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( Myosin)
Myosins are a large family of motor proteins found in eukaryotic tissues. They are responsible for actin-based motility. Most myosin molecules are composed of a head, neck, and tail domain. The wide variety of myosin genes found throughout the eukaryotic phyla were named according to different schemes as they were discovered. The nomenclature can therefore be somewhat confusing when attempting to compare the functions of myosin proteins within and between organisms. Skeletal muscle myosin, the most conspicuous of the myosin superfamily due to its abundance in muscle fibers, was the first to be discovered. This protein makes up part of the sarcomere and forms macromolecular filaments composed of multiple myosin subunits. Similar filament-forming myosin proteins were found in cardiac muscle, smooth muscle, and non-muscle cells. However, beginning in the 1970s researchers began to discover new myosin genes in simple eukaryotes [2] encoding proteins that acted as monomers and were therefore entitled Class I myosins. These new myosins were collectively termed "unconventional myosins" [3] and have been found in many tissues other than muscle. These new superfamily members have been grouped according to phylogenetic relationships derived from a comparison of the amino acid sequences of their head domains, with each class being assigned a Roman numeral [4][5][6][7](see phylogenetic tree). The unconventional myosins also have divergent tail domains, suggesting unique functions[8]. The now diverse array of myosins likely evolved from an ancestral precursor (see picture).
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