Alpha solenoid

An alpha solenoid (sometimes also known as an alpha horseshoe or as stacked pairs of alpha helices, abbreviated SPAH) is a protein fold composed of repeating alpha helix subunits, commonly helix-turn-helix motifs, arranged in antiparallel fashion to form a superhelix. Alpha solenoids are known for their flexibility and plasticity. Like beta propellers, alpha solenoids are a form of solenoid protein domain commonly found in the proteins comprising the nuclear pore complex. They are also common in membrane coat proteins known as coatomers, such as clathrin, and in regulatory proteins that form extensive protein-protein interactions with their binding partners. Examples of alpha solenoid structures binding RNA and lipids have also been described.

Terminology and classification

The term "alpha solenoid" has been used somewhat inconsistently in the literature. As originally defined, alpha solenoids were composed of helix-turn-helix motifs that stacked into an open superhelix. However, protein structural classification systems have used varying terminology; the Structural Classification of Proteins (SCOP) database describes these proteins using the term "alpha alpha superhelix". The CATH database uses the term "alpha horseshoe" for these proteins, and uses "alpha solenoid" for a somewhat different and more compact structure exemplified by the peridinin-chlorophyll binding protein.

Structure

Alpha solenoid proteins are composed of repeating structural units containing at least two alpha helices arranged in an antiparallel orientation.