Abstract
The structure and assembly of bacteriophage T4 has been extensively studied. However, the detailed structure of the portal protein remained unknown. Here we report the structure of the bacteriophage T4 portal assembly, gene product 20 (gp20), determined by cryo-electron microscopy (cryo-EM) to 3.6 Å resolution. In addition, analysis of a 10 Å resolution cryo-EM map of an empty prolate T4 head shows how the dodecameric portal assembly interacts with the capsid protein gp23 at the special pentameric vertex. The gp20 structure also verifies that the portal assembly is required for initiating head assembly, for attachment of the packaging motor, and for participation in DNA packaging. Comparison of the Myoviridae T4 portal structure with the known portal structures of φ29, SPP1 and P22, representing Podo- and Siphoviridae, shows that the portal structure probably dates back to a time when self-replicating microorganisms were being established on Earth.
Highlights
The structure and assembly of bacteriophage T4 has been extensively studied
The portal assembly serves at least three functions in the life cycle of most of the tailed phages[3]: (i) it initiates head assembly; (ii) it provides a platform for the packaging motor during DNA packaging into the prohead; and (iii) it is required for tail attachment
The ‘small terminase’ of T4-like bacteriophages initiates DNA packaging into the prohead, a process that is powered by five copies of a virally coded ‘large terminase’ ATPase[2,3,4]
Summary
The structure and assembly of bacteriophage T4 has been extensively studied. the detailed structure of the portal protein remained unknown. The gp[20] structure verifies that the portal assembly is required for initiating head assembly, for attachment of the packaging motor, and for participation in DNA packaging. Many of the proteins that form the structure of the T4 capsid[17,18,19,20], packaging motor[6,21] and tail have been determined[22,23,24]. The portal protein assembly of phage P22 has an additional 200 Å-long carboxy-terminal, a-helical, coiled coil domain inside the head that might facilitate genome spooling onto the interior surface of the capsid during genome packaging and facilitate genome ejection into the host cell during infection[28,29]
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