Beta-propeller
In structural biology, a beta-propeller (β-propeller) is a type of all-β protein architecture characterized by 4 to 8 highly symmetrical blade-shaped beta sheets arranged toroidally around a central axis. Together the beta-sheets form a funnel-like active site. Structure. Each beta-sheet typically has four anti-parallel β-strands arranged in the beta-zigzag motif. The strands are twisted so that the first and fourth strands are almost perpendicular to each other. There are five classes of beta-propellers, each arrangement being a highly symmetrical structure with 4–8 beta sheets, all of which  generally form a central tunnel that yields pseudo-symmetric axes. While, the protein's official active site for ligand-binding is formed at one end of the central tunnel by loops between individual beta-strands, protein-protein interactions can occur at multiple areas around the domain. Depending on the packing and tilt of the beta-sheets and beta-strands, the beta-propeller may have a central pocket in place of a tunnel. The beta-propeller structure is stabilized mainly through hydrophobic interactions of the beta-sheets, while additional stability may come from hydrogen bonds formed between the beta-sheets of  the C- and N-terminal ends. In effect this closes the circle which can occur even more strongly in 4-bladed proteins via a disulfide bond. The chaperones Hsp70 and CCT have been shown to sequentially bind nascent beta-propellers as they emerge from the ribosome. These chaperones prevent non-native inter-blade interactions from forming until the entire beta-propeller is synthesized. Many beta-propellers are dependent on CCT for expression. In at least one case, ions have been shown to increase stability by binding deep in the central tunnel of the beta-propeller. Murzin proposed a geometric model  to describe the structural principles of the beta propeller. According to this model the seven bladed propeller was the most favored arrangement in geometric terms. Despite its highly conserved nature, beta-propellers are well known for their plasticity. Beyond having a variety of allowed beta-sheets per domain, it can also accommodate other domains into its beta-sheets. Additionally, there are proteins that have shown variance in the number of beta-strands per beta-sheet. Rather than having the typical four beta-strands in a sheet, beta-lactamase inhibitor protein-II only has three beta-strands per sheet while the  phytase of "Bacillus subtilis" has five beta-strands per beta-sheet. Function. Due to its structure and plasticity, protein-protein interactions can form with the top, bottom, central channel, and side faces of the beta-propeller. The function of the propeller can vary based on the blade number. Four-bladed beta-propellers function mainly as transport proteins, and because of its structure, they have a conformation that is favorable for substrate binding. Unlike larger beta-propellers, four-bladed beta-propellers usually cannot perform catalysis themselves, but act instead to aid in catalysis by performing the aforementioned functions. Five-bladed  propellers can act as transferases, hydrolases, and sugar binding proteins. Six- and seven-bladed propellers perform a much broader variety of functions in comparison to four- and five-bladed propellers. These functions can include acting as ligand-binding proteins, hydrolases, lyases, isomerases, signaling proteins, structural proteins, and oxidoreductases. Variations in the larger (five- to eight-bladed) beta-propellers can allow for even more specific functions. This is the case with the C-terminal region of GyrA which expresses a positively charged surface ideal for binding DNA. Two alpha-helices coming out of the six-bladed beta-propeller of serum paraoxonase  may provide a hydrophobic region ideal for anchoring membranes. DNA damage-binding protein 1 has three beta-propellers, in which the connection between two of the propellers is inserted into the third propeller potentially allowing for its unique function. Examples. Domains. Repeat domains known to fold into a beta-propeller include WD40, YWTD, Kelch, YVTN, RIVW (PD40), and many more. Their sequences tend to group together, suggesting a close evolutionary link. They are also related to many beta-containing domains.