Structure of the 13-fold symmetric portal protein of bacteriophage SPP1

We have determined the three-dimensional structure of bacteriophage SPP1 portal protein (gp6) using electron microscopy at liquid-helium temperatures and angular reconstitution. The 13-fold symmetric gp6 oligomer is a turbine-shaped structure with three distinct regions: a conical stem with a central channel; the turbine wings region; and a fringe of small 'tentacles' at the end of the channel exposed to the viral head interior. The tentacle region appears flexible and may be associated with a particular function — sensing when the correct amount of DNA has been packaged. The three-dimensional structure of the gp6 SizA mutant, which packages a smaller chromosome, reveals significant differences in that region.