The SPP1 connection

Abstract

The connector of the virulent Bacillus subtilis bacteriophage SPP1 (Styloviridae) is a structure localized at the phage head vertex which attaches the tail. It is formed by oligomerization of SPP1 gene product 6 (gp6; portal protein). The purified protein is found in solution essentially as a homo-tredecamer. Its assembly pattern resembles the turbine-like organization found for other portal proteins and has a defined handedness (Dube et al. (1993) EMBO J. 12, 1303-1309. A preliminary reconstruction of the structure shows that gp6 is composed of a lower ring connected by a narrow region to the upper area consisting of 13 lobes radiating from an inner ring. The assembly is organized around a central channel which spans its full height. A functional characterization of gp6 mutants showed that substitutions of defined amino acids by more basic residues lead to packaging of reduced amounts of DNA into the phage head (Tavares et al. (1992) J. Mol. Biol. 225,[81][82][83][84][85][86][87][88][89][90][91][92]. Since SPP1 encapsidates its DNA by a headful mechanism, these mutations (siz) affect most probably a function on the headful sensor-signal transduetion-headful cut system. Combination of siz alleles has severe effects in packaging. The resulting gp6 versions lead to the encapsidation of shorter DNA molecules at a lower efficiency than single siz mutants. Gene 6 is expressed late during SPP1 infection. Interestingly, the mass of portal protein inside the ceil then increases continuously until lysis, reaching a level several fold higher than the amount required to accomplish its role as a structural component of the virion.

Effect of the ionic environment on the molecular structure of bacteriophage SPP1 portal protein

“…Microbiological methods, DNA manipulations and analysis, media and enzymes were as described by Chai et al [16]. Rabbit polyclonal antibodies against gp6 were raised and used for Western blot as described [11].…”

“…Considering the three-dimensional structure of the hollow oligomer [10], it is possible that this is caused by the additional solvent-accessible surface contributed by the central channel and the possible solvent stream through it. The channel is expected to be hydrophilic considering its putative role in viral DNA transit [10,11].…”

“…Mg 2+ is the most abundant intracellular bivalent cation present at concentrations (< 50 mm in the cytoplasm of exponentially growing B. subtilis [39]) that stabilize the association state of annelid hemoglobins [40], GroEL [7], GroES [8] and gp6 (this work). We have estimated that there are <130 000 gp6 protomers in the host cell at the late phase of bacteriophage SPP1 infection during which viral particles are assembled [11]. This implies a micromolar concentration of gp6 in the B. subtilis cytoplasm (< 16.6 mm in a cell volume of 10 215 L [41]).…”

“…Gp6 is a homo-oligomer with 13-fold cyclical symmetry encoded by gene 6 of the phage genome. A central channel follows the full height of the turbine-like molecule, the 13 wings of which show a defined handedness [10,11]. This assembly is located at the unique`portal' vertex of the viral capsid through which DNA movement occurs and where the phage tail is bound [12].…”

“…This assembly is located at the unique`portal' vertex of the viral capsid through which DNA movement occurs and where the phage tail is bound [12]. Functionally, gp6 participates in the assembly of procapsids, is essential for recognition/binding of other phage structures, is believed to be a rotary element of the viral DNA-translocation machinery [10], and is part of the sensor system that measures the level of headfilling during DNA packaging [11,13].…”