A structure named for this letter is used by retinol-binding protein to carry retinol and is used by porins to assist diffusion across the cell membrane. This letter partly names fragments of the membrane receptor APP (10[1])that accumulate in gray matter. Adrenergic receptors denoted by this letter (10[1])are antagonized (10[1])by medications that treat abnormal heart (-5[1])rhythms (10[1])by suppressing (10[1])the fight-or-flight response. The center of green fluorescent protein contains a "barrel" structure (-5[1])named for this letter. The top left of a Ramachandran plot represents a motif named for this letter which is abundant in amyloid plaques. That secondary structure denoted by this letter consists of several (10[1])adjacent polypeptide strands held together by hydrogen bonds. For 10 points, what Greek letter names a "sheet" protein motif (10[1])that's contrasted (10[1])with the alpha helix? ■END■ (10[1])