Final answer:
In skeletal muscle, troponin does not bind along the entire length of the thin filament, nor does it block the myosin binding sites; rather, it participates in the mechanism that regulates the exposure of these sites.
Step-by-step explanation:
The major protein components of skeletal muscle, matched with their functions, can be carefully examined to determine which pairing is incorrect. Let's review the functions of the listed proteins:
- Titin: A protein that connects thick filaments to the Z line in muscle fibers, and helps maintain the structure of the sarcomere during muscle contraction and relaxation.
- Nebulin: Serves as a molecular ruler that regulates the length of thin filaments and contributes to the structural scaffold of the sarcomere.
- Myomesin: Found at the M line, it helps to link myosin bipolar filaments at the center of the sarcomere.
- Troponin: It is a complex of three proteins that, when bound to calcium ions, cause a change in tropomyosin position on the thin filament, exposing myosin-binding sites on actin. It does not bind along the entire length of the thin filament but is rather spaced at regular intervals.
- Actin: The primary structural component of thin filaments and has binding sites for myosin.
Based on this information, the choice D ('troponin-binds along the length of the thin filaments and blocks myosin binding sites') is incorrectly matched with its function because troponin does not block the myosin binding sites on its own; it is the tropomyosin that blocks these sites when muscle is at rest.