The structure of human plasma retinol-binding protein (RBP) is an up-and-down barrel. A retinol molecule, vitamin A (yellow), is bound inside the barrel, between the two sheets, such that its only hydrophilic part (an OH tail) is at the surface of the molecule .
This motif is formed when one of the connections of four antiparallel strands is not a hairpin connection.
The motif occurs when strand number n is connected to strand n + 3 (a) or n - 3 (b) instead of n + 1 or n - 1 in an eight-stranded antiparallel sheet or barrel. The two different possible connections give two different hands of the Greek key motif.
In all protein structures known so far, only the hand shown in (a) has been observed.
Beta strands labeled A-G of the constant and variable domains of immunoglobulins have the same topology and similar structures. There are two extra strands, C' and C'' (red) in the variable domain. The loop between these strands contains the hyper-variable region CDR2. The remaining CDR regions are at the same end of the barrel in the loops connecting strands B and C and strands F and G.
The two domains of the complete molecule have the same topology; each is composed of two Greek key motifs that are joined by a short loop region.
There is a greater amino acid sequence homology between the domains than the motifs within each domain, suggesting that the four Greek Key motifs in -crystallin are evolutionarily related by gene duplication and fusion.
Evidence for two gene duplication events in -crystallin evolution
Two domains have about 40% sequence identity
Two motifs within the domain share 20-30% sequence identity
Barrel is distorted and with helices instead of some loops
Example: Rhinovirus (common cold)
The Globular Head of the Hemagglutinin Subunit is a Distorted Jelly Roll Structure
strand 1 contains a long insertion, and strand 8 contains a bulge in the corresponding position. Each of these two strands is therefore subdivided into shorter strands. The loop region between strands 3 and 4 contains a short helix, which forms one side of the receptor binding site (yellow circle).
Comparison of all those -barrels Up-and-down -crystallin-like jelly-roll
As shown in (a), two of the sheets (blue and yellow) are parallel to each other and are perpendicular to the third (green). In (b), each structural unit is composed of three strands connected by three loop regions (labeled a, b and c).
Loop a (red) is invariably composed of only two residues, whereas the other two loop regions vary in length .
Unlike two-sheet beta helices, there are no repetitive sequence patterns