Which proteins regulate the bridging of actin and myosin?

Troponin and tropomyosin are critical regulatory proteins in the process of muscle contraction, specifically in the interaction between actin and myosin filaments within the muscle fibers. Troponin is a complex of three proteins that binds to calcium ions, while tropomyosin is a long, coiled protein that winds around the actin filament. Together, these proteins play a pivotal role in regulating muscle contraction. When calcium binds to troponin, it causes a conformational change that moves tropomyosin away from the binding sites on actin. This exposure allows myosin heads to attach to actin, facilitating the cross-bridge cycling that ultimately leads to muscle contraction.

The other choices include proteins that do not participate in the regulation of actin and myosin interaction. For example, myosin is indeed involved in muscle contraction but is not a regulatory protein. Elastin and collagen are structural proteins primarily found in connective tissues, while keratin is a structural protein found in hair, nails, and the outer layer of skin, which also does not play a role in muscle contraction mechanics. Thus, focusing on the regulation of actin and myosin indicates that troponin and tropomyosin are the correct proteins responsible for this process.