wac (whisker antigen control) encodes fibritin (gpwac; 487 amino acids, ~53.1 kDa), a parallel homotrimeric alpha-helical coiled-coil structural protein that forms the collar and six ~53 nm whiskers radiating from the head-to-tail connector (neck) of bacteriophage T4 (12 trimers or 36 monomers per virion). The N-terminal domain anchors each fibritin trimer to the gp13/gp14 neck complex, twelve segmented alpha-helical coiled-coil rods form the elongated shaft, and the globular C-terminal beta-propeller-like "foldon" domain (residues 457-486) drives rapid autonomous trimerization and folding of the entire molecule. In the mature virion, the collar/whisker assembly catalyzes the attachment of assembled long tail fibers (LTFs) to the baseplate during morphogenesis and acts as an environmental sensor that binds the gp35-gp36-gp37 knee region of the LTFs to hold them in a retracted, non-infectious conformation under unfavorable pH, ionic strength, or temperature conditions.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0039660 structural constituent of virion | IEA file:BPT4/wac/wac-uniprot.txt | NEW | Summary: Fibritin (gpwac) is a homotrimeric structural protein that forms the collar and whiskers at the neck of the mature bacteriophage T4 virion (12 trimers per virion). Reason: The GOA file for wac currently contains no annotations. Cryo-EM, X-ray crystallographic, and biochemical studies establish that 12 fibritin homotrimers assemble around the gp13/gp14 neck to form the collar and whiskers of the mature virion, supporting GO:0039660 (structural constituent of virion) as its core molecular function. Supporting Evidence: file:BPT4/wac/wac-uniprot.txt six molecules forming the whiskers. file:BPT4/wac/wac-deep-research-falcon.md The bacteriophage T4 gene **wac** (UniProt P10104) encodes **fibritin** (also called **Wac** or **gpwac**), a trimeric fibrous structural protein that decorates the **virion neck** as the **collar and whiskers**. |
| GO:0098004 virus tail fiber assembly | IEA file:BPT4/wac/wac-uniprot.txt | NEW | Summary: Neck-assembled fibritin whiskers bind the knee region of mature long tail fibers to align and accelerate their docking onto the gp9 baseplate sockets during late virion morphogenesis. Reason: During the final stage of T4 morphogenesis, the collar and whiskers formed by gpwac interact transiently with the knee region of assembled long tail fibers, positioning the proximal gp34 end for efficient attachment to gp9 on the baseplate. Mutants lacking wac exhibit severely impaired long tail fiber attachment rates, supporting GO:0098004 (virus tail fiber assembly). Supporting Evidence: file:BPT4/wac/wac-uniprot.txt Acts as a chaperone helping to attach the long tail fibers to the virus file:BPT4/wac/wac-deep-research-falcon.md The bacteriophage T4 gene **wac** (UniProt P10104) encodes **fibritin** (also called **Wac** or **gpwac**), a trimeric fibrous structural protein that decorates the **virion neck** as the **collar and whiskers**. |
| GO:0044423 virion component | IEA file:BPT4/wac/wac-uniprot.txt | NEW | Summary: Fibritin localizes to the neck collar and whiskers of the mature T4 virion. Reason: Because GO lacks a dedicated cellular component term for the phage neck collar or whiskers (which are distinct from both the icosahedral capsid shell and the tail tube/fiber), GO:0044423 (virion component) accurately captures the localization of gpwac in the mature virion. Supporting Evidence: file:BPT4/wac/wac-uniprot.txt six molecules forming the whiskers. file:BPT4/wac/wac-deep-research-falcon.md The bacteriophage T4 gene **wac** (UniProt P10104) encodes **fibritin** (also called **Wac** or **gpwac**), a trimeric fibrous structural protein that decorates the **virion neck** as the **collar and whiskers**. |
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Download this section (compressed HTML)Q: Which specific residues of the fibritin whisker shaft or C-terminal foldon domain contact the gp35-gp36-gp37 knee region of the retracted long tail fiber, and how do changes in ionic strength or pH modulate the affinity of this interaction?
Q: How do the twelve N-terminal domains of fibritin accommodate the symmetry mismatch and interface geometry at the dodecameric gp13/hexameric gp14 neck complex to segregate into six upward-pointing collar trimers and six downward-pointing whisker trimers?
Experiment: Determine the cryo-EM structure of the intact T4 neck-collar-whisker complex with retracted long tail fibers using focused classification and local refinement around the whisker-to-LTF knee contact site.
Experiment: Measure the binding affinity and kinetics between purified fibritin trimers and reconstituted gp35-gp36-gp37 knee complexes across varying pH, temperature, and ionic strength conditions using surface plasmon resonance (SPR) or biolayer interferometry (BLI).
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