The head binding domain found in the Phage P22 tailspike protein contains two regular beta-sheets, A and B, oriented nearly perpendicular to each other and composed of five and three strands respectively. The topology of the strands is exclusively antiparallel. The tailspike protein trimerises through this domain, and the direction of the strands with respect to the molecular triad is almost parallel for beta-sheet A, whereas beta-sheet B is perpendicular to the triad, forming a dome-like structure. This domain is dispensable for thermostability and SDS resistance of the intact protein, and its deletion has only minor effects on tailspike folding kinetics [1].

1: Phage P22 tailspike protein: crystal structure of the head-binding domain at 2.3 A, fully refined structure of the endorhamnosidase at 1.56 A resolution, and the molecular basis of O-antigen recognition and cleavage. Steinbacher S, Miller S, Baxa U, Budisa N, Weintraub A, Seckler R, Huber R; J Mol Biol. 1997;267:865-880. PMID:9135118