BIOL4510 / KINE 4510
Contractile Proteins 1
Peter Backx 2012
There are two properties of muscle you need to fully understand:
1) The force-length relationship
Tension (% Max) (Starling)
1.0 1.4 1.8 2.2 2.6 3.0 3.4 3.8
Sarcomere Length (m)
2) The dependence of force generation on the intracellular Ca 2+ levels. Ca 2+
acts as a catalyst for activating the ATPase in muscle for the purposes of
conversion of chemical energy into force.
1 Heavy Filament Myosin
Myosin is the molecular motor of the contractile protein machinery. It is the protein that
converts chemical energy from ATP to mechanical energy, thus generating force and
There are least 15 different classes of myosin;
the myosins involved in force generation and
movement in muscle (cardiac, skeletal and
smooth muscle are comprised of Type II.
Type II dimerizes…two myosin moleculae are
connected by coil-coil structures in the tail
Type I is the most primitive2+yosin: moves
molecular cargos….are Ca -dependent via
Calcium/calmodulin binding in neck region
Myosins are generally large proteins with a total molecular mass of ~500,000 Da.
Each molecular unit of the thick filament is comprised of:
a tail, a neck and a head region
The head region is the site of force generation and ATPase activity
An -helix with sequences that bind light chains
The light chains contribute to the stability of the helix in the neck region and modulate
ATPase activity. The head and neck domains are referred to as the S1 domain, also
Myosin heavy chain tails form an -helical coiled-coil tail domain (tail).
2 Individual long tails of myosin interact with each other allowing myosin molecules to
assemble into thick filaments. About 150 myosin dimers molecules polymerize in a
thick filament (about 300 myosin heads/thick filament)
Myosin Heavy Chain (MHC)
There are two types of cardiac myosin heavy chains (MHC and MHC isoforms)
MHC = fast
MHC = slow
myosin ATPase = 5 * myosin ATPase
myosin ATPase is thought to be more “energetically efficient”
and -myosin heavy chains are separate gene products
These two isoforms can co-assemble in 3 possible ways:
V = intermediate)
3 V 3 (slow)
In the human heart, the myosin heavy chains, MHC and MHC, are expressed in a
tissue-specific manner. The MHC is preferentially expressed in atria. ~90% of
ventricle is MHC (rest is MHC ). In the fetal and neonatal heart, MHC is expressed
in the ventricles and atria.
There are species differences in MHC expression. The rat ventricle predominantly
expresses MHC rabbit hearts expresses MHC and MHC
MHC is found primarily in large mammals, which correlates with speed of contraction
and heart rate; large mammals have slower heart rates and slower contraction.
- slower myosin is more energy efficient (general principle in biology, the faster
you want reactions to occur the more inefficient the system)
- human heart disease levels of MHC,go up and MHC gprs down,.
- speed of contraction slows in heart disease
- mutations in myosin genes were the first to be associated with inherited hypertrophic
- heart on the left (above) is hypertrophic cardiomyopathy, heart on right is normal.
- Mutations of contractile proteins can cause massive hypertrophy. Why would the
heart do this? Answer compensation
In developing skeletal muscle, MHC-emb (embryonic) and MHC-neo (neonatal) are
In post-natal skeletal muscle there are 4 MHC isoforms
• MHC (also referred to as MHC slow, same as cardiac MHC), IIA, IIB, and IIX
4 • MHC/slow, IIA, and IIX (humans although IIB gene is present, it does not seem
to be expressed)
These isoforms >80% identical (i.e., their amino acid sequences are < 20% different)
MHC IIa MHC IIB MHC
The type of myosin matches up with type of myosin light
chains: different myosin light chains for MHC versus
MHC myosin in the human heart.
Human MHC contains 1939 amino acids residues while
MHC contains 1935 amino acids. They only differ by 131
amino acids, mainly confined to regions of biological
significance in the head domain such as the N-terminus, the
ATP binding pocket, the actin binding cleft, the light chain
binding domain and in the two hinge regions further down in
the rod (tail) domain.
ATPase of MHC is modulated by actin and Ca
Ca 2+ (1-3 µM) stimulates ATPase in presence of actin
Ca 2+ influences the rate of movement
Increase of Ca to >~10 µM decreases velocity
immediately followed by a gradual decay
Myosin Light Chain
There are two types of myosin light chains (MLC), essential and regulatory, which are
associated with the neck region of MHC myosin. The essential MLC is also referred to
as MLC-1 or alkali MLC. It is called essential because myosin head will not work well
without this chain. The regulatory MLC is also known as MLC-2 or phosphorylatable
MLC. It is phosphorylated by myosin light chain kinase. Phosphorylation modulates
myosin ATPase rate and increases Ca 2+ sensitivity (although most of Ca2+ regulation
resides on thin filament (more later)
Crystal structure of the myosin head and neck
5 Cardiac Isoforms
In the human heart, there are two essential MLC isoforms, the MLC1sb (ventricle) and
MLC1sa atrium). Both are regarded as slow.
- the developing human embryo expresses MLCemb/atria in the whole heart and
skeletal muscle. MLC-1emb/atrial protein levels decrease in ventricles to undetectable
levels during early postnatal development (due to thyroid hormone) but persistent
expression occurs in the atrium throughout adult life.