This is a beta-sandwich immunoglobulin fold domain, which resembles the plexin-A2 C-terminal domain in structure. In baseplate upper protein (BppU, also known as ORF48) trimer, this domain plays part in surrounding the Dit hexameric core [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