Question

Actin filaments that are held together by the cross-linking protein fimbrin are not contractile. This is probably because ...

A. the very weak cross-linking by this protein cannot convert myosin II activity into a coherent contraction.
B. fimbrin arranges the actin filaments in parallel bundles in which all the plus ends point to the same direction.
C. the very tight packing of actin filaments by this small protein excludes myosin II filaments and other large proteins.
D. fimbrin arranges the actin filaments into gel-like networks in which myosin II activity does not produce contraction.
E. fimbrin is a large protein that binds to several actin filaments and resists contraction.

Answer 1

Actin filaments bound by fimbrin are non-contractile because the tight packing excludes myosin II, which is essential for muscle contraction (option C).

Step-by-step explanation:

The reason why actin filaments that are held together by the cross-linking protein fimbrin are not contractile is likely due to option C, which states that the tight packing of actin filaments by fimbrin excludes myosin II filaments and other large proteins. Myosin II is essential for contraction because it binds to the actin filaments and uses ATP to generate force, causing the filaments to slide against each other and contract. If fimbrin packs the actin filaments too tightly, there is no space for myosin II to interact with the actin, thereby preventing the contraction process.

Hence, the answer is option C.