Troponin, the On-Off Switch of Muscle Contraction in Striated Muscle

Abstract

While it is now well known that a rise in the intracellular free Ca²⁺ concentration precedes muscle contraction, it is still puzzhng how exactly the contractile machinery is switched on by calcium. Much progress was achieved, however, by studying the calcium-binding protein troponin, which proved to be the intracellular calcium switch regulating the activity of the contractile machinery in vertebrate striated muscle and many types of invertebrate muscle. Thus, troponin senses the free Ca²⁺ concentration in the myoplasm and transmits the calcium signal to the actin filaments (Sect. 4.1). By influencing the filament structure troponin regulates the availabihty of potential crossbridge-binding sites on a segment of actin monomers and, hence, the activity of the contractile ATPase (Sect. 4.2). This calcium activation may be modulated indirectly by ancillary calcium-binding proteins, such as calmodulin and parvalbumin (Sect. 4.3).