Beta barrel
In protein structures, a beta barrel (β barrel) is a
In many cases, the strands contain alternating
All beta-barrels can be classified in terms of two integer parameters: the number of strands in the beta-sheet, n, and the "shear number", S, a measure of the stagger of the strands in the beta-sheet.[3] These two parameters (n and S) are related to the inclination angle of the beta strands relative to the axis of the barrel.[4][5][6]
Types
Up-and-down
Up-and-down barrels are the simplest barrel topology and consist of a series of beta strands, each of which is hydrogen-bonded to the strands immediately before and after it in the
Jelly roll
The jelly roll fold or barrel, also known as the Swiss roll, typically comprises eight beta strands arranged in two four-stranded sheets. Adjacent strands along the sequence alternate between the two sheets, such that they are "wrapped" in three dimensions to form a barrel shape.
Examples
Porins
Sixteen- or eighteen-stranded up-and-down beta barrel structures occur in porins, which function as transporters for ions and small molecules that cannot
Preprotein translocases
Beta barrels also function within endosymbiont derived organelles such as mitochondria and chloroplasts to transport proteins.[8] Within the mitochondrion two complexes exist with beta barrels serving as the pore forming subunit, Tom40 of the Translocase of the outer membrane, and Sam50 of the Sorting and assembly machinery. The chloroplast also has functionally similar beta barrel containing complexes, the best characterised of which is Toc75 of the TOC complex (Translocon at the outer envelope membrane of chloroplasts).
Lipocalins
Lipocalins are typically eight-stranded up-and-down beta barrel proteins that are secreted into the extracellular environment. A distinctive feature is their ability to bind and transport small hydrophobic molecules in the barrel
Shear number
A piece of paper can be formed into a cylinder by bringing opposite sides together. The two edges come together to form a line. Shear can be created by sliding the two edges parallel to that line. Likewise, a beta barrel can be formed by bringing the edges of a beta sheet together to form a cylinder. If those edges are displaced, shear is created.
A similar definition is found in geology, where
The determination of shear number requires the assumption that each amino acid in one strand of a beta sheet is adjacent to just one amino acid in the neighboring strand (this assumption may not hold if, for example, a beta bulge is present).[12] To illustrate, S will be calculated for green fluorescent protein. This protein was chosen because the beta barrel contains both parallel and antiparallel strands. To determine which amino acid residues are adjacent in the beta strands, the location of hydrogen bonds is determined.
The inter-strand hydrogen bonds can be summarised in a table. Each column contains the residues in one strand (strand 1 is repeated in the last column). The arrows indicate the hydrogen bonds that were identified in the figures. The relative direction of each strand is indicated by the "+" and "-" at the bottom of the table. Except for strands 1 and 6, all strands are antiparallel. The parallel interaction between strands 1 and 6 accounts for the different appearance of the hydrogen bonding pattern. (Some arrows are missing because not all of the hydrogen bonds expected were identified. Non-standard amino acids are indicated with "?") The side chains that point to the outside of the barrel are in bold.
If no shear were present in this barrel, then residue 12 V, say, in strand 1 should end up in the last strand at the same level as it started at. However, because of shear, 12 V is not at the same level: it is 14 residues higher than it started at, so its shear number, S, is 14.
See also
- OMPdb (2011)
References
Further reading
- Branden C, Tooze J (1999). Introduction to protein structure (2nd ed.). New York: Garland Pub. ISBN 978-0-8153-2304-4.
- Michalik M, Orwick-Rydmark M, Habeck M, Alva V, Arnold T, Linke D (2017). "An evolutionarily conserved glycine-tyrosine motif forms a folding core in outer membrane proteins". PLOS ONE. 12 (8): e0182016. PMID 28771529.
- Hayward S, Milner-White EJ (October 2017). "Geometrical principles of homomeric β-barrels and β-helices: Application to modeling amyloid protofilaments" (PDF). Proteins. 85 (10): 1866–1881. S2CID 206410314.
External links
- Explanation of all-beta topologies: "orthogonal SCOPclassification.
- all-beta folds in SCOP database (folds 54 to 100 are water-soluble beta-barrels).
- CATH database - folds and homologous superfamilies within the beta-barrel architecture.
- General classification and images of protein structures from Jane Richardson lab
- Images and examples of transmembrane beta-barrels
- Stockholm Bioinformatics Center review of transmembrane proteins
- The Lipocalin Website
- The OMPdb database for beta-barrel proteins