Tropomyosin

Tropomyosin is a two-stranded alpha-helical coiled coil protein found in cell cytoskeletons.

All organisms contain structures that provide physical integrity to their cells. These structures are collectively known as the cytoskeleton, and one of the most ancient systems is based on filamentous polymers of the protein actin. A second polymer of the 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 all cells, 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 4 actin filament isoforms and 5 tropomyosin isoforms, whereas the actin filament system of the cytoskeleton uses 2 actin filament isoforms and over 40 tropomyosin isoforms.

In direct contrast of the ‘one gene, one polypeptide’ rule, we now know 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 5 times as many different proteins (isoforms) through alternative splicing than they have genes. 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 4 of the potential genes produced by the dual genomic duplication event that took place in early eukaryotic evolution.

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