Regulatory Proteins
Tropomyosin and troponin are two main proteins that
regulate muscle contraction and relaxation (Choi and Kim,
2009; Zot and Potter, 1987). They prevent the activation of
actomyosin ATPase in the absence of calcium ions by
interacting with actin filaments to block the myosin binding
site. Tropomyosin is a long, coiled protein (MW 65 kDa) that
comprises two α-helix polypeptide subunits, called α- and β-
tropomyosin. Tropomyosin molecules bind head-to-tail along
the F-actin filament. Each tropomyosin molecule is attached
to a troponin complex (MW 80 kDa) which is made up of
troponin C (MW 18 kDa), troponin I (MW 21 kDa) and
troponin T (MW 31 kDa) (Fig. 5). Troponin C acts as the
calcium binding site; troponin T connects troponin complex
to tropomyosin while troponin I inhibits actomyosin ATPase
activity when it is bound to actin (Lehman and Craig, 2008).
At high calcium ion concentration, calcium ions bind to
Troponin C, which initiates a conformation change in the
tropomyosin-troponin complex, dislocating troponin I,
allowing the action of actomyosin ATPase for muscle
contraction.
Structural Proteins
The structural proteins control the filamentous structure and
integrity of myofibrils . Titin, also known as connectin, with a
molecular weight of , serves as the backbone of thick
filaments in the A-band. It also acts as a molecular spring in
the I-band, which provides elasticity to the sarcomere during
muscle contraction (Fig. 3) (Labeit and Kolmerer, 1995).
Nebulin (MW 800 kDa) is a structural protein that regulates
the length of the thin filaments (McElhinny et al., 2003;
Strasburg et al., 2008).
α-Actinin (MW 95 kDa) is the principal constituent of the Z-
disk, supporting and attaching actin to the Z-disk (Obinata et
al., 1981).
β-Actinin, also known as CapZ protein, is a heterodimer
consisting of α- and β-subunits (MW 37 and 34 kDa
respectively). It binds α-actinin in the Z-disk and prevents
network formation between the actin filaments (Swartz et
al., 2009).
Tropomodulin (MW 40 kDa) binds tropomyosin and actin as
well as controlling the length of thin filaments by maintaining
the number of G-actin monomers (Clark et al., 2002).
Both desmin (MW 55 kDa) and filamin (MW 300 kDa), play a
major role in linking the myofibrils to the sarcolemma as well
as stabilizing muscle structure (Capetanaki et al., 1997;
Strasburg et al., 2008; Vander Ven et al., 2000).
C-protein (MW 140 kDa) and H-protein (MW 58 kDa) are
myosin-binding proteins that are found in the A-band of thick
filaments (Koretz et al., 1993; Xiong, 1997). These proteins
are believed to contribute to the alignment and stabilization
of the thick filaments.
Myomesin (MW 185 kDa) is the major protein in the M-line.
It is responsible for the binding of titin and myosin and for
maintaining the structure of the thick filaments.
