Smooth muscle myosin is a specialized motor protein found within smooth muscle cells throughout the body. It converts chemical energy into mechanical force, driving muscle contraction. This protein is fundamental to smooth muscle’s ability to perform its various functions. Its precise actions allow for sustained and controlled contractions, distinct from those in skeletal or cardiac muscles.
Understanding Smooth Muscle Myosin
Smooth muscle is an involuntary muscle tissue found in the walls of many internal organs, including arteries, veins, the digestive tract, urinary bladder, and lung airways. Unlike skeletal muscle, smooth muscle lacks a striated appearance due to its less organized contractile proteins.
Smooth muscle myosin, specifically Myosin II, is the primary force-generating protein in these cells. Each Myosin II molecule has two heavy chains and four light chains. The heavy chains form a globular head, a neck, and a long coiled-coil tail. The head binds and hydrolyzes ATP for movement, the neck acts as a lever arm, and the tail domains allow myosin molecules to assemble into thick filaments.
The Contraction Mechanism
The contraction of smooth muscle, driven by smooth muscle myosin, involves a unique regulatory mechanism centered on phosphorylation. Unlike skeletal muscle, where calcium directly binds to troponin, smooth muscle contraction is initiated by an increase in intracellular calcium levels. This calcium then binds to a protein called calmodulin. The calcium-calmodulin complex subsequently activates myosin light chain kinase (MLCK).
MLCK phosphorylates the regulatory light chain of smooth muscle myosin at serine 19. This phosphorylation causes a conformational change in the myosin head, enabling it to bind to actin filaments and initiate the cross-bridge cycle. The myosin heads then pull on the actin filaments, leading to muscle shortening. This process is slower than in skeletal muscle, allowing for sustained contractions with less energy expenditure.
Following phosphorylation, the myosin head undergoes a power stroke, pulling the actin filament. ATP then binds to the myosin head, causing it to detach from actin. ATP is hydrolyzed into ADP and inorganic phosphate, re-energizing the myosin head. The phosphate is released, allowing the myosin head to reattach to a new site on the actin filament.
The cycle continues as long as the myosin light chain remains phosphorylated. This sustained attachment and slow cycling contribute to the “latch” state, where smooth muscle maintains tension with minimal ATP consumption.
Essential Bodily Functions
Smooth muscle myosin plays a role in many involuntary bodily functions.
Cardiovascular System
In the cardiovascular system, smooth muscle in blood vessel walls contracts and relaxes, regulating blood pressure and blood flow. Contraction reduces vessel diameter, increasing resistance and blood pressure, while relaxation widens vessels.
Digestive System
In the digestive system, smooth muscle myosin facilitates peristalsis, the wave-like contractions that propel food through the esophagus, stomach, and intestines. This action ensures efficient digestion and nutrient absorption.
Respiratory System
Rhythmic contractions of smooth muscle in the lung bronchioles control airway diameter, regulating airflow during breathing.
Childbirth
During childbirth, powerful, sustained contractions of uterine smooth muscle, driven by smooth muscle myosin, expel the baby.
Implications in Health and Disease
Dysregulation of smooth muscle myosin function can contribute to various health conditions.
Hypertension
In hypertension, or high blood pressure, excessive contraction of smooth muscle in arterial walls narrows blood vessels, leading to elevated systemic pressure. This persistent vasoconstriction strains the heart and blood vessels.
Respiratory Conditions
Respiratory conditions like asthma involve abnormal smooth muscle contraction in the airways, causing bronchoconstriction and making breathing difficult. This narrowing restricts airflow, leading to symptoms such as wheezing and shortness of breath.
Gastrointestinal Disorders
Gastrointestinal disorders, including irritable bowel syndrome (IBS), can involve altered smooth muscle motility, resulting in symptoms like abdominal pain, cramping, and irregular bowel habits.
Bladder Dysfunction
Bladder dysfunction, such as overactive bladder, may arise from abnormal smooth muscle contraction in the bladder wall, leading to frequent and urgent urination.