Final answer:
Actin filaments cross-linked by fimbrin are not contractile because fimbrin creates a rigid structure, inhibiting the actin-myosin sliding necessary for contraction. Stress fibers provide structural support rather than enabling muscle cell-like contraction.
Step-by-step explanation:
Actin filaments that are held together by the cross-linking protein fimbrin are not contractile probably because the fimbrin cross-links actin filaments too rigidly, preventing the actin-myosin sliding mechanism necessary for contraction. Muscle contractions depend on the interaction between actin filaments, composed of actin monomers, and the motor protein myosin. This interaction is facilitated by the specific structural arrangement found in muscle cells where thin actin filaments slide past thick myosin filaments, a dynamic process powered by ATP.
In non-muscle cells, the presence of proteins like fimbrin creates stable actin filament bundles that do not readily engage with myosin in the same sliding mechanism. Consequently, while fimbrin-stabilized filament bundles provide structural support and help maintain cell shape, they lack the necessary contractile property.
The proteins and filaments in stress fibers resemble premuscle sarcomeres, suggesting a possible evolutionary relationship. However, the organization in stress fibers is less complex, and they do not form the specialized Z- or M-line structures that characterize muscle cell sarcomeres. Stress fibers' primary role includes cellular tension and structure, not contraction.