Changes in bacteriophage T7 virion structure at the …

Abstract

Five proteins are ejected from the bacteriophage T7 virion at the initiation of infection. The three known proteins of the internal core enter the infected cell; all three must both disaggregate from their structure in the mature virion and also almost completely unfold in order to leave the head and pass through the headtail connector. Two small proteins, the products of genes 6.7 and 7.3, also are ejected from the infecting virion. Gp6.7 and gp7.3 were not previously described as structural virion components, leading to a re-appraisal of the stoichiometry of virion proteins. Gp6.7 is found in tail-less particles and is defined as a head protein, whereas gp7.3 is localized in the tail. Gene 6.7 may be important in morphogenesis; mutants defective in this late gene yield a reduced burst of progeny. Gene 7.3 is essential for virion assembly but, although normally present, its product gp7.3 is not required in a mature particle. Particles assembled in the absence of gp7.3 contain tail fibers but fail to adsorb to cells.

The virions of bacteriophages T3 and T7 are very similar and have a fairly simple structure (Fig. 1). The icosahedral head has a diameter of 60 to 61 nm with a shell that is 2 nm thick (Stroud et al., 1981andRont et al., 1983). The outer shell is composed of two forms of the gene 10 protein, which are made via a programmed translational frameshift near the 3 end of the shorter gene 10A ( Dunn and Studier, 1983, Condreay et al., 1989, Condron et al., 1991aandCondron et al., 1991b). Inserted at one vertex is the headtail connector, composed of 12 gp8 molecules ( Carazo et al., 1986, Cerritelli and Studier, 1996, Kocsis et al., 1995andValpuesta et al., 1992). The connector has a 12-lobed wide domain inserted into the head cavity and a narrower domain that interacts with the tail ( Valpuesta et al., 1992andValpuesta et al., 2000). A channel, which is closed in mature virions, runs through the center of the connector (Donate et al., 1988). Inside the head, and attached to the headtail connector in the coaxial orientation, is a 26 nm21 nm cylindrical structure that is usually referred to as the internal core ( Serwer, 1976, Serwer et al., 1997andSteven and Trus, 1986). The core has recently been shown to exhibit 8-fold symmetry (Cerritelli et al., 2003), it consists of stacked rings and contains three distinct proteins, the products of genes 14, 15, and 16. The 40-kb genome is spooled around the internal core in six coaxial shells ( Cerritelli et al., 1997). Unlike most other tailed phages, the T7 tail is not assembled as a separate structure but forms directly on the DNA-filled head ( Studier, 1972, Serwer, 1976, Roeder and Sadowski, 1977andMatsuo-Kato et al., 1981). The stubby tail is 23 nm long, tapering from a diameter of 21 nm at the connector to 9 nm at its distal end, and is known to consist of two major proteins gp11 and gp12 ( Studier, 1972andSteven and Trus, 1986). Attached near the head proximal end of the tail are six symmetrically positioned tail fibers. Each fiber is composed of a trimer of gp17 that forms a kinked structure ( Kato et al., 1985, Kato et al., 1986andSteven et al., 1988). The N-terminal 150 residues of gp17 link the fiber to the tail, the next 117 residues fold into an -helix, forming a 16.4-nm rod that is flexibly joined to the 15.5-nm distal half fiber. The latter consists of a linear array of four globules that is thought to bind directly to the bacterial cell.

Bacteriophage T7 initiates an infection of Escherichia coli by the interaction of its tail fibers with the lipopolysaccharide (LPS) on the cell surface. Interaction of all six tail fibers with LPS would orient the phage tail perpendicular to the cell surface, conferring efficiency to subsequent stages of infection. However, the stubby T7 tail is too short to span the E. coli cell envelope and a channel needs to be made to allow the phage genome to travel from the virion into the cytoplasm. It was proposed that virion proteins are ejected into the cell, functionally endowing T7 with an extensible tail, in contrast to the well-known contractile tail of T4 and other Myoviridae ( Molineux, 2001). In testing this suggestion, we show here that five T7 proteins are ejected from the virion into the cell at the initiation of infection. Two of these proteins were not previously known to be part of the virion, an observation that prompted a re-evaluation of the protein composition of the T7 particle.

In order to determine which T7 virion proteins become irreversibly associated with the infected cell, adsorbed particles were gently eluted by extensive washing in pure water. Prior to infection, cells of the E. coli K-12 strain IJ1133 were treated with rifampicin so that phage development would be inhibited after the first 850 bp of the T7 genome had been translocated into the cell and there would be no phage gene expression ( Garca and Molineux, 1995, Garca and Molineux, 1996andStruthers-Schlinke et al., 2000). Adsorption of the T7 tail fibers to E. coli LPS occurs through electrostatic interactions ( Puck et al., 1951, Luria, 1953andTolmach, 1957), and the reduction in ionic strength during washing causes dissociation of the phage from the cell. A similar protocol was used to identify P22 proteins that are ejected into the host cell (Israel, 1977). Elution results in the removal of the majority of the major and minor forms of the T7 capsid protein gp10, the headtail connector gp8, and the tail proteins gp11, gp12, and gp17 (Fig. 2A). In contrast, the majority of the internal core proteins gp14, gp15, and gp16 remain stably associated with the cell during the elution regimen. At the initiation of infection, the internal core must therefore disaggregate in order to allow its constituent proteins to be ejected from the virion into the cell. Two small virion proteins, gp6.7 and gp7.3, are also ejected from the virion but are degraded in infected wild-type cells. The proteins can be stabilized when the multiply protease-deficient strain HM130 is infected (data not shown). It is not yet known which of degP, lon, tsp, or ptr, missing in strain HM130 ( Meerman and Georgiou, 1994), is responsible.

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Changes in bacteriophage T7 virion structure at the ...