THE UP-AND-DOWN BETA-BARREL PROTEINS

The up-and-down beta-barrel is a common folding motif found frequently in proteins that bind and transport hydrophobic ligands. It is formed by an array of beta-strands arranged in an antiparallel manner with e ach strand hydrogen-bonded to neighboring strands nearly always adjace nt in the amino acid sequence. The arrangement is completed by forming hydrogen bonds between the first and last strands. The barrel motif s o formed produces interior and exterior components. Proteins belonging to this class of up-and-down beta-barrels are found typically to be l ipid-binding proteins in which the interior surface forms a cavity or pit that serves as the ligand binding region. Two evolutionarily disti nct but structurally related families of such carriers have been ident ified by comparing known crystal structures. One group found intracell ularly uses a 10-stranded beta-structure and a second family of protei ns typically found extracellularly utilizes an 8-stranded motif. The 1 0-stranded beta-barrels have a large, hydrophilic water-filled interio r cavity that serves as the ligand-binding domain. Hydophobic lipids s uch as fatty acids and retinoids bind within the cavity, totally seque stered from the external milieu. The 8-stranded beta-barrel proteins h ave a hydrophobic pit, which serves as the ligand-binding domain for c ompounds such as bilins and retinoids. The up-and-down beta-barrel mot if appears to be one of nature's primary choices for hydrophobic ligan d transport proteins.-LaLonde, J. M., Bernlohr, D. A., Banaszak, L. J. The up-and-down beta-barrel proteins.