This is the N-terminal domain of the major capsid protein in several dsDNA viruses [1].

1: The structure and evolution of the major capsid protein of a large, lipid-containing DNA virus. Nandhagopal N, Simpson AA, Gurnon JR, Yan X, Baker TS, Graves MV, Van Etten JL, Rossmann MG; Proc Natl Acad Sci U S A. 2002;99:14758-14763. PMID:12411581