Myosin

Myosins are a family of motor proteins that help cells move and change shape. They play a big role in muscle contraction, which is how our muscles work when we move. These proteins need a special energy source called ATP to do their job, and they work together with another protein called actin to create movement in cells.

The first myosin was discovered in 1864 by a scientist named Wilhelm Kühne. He found a sticky protein in skeletal muscle that helped keep the muscle tight, and he named it myosin. Since then, scientists have found many different types of myosin in many kinds of living cells.

In 1973, scientists found enzymes that work like myosin in a tiny organism called Acanthamoeba castellanii. This discovery showed that myosin is found in many different types of cells, not just muscle cells. Today we know that almost every cell in our body has some form of myosin, and these proteins look and work similarly in all kinds of animals and other living things.

Structure and functions

Most myosin molecules have three main parts: a head, a neck, and a tail. The head domain binds to actin filaments and uses ATP to generate force, allowing it to move along the filament. The neck domain connects the head to the tail and can also bind to special proteins that help control the motor. The tail domain helps the myosin interact with other molecules or parts of itself.

In skeletal muscle, many myosin II molecules work together to create contraction through a process called the power stroke. This process uses energy from ATP to pull on actin filaments, causing the muscle to shorten. After releasing some molecules, myosin can bind to actin again and repeat the movement, helping muscles contract and move.

Nomenclature, evolution, and the family tree

The many types of myosin proteins found in eukaryotic organisms have different names, making it tricky to compare them. The most well-known myosin is the one in skeletal muscle, which was the first to be discovered. This myosin is a key part of muscle fibers and helps form structures called sarcomeres. Similar types of myosin are found in heart muscle, smooth muscle, and even in cells that aren’t muscle.

Starting in the 1970s, scientists found new types of myosin in simple organisms. These are called "unconventional myosins" and they act alone rather than in groups. They have been found in many types of tissues, not just muscle. These myosins are grouped by their genetic relationships, and each group is given a Roman numeral, like Myosin I, Myosin II, and so on. They also have different tail parts, which suggest they have unique jobs. Over 40 different myosin genes exist in the human genome.

Myosin proteins can move at different speeds depending on their shape. When they break down ATP, it causes a "power stroke" that moves the protein. The length of a part called the lever arm decides how far the cargo moves with each step. Longer lever arms move cargo farther with each step. The speed of myosin depends on how quickly it goes through the steps of using ATP.

Myosin classes

Myosin I

Myosin I is found everywhere in cells and works alone. It helps move small packages called vesicles and is important for how our ears hear.

Myosin II

Myosin II is the type that makes muscles contract. It is found in muscle cells and also in non-muscle cells. Myosin II has two long chains that form heads and tails. These tails twist together like snakes. It also has smaller chains called light chains that help it work.

Myosin III

Myosin III is not well understood. It has been studied in the eyes of fruit flies and may help with seeing. A similar version exists in humans and is found in the retina and inner ear.

Myosin V

Myosin V moves things like RNA and organelles from the center of the cell to the edges. It walks along special strings inside cells called actin filaments.

Myosin VI

Myosin VI also moves along actin filaments but in the opposite direction compared to Myosin V. It is thought to help move small packages into cells.

Myosin VII

Myosin VII has special parts in its tail and is needed for certain processes in cells, like helping cells eat other cells and forming structures in the ears of animals.

Myosin VIII

Myosin VIII is found in plants and helps with cell division and moving things between cells.

Myosin IX

Myosin IX is a single-headed motor protein. Its direction of movement along actin filaments is not fully understood.

Myosin X

Myosin X works in pairs and is found in the finger-like projections of cells. It moves towards the ends of actin filaments and may prefer to move on groups of these filaments.

Myosin XI

Myosin XI moves organelles like chloroplasts and mitochondria in plant cells. It helps move chloroplasts based on light and is important for root growth in plants.

Myosin XIV

Myosin XIV is found in certain parasites and helps them invade host cells. It is also found in a type of single-celled organism where it helps move certain structures.

Myosin XV

Myosin XV is needed for the development of structures in the inner ear that help with hearing.

Myosin XVIII

Myosin XVIII may help keep certain cells connected together.

Myosin XIX

Myosin XIX is linked to mitochondria, the powerhouses of cells.

Genes in humans

Humans have many genes related to myosin, but not all of them are active. These genes are grouped into classes, such as Class I, Class II, and others, each with specific names like MYO1A and MYO5A.

Myosin also includes special partners called light chains, which have their own unique properties. These light chains, like MYL1 and MYL2, work together with myosin to help it function properly.

Paramyosin

Paramyosin is a large protein found in many different animals without backbones, such as clams and insects. It works together with myosin, a protein that helps muscles contract, by forming a core inside muscle fibers. This allows some animals, like clams, to stay closed for long periods without using much energy.

Paramyosin can also be found in seafood. Recent studies have shown that when certain sea foods are digested, the paramyosin can break down into small pieces that may affect how some enzymes in our bodies work.