Armored Tail Fiber Structure

Armored tail fibers are elongated, flexible, trimeric receptor-binding proteins with modular domains that mediate host recognition in bacteriophages.

General Architecture

Armored tail fibers, a type of receptor-binding protein (RBP), are typically long, trimeric complexes that attach to the phage tail, often at the baseplate or neck region. They are composed of multiple structurally distinct domains, which can include N-terminal attachment regions, central shaft domains, and C-terminal receptor-binding modules. These fibers are highly modular, allowing for domain swapping, sequence divergence, and convergence, which contributes to phage adaptability and host specificity .

Structural Prediction and Modeling

Due to their length and flexibility, obtaining high-resolution experimental structures of full-length tail fibers is challenging. Recent advances in deep learning-based protein structure prediction, such as ESMFold and AlphaFold2-multimer (AF2M), have enabled high-confidence modeling of complete tail fibers. The RBPseg pipeline segments tail fiber sequences into smaller fractions using a sigmoid distance pair (sDp) function, predicts each segment in parallel, and assembles them into full-length models. This approach improves computational efficiency and model reliability, allowing structural classification of fibers into 16 classes and 89 domains, representing at least 24% of known tail fiber diversity .

Functional Domains

Tail fibers generally include:

• N-terminal attachment domains: mediate connection to the phage tail or baseplate.
• Central shaft domains: provide structural rigidity and flexibility.
• C-terminal receptor-binding domains: recognize specific host receptors such as outer membrane proteins, exopolysaccharides, or flagella. These domains often determine host range and specificity .

Example: Mu Phage Tail Fiber

The Mu phage possesses two types of tail fibers, each recognizing different host surfaces. Structural studies of subunits gp49 and gp52 reveal that Mu phage employs distinct structural motifs for each fiber, highlighting the modular and adaptable nature of tail fiber architecture. These fibers enable the phage to expand its host range and facilitate horizontal gene transfer .

Evolutionary and Functional Insights

The modularity of armored tail fibers allows horizontal gene transfer between phages, contributing to structural diversity and host adaptation. Domain swapping and sequence divergence enable phages to evolve new receptor specificities while maintaining the overall trimeric architecture. This structural versatility is critical for phage survival and has implications for phage therapy and bioengineering applications . In summary, armored tail fibers are trimeric, modular proteins with distinct domains for attachment, structural support, and receptor recognition. Advances in computational modeling and cryo-EM validation have provided detailed insights into their structural diversity, evolutionary adaptability, and functional roles in host recognition.