Myosin - essential motor protein
Myosins are a superfamily of motor proteins found in nearly all eukaryotic cells. They play vital roles in a wide range of cellular processes by converting chemical energy from ATP hydrolysis into mechanical force. This enables them to move along actin filaments, a component of the cytoskeleton. While best known for their role in muscle contraction in animals, myosins are also crucial for intracellular transport, cell division, and other non-muscle functions across different species.
Structure and Function:
Most myosins share a common structure consisting of:
Head (motor) domain: Binds to actin and hydrolyzes ATP to produce movement.
Neck domain: Acts as a lever arm and often contains light chains (like calmodulin).
Tail domain: Determines cargo specificity and cellular localization.
Types and Classes:
Myosins are classified into multiple families (currently over 35 classes), based on sequence and structural features. Key examples:
Myosin II: Found in muscle and non-muscle cells; responsible for contraction. The classic example in humans is skeletal muscle myosin.
Myosin I: Involved in membrane tension and endocytosis.
Myosin V: A processive motor important in organelle and vesicle transport.
Myosin VI: Unique in moving toward the minus-end of actin filaments, involved in endocytosis.
Myosin X: Functions in filopodia formation and cell motility.
Human Myosins:
In humans, myosins are essential for:
Muscle contraction (e.g., Myosin II in skeletal, cardiac, and smooth muscle)
Cell migration and adhesion
Intracellular trafficking of organelles and vesicles
Hearing (e.g., Myosin VIIa in stereocilia)
Cell division (cytokinesis)
Mutations in myosin genes are linked to several diseases, including:
Hypertrophic cardiomyopathy
Deafness
Neurological disorders
Non-Human Myosins:
1. Invertebrates (e.g., Drosophila, C. elegans):
Possess homologous myosins with similar roles in development, neuronal transport, and morphogenesis.
Some unique isoforms are tailored to specialized tissues like the flight muscles of insects.
2. Fungi (e.g., Saccharomyces cerevisiae):
Use Myosin I and Myosin V for cargo transport and organelle positioning.
Fungal myosins help in polarized growth during budding and mating.
3. Plants (e.g., Arabidopsis thaliana):
Lack myosin II but have expanded families of Myosin XI and VIII.
These myosins support cytoplasmic streaming, vesicle movement, and development of root hairs and pollen tubes.
4. Protozoa (e.g., Plasmodium falciparum, Trypanosoma brucei):
Myosins are involved in gliding motility, host cell invasion, and division.
Targeting parasite-specific myosins offers potential in drug development.
5. Vertebrates (e.g., mice, zebrafish, birds):
Myosins are conserved and studied extensively in model organisms for insights into muscle biology, development, and genetic diseases.
Zebrafish models are used to study myosin mutations affecting cardiac or skeletal muscle.
Evolutionary Insight:
The diversification of myosin classes reflects evolutionary adaptation to organism-specific needs. Despite sequence divergence, the motor mechanism is largely conserved, underlining the ancient origin of this protein family.
Myosins are essential, versatile proteins found across eukaryotes. Their functions extend far beyond muscle contraction, encompassing nearly every aspect of cell structure and movement. The study of both human and non-human myosins continues to advance our understanding of cell biology, evolution, and disease mechanisms.