Bacteriophage T4 Tail Fibers as a Basis for Structured Assemblies
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By Paul Hyman and Timothy Harrah
Bacteriophages, viruses that infect bacteria, have evolved a variety of complex protein structures to carry their genomes between host cells. These proteins form the virion particle which can be considered a mostly self-assembled protein machine that protects the genome and effects genome entry into new cells. Because bacteriophages (phages) are often found in harsh environments including animal digestive tracts, sewage, and sea water, virion particle proteins are typically very stable and resistant to changes in pH, salts, digestive proteases, and other agents that typically denature or degrade proteins. Bacteriophage T4 long tail fibers are specialized proteins that bind to the host cell surface. They are very long (≈160 nm) and thin (≈3-5 nm) rigid fibrous multiprotein structures. The high length to width ratio of the long tail fibers (LTFs), rigidity, self-assembling properties plus chemical durability suggest that LTFs could be adapted into a self-patterning nanoscale protein structure or system. The long tail fibers of T4 are composed of 10 proteins, 3 copies each of gene product (gp) 34, gp 36, and gp 37 plus a single copy of gp 35 which forms the hinged "knee" of the tail fiber. Although crystallizing whole tail fibers remains a challenge, other structural data on fiber fragments, related trimeric protein fibers, and other data suggest that some type of repetitive beta secondary structure comprise the rigid rod portions of the tail fibers.
The presence of large segments of beta structure arising from mostly local interactions also support the proposition that tail fibers can withstand a variety of modifications without compromising the overall structure and function of the bacteriophage. Toward the end of creating structured assemblies we have constructed and tested a variety of tail fibers altered in gp 37. These include deletions to alter the overall length and modifications to a key segment where assembly is initiated to improve assembly in vitro. We have developed an improved purification method for the assembled tail fiber. We have also added a variety of attachment sites to several locations in the gp 37 including a biotinylation site and an antibody binding epitope. These insertions do not appear to disrupt the gp 37 structure in any way and phages carrying these gp 37 modifications remain viable. In this monograph, we will review what is known about the structure of the bacteriophage T4 tail fiber system and present a model of how it can be adapted into a controlled self-assembling system. We further review the published and unpublished work we have completed on tail fiber purification and modifications.