In prokaryotic flagella (bacterial locomotion), the hook and filament are primarily composed of which structural protein?

Introduction / Context:
Bacterial flagella are rotary nanomachines with distinct parts: basal body, hook, and filament. Recognizing the protein composition of these structures is fundamental for understanding motility and antigenic variation (e.g., H antigens in Enterobacteriaceae).

Given Data / Assumptions:

• The question concerns the protein building blocks of the hook and filament (the external, helical portion).
• We are discussing bacteria, not eukaryotic flagella or cilia.
• Answer choices include common proteins unrelated to bacterial flagella.

Concept / Approach:
The bacterial flagellar filament and hook are made primarily of the protein flagellin. Flagellin polymerizes to form the long helical filament; related proteins form the hook that connects the filament to the basal body. This protein is immunogenic and forms the basis of the H antigen used in serotyping.

Step-by-Step Solution:

Verification / Alternative check:
Biochemical purification and electron microscopy studies consistently show the filament is a polymer of flagellin monomers arranged in a helical pattern.

Why Other Options Are Wrong:
Keratin: Eukaryotic intermediate filament protein in animals; not used by bacteria for flagella. Gelatin: Denatured collagen; not a native bacterial protein. Casein: Milk protein; irrelevant to bacterial flagellar structure.

Common Pitfalls:
Confusing bacterial flagella with eukaryotic flagella/cilia, which are composed of microtubules (tubulin) and dynein arms, not flagellin.

Final Answer:
flagellin