This is the N-terminal domain of the major tail protein, or knob protein, from Caudovirales [1].

1: Structural investigations of a Podoviridae streptococcus phage C1, implications for the mechanism of viral entry. Aksyuk AA, Bowman VD, Kaufmann B, Fields C, Klose T, Holdaway HA, Fischetti VA, Rossmann MG; Proc Natl Acad Sci U S A. 2012;109:14001-14006. PMID:22891295