Tailed bacteriophages and herpesviruses load their capsids with DNA through a tunnel formed by the portal protein assembly. Here we describe the X-ray structure of the bacteriophage SPP1 portal protein in its isolated 13-subunit form and the pseudoatomic structure of a 12-subunit assembly. The first defines the DNA-interacting segments (tunnel loops) that pack tightly against each other forming the most constricted part of the tunnel; the second shows that the functional dodecameric state must induce variability in the loop positions. Structural observations together with geometrical constraints dictate that in the portal-DNA complex, the loops form an undulating belt that fits and tightly embraces the helical DNA, suggesting that DNA translocation is accompanied by a 'mexican wave' of positional and conformational changes propagating sequentially along this belt.

Electron microscopy in combination with image processing is a powerful method for obtaining structural information on non‐crystallized biological macromolecules at the 10–50 A resolution level. The processing of noisy microscopical images requires advanced data processing methodologies in which one must carefully avoid the introduction of any form of bias into the data set. Using a novel multivariate statistical approach to the analysis of symmetry, we studied the structure of the bacteriophage SPP1 portal protein oligomer. This portal structure, ubiquitous in icosahedral bacteriophages which package dsDNA, is located at the site of symmetry mismatch between a 5‐fold vertex of the icosahedral shell and the 6‐fold symmetric (helical) tail. From previous studies such ‘head‐to‐tail connector’ structures were generally accepted to be homododecamers assembled in a 12‐fold symmetric ring around a central channel. Using a new analysis methodology we have found that the phage SPP1 portal structure exhibits 13‐fold cyclical symmetry: a new point group organization for oligomeric proteins. A model for the DNA packaging mechanism by 13‐fold symmetric portal protein assemblies is presented which attributes a coherent functional meaning to their unusual symmetry.

The majority of known bacteriophages have long noncontractile tails (Siphoviridae) that serve as a pipeline for genome delivery into the host cytoplasm. The tail extremity distal from the phage head is an adsorption device that recognises the bacterial receptor at the host cell surface. This interaction generates a signal transmitted to the head that leads to DNA release. We have determined structures of the bacteriophage SPP1 tail before and after DNA ejection. The results reveal extensive structural rearrangements in the internal wall of the tail tube. We propose that the adsorption device-receptor interaction triggers a conformational switch that is propagated as a domino-like cascade along the 1600 Å-long helical tail structure to reach the head-to-tail connector. This leads to opening of the connector culminating in DNA exit from the head into the host cell through the tail tube.