Tropomyosin is a two-stranded alpha-helical, coiled coil protein found in many animal and fungal cells. In animals, it is an important component of the muscular system which works in conjunction with troponin to regulate muscle contraction. It is present in smooth and striated muscle tissues, which can be found in various organs and body systems, including the heart, blood vessels, respiratory system, and digestive system. In fungi, tropomyosin is found in cell walls and helps maintain the structural integrity of cells.
Tropomyosin is also found in other eukaryotes, but not in plants. Overall, tropomyosin is an important protein that plays a vital role in the proper functioning of many different organisms.
Tropomyosin and the actin skeleton
thumb|336x336px|Cardiac sarcomere structure featuring tropomyosin
All organisms contain organelles that provide physical integrity to their cells. These types of organelles are collectively known as the cytoskeleton, and one of the most ancient systems is based on filamentous polymers of the protein actin. A polymer of a second protein, tropomyosin, is an integral part of most actin filaments in animals.
Tropomyosins are a large family of integral components of actin filaments that play a critical role in regulating the function of actin filaments in both muscle and nonmuscle cells. These proteins consist of rod-shaped coiled-coil hetero- or homo-dimers that lie along the α-helical groove of most actin filaments. Interaction occurs along the length of the actin filament, with dimers aligning in a head-to-tail fashion.
Tropomyosins are often categorised into two groups: muscle tropomyosin isoforms and nonmuscle tropomyosin isoforms. Muscle tropomyosin isoforms are involved in regulating interactions between actin and myosin in the muscle sarcomere and play a pivotal role in regulated muscle contraction. Nonmuscle tropomyosin isoforms function in both muscle and nonmuscle cells, and are involved in a range of cellular pathways that control and regulate the cell's cytoskeleton and other key cellular functions.
The actin filament system that is involved in regulating these cellular pathways is more complex than the actin filament systems that regulates muscle contraction. The contractile system relies upon four actin filament isoforms and five tropomyosin isoforms, whereas the actin filament system of the cytoskeleton uses two actin filament isoforms and over 40 tropomyosin isoforms.
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Isoforms and evolution
In direct contrast with the 'one gene, one polypeptide' rule, it is now known from a combination of genomic sequencing, such as the Human Genome Project and EST data of expressed proteins, that many eukaryotes produce a range of proteins from a single gene. This plays a crucial role in the functionality of higher eukaryotes, with humans expressing more than five times as many different proteins (isoforms) as genes through alternative splicing. From a mechanistic point of view, it is much easier for an organism to expand on a current gene/protein family (creating protein isoforms) than it is to create an entirely new gene.
From an evolutionary point of view, tropomyosins in higher eukaryotes are notable in retaining all four of the potential genes produced by the dual genomic duplication event that took place in early eukaryotic evolution.