritelli et al, 1997). Similar conformations were revealed by higher-resolution 3D reconstructions on phages ε15 and P22 Lander et al, 2006), which are both isomorphous to T7 (Agirrezabala et al, 2005). 3D density reconstruction of prolate ϕ29 showed a set of concentric ellipsoidal shells of DNA density (Tao et al, 1998;Xiang et al, 2006) but failed to resolve individual DNA strands.…”

“…ε15 , T7 (Agirrezabala et al, 2005) and P22 Lander et al, 2006), that contain a large protein portal complex located at one of the capsid's vertices. Inspired by these results we have simulated the packaging of dsDNA into a detailed model for ε15 .…”

Visualization of uncorrelated, tandem symmetry mismatches in the internal genome packaging apparatus of bacteriophage T7

“…Symmetry mismatches also occur in the capping and branching of actin filaments to control cell shapes (7), and the reciprocating conformational changes of the two barrels of GroEL (Ping-Pong mode) to fold protein substrates (8). Although the details of symmetric structures have been routinely visualized, the details of both asymmetry and symmetry mismatch have only recently become visualizable with the development of single-particle cryo-EM and associated asymmetric 3D reconstruction methods (9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20). Both the symmetry-mismatched, coaxial 12-fold portal ring and the 5-fold capsid of several bacteriophages (9-11, 13, 15-18) were resolved simultaneously in asymmetric reconstructions, i.e., reconstructions in which no symmetry was imposed.…”