Contractile Proteins and Calcium

A number of proteins are involved in contraction including contractile, regulatory, and structural proteins

Contractile Proteins:

• Myosin: This thick filament contains the myosin heads that bind to actin.
• Actin: This thin filament consists of 2 actin units that are intertwined in a helical pattern, both being carried on a heavier tropomyosin molecule that functions as a backbone. The actin units contain sites that bind to the myosin heads and contain the regulatory proteins.

Regulatory Proteins:

• Tropomyosin: Part of the thin filament that acts as a structural backbone to the actin helix. At rest, tropomyosin is structurally positioned to obstruct the interaction of myosin heads with actin binding sites.
• Troponin-I (Tn-I): Part of the troponin complex, positioned on the tropomyosin protein. I stands for inhibition. When Tn-I is not bound tightly to Tn-C (tight binding occurs as a result of Ca²⁺ activation), Tn-I promotes a tight interaction between Tn-T and tropomyosin that enables the position of tropomyosin to restrict the interaction between actin and the myosin heads. When Ca²⁺ binds to Tn-C, activated Tn-C binds tightly to Tn-I. Tn-I now moves to a new position on the thin filament that causes a weakening of the interaction between Tn-T and tropomyosin. This promotes a conformational change (repositioning of tropomyosin on the thin filament) that exposes more actin binding sites to myosin heads.
• Troponin-C (Tn-C): Part of the troponin complex, positioned on the tropomyosin protein. C stands for calcium. Tn-C is activated by Ca²⁺ binding (released by the sarcoplasmic reticulum). This starts the contraction process (cross bridge cycling). Once activated by Ca²⁺, Tn-C binds to Tn-I. Tn-I now moves to a new position on the thin filament causing a weakening of the interaction between Tn-T and tropomyosin. Tropomyosin now moves to expose more actin binding sites to cross bridge with myosin heads.
• Troponin-T (Tn-T): Part of the troponin complex, positioned on the tropomyosin protein. T stands for tropomyosin binding. Tn-T links the whole troponin complex to tropomyosin. When Tn-T is tightly bound to tropomyosin, tropomyosin takes up a position on the thin filament that blocks most of the actin sites that could bind to myosin heads.

Structural Proteins:

• Titin: A very large molecule consisting of 2 segments, an anchoring segment and an elastic segment. Mutations in the titin gene have been implicated in familial dilated cardiomyopathy in the Doberman pinscher.
• It has 2 major roles:
  • It tethers the myosin molecule to the Z line.
  • The folded elastic segment is important to allow the myocardium to regain its original shape in diastole by stretching during systole.

Calcium-induced calcium release

The calcium that enters the cell as a result of the action potential induces an avalanche of Ca²⁺ to be released from the sarcoplasmic reticulum (SR) (a tenfold increase in cytosolic Ca²⁺ compared to the flux of Ca²⁺ across the cell membrane).

The Ca that enters the cell during the action potential binds to the ryanodine receptor of the terminal end of the SR. Binding to this receptor causes an avalanche of Ca²⁺, previously stored in the SR, to be released into the cytosol. This is called calcium-induced calcium release.

Role of calcium in mediating contraction

The massive increase in free cytosolic Ca²⁺ results in Ca²⁺ binding to Tn-C of the troponin complex, resulting in a conformational change in the tropomyosin protein thereby releasing the inhibition of tropomyosin on actin such that the actin sites are now exposed for binding to myosin heads and enabling cross-bridging and contraction.

At rest, tropomyosin lays across the actin sites hiding them from the myosin heads to which they attach for contraction to occur.

Interaction of contractile and regulatory proteins during contraction

The thin actin fibers slide between the thicker myosin fibers as a result of repetitive movements of the myosin heads. The linkage between the myosin head and the actin filament is the crossbridge. The crossbridge cycle is the repetitive attachment and detachment of myosin heads to and from actin filaments.

ATP is required for the process of attachment and detachment. The terminal phosphate bond of ATP is split off by myosin ATPase releasing energy for attachment.

Crossbridging is inhibited at rest by both the avid binding of Tn-T and tropomyosin that promotes the positioning of the tropomyosin molecule which is twisted in such a way blocking the interaction of the myosin heads with actin.

The binding of Ca²⁺ to the Tn-C results in:

• binding of activated Tn-C with Tn-I is strengthened, Tn-I moves, and this weakens the interaction between Tn-T and tropomyosin causing the tropomyosin to be repositioned on the thin filament and
• the repositioning of the tropomyosin molecule exposes more binding sites on the actin molecule to myosin heads.

Differences in vascular smooth muscle contraction compared to myocardial contraction

• The production of cAMP in the myocardium induces contraction, but it inhibits myosin light chain kinase causing dilation in vascular smooth muscle.
• Depolarization is not essential for the initiation of the contractile cycle in vascular smooth muscle, because it can be triggered by the increase in Ca²⁺ released from the SR by inositol triphosphate or increased Ca²⁺ entry into the cell.
• Peripheral vascular contraction is tonic, whereas cardiac contraction is short-lived and generates considerable force.
• In vascular smooth muscle, Tn-C is absent from the actin filaments.
• In vascular smooth muscle, Ca²⁺ regulates actin-myosin interaction by binding to calmodulin which promotes phosphorylation of the myosin light chain kinase which promotes cross bridging between actin and myosin.