This is the N-terminal domain of baseplate upper proteins (BppU, also known as ORF48) which is found in phage from a number of different bacteria. The N-terminal domain exhibits a beta-sandwich immunoglobulin fold [1].

1: Structure of the phage TP901-1 1.8 MDa baseplate suggests an alternative host adhesion mechanism. Veesler D, Spinelli S, Mahony J, Lichiere J, Blangy S, Bricogne G, Legrand P, Ortiz-Lombardia M, Campanacci V, van Sinderen D, Cambillau C; Proc Natl Acad Sci U S A. 2012;109:8954-8958. PMID:22611190