THE TYROSINE CORNER - A FEATURE OF MOST GREEK KEY BETA-BARREL PROTEINS

The Tyr corner is a conformation in which a tyrosine (residue ''Y'') n ear the beginning or end of an antiparallel beta-strand makes an H bon d from its side-chain OH group to the backbone NH and/or CO of residue Y - 3, Y - 4, or Y - 5 in the nearby connection. The most common ''cl assic'' case is a Delta 4 Tyr corner (more than 40 examples listed), i n which the H bond is to residue Y - 4 and the Tyr (chi)1 is near -60 degrees. Y - 2 is almost always a glycine, whose left-handed beta or v ery extended beta conformation helps the backbone curve around the Tyr ring. Residue Y - 3 is in polyproline II conformation (often Pro), an d residue Y - 5 is usually a hydrophobic (often Leu) that packs next t o the Tyr ring. The consensus sequence, then, is LxPGxY, where the fir st x (the H-bonding position) is hydrophilic. Residues Y and Y - 2 bot h form narrow pairs of beta-sheet H-bonds with the neighboring strand. Delta 5 Tyr corners have a 1-residue insertion between the Gly and th e Tyr, forming a beta-bulge. One protein family has a Delta 4 corner f ormed by a His rather than a Tyr, and several examples use Trp in plac e of Tyr. For almost all these cases, the protein or domain is a Creek key beta-barrel structure, the Tyr corner ends a Creek key connection , and it is well-conserved in related proteins. Most low-twist Greek k ey beta-barrels have 1 Tyr corner. ''Reverse'' Delta 4 Tyr corners (H bonded to Y + 4) and other variants are described, all less common and less conserved. It seems likely that the more classic Tyr corners (De lta 4, Delta 5, and Delta 3 Tyr, Trp, or His) contribute to the stabil ity of a Creek key connection over a hairpin connection, and also that they may aid in the process of folding up Creek key structures.