Summary The extra-terminal (ET) domain of BRD3 is conserved among BET proteins (BRD2, BRD3, BRD4), interacting with multiple host and viral protein-protein networks. Solution NMR structures of complexes formed between BRD3-ET domain with either the 79-residue murine leukemia virus integrase (IN) C-terminal domain (IN 329-408 ), or its 22-residue IN tail peptide (TP) (IN 386-407 ) alone, reveal similar intermolecular three-stranded β-sheet formation. 15 N relaxation studies reveal a 10-residue linker region (IN 379-388 ) tethering the SH3 domain (IN 329-378 ) to the ET-binding motif (IN 389-405 )-ET complex. This linker has restricted flexibility, impacting its potential range of orientations in the IN - nucleosome complex. The complex of the ET-binding peptide of host NSD3 protein (NSD3 148-184 ) and BRD3-ET domain includes a similar three-stranded β-sheet interaction, but the orientation of the β−hairpin is flipped compared to the two IN : ET complexes. These studies expand our understanding of molecular recognition polymorphism in complexes of ET-binding motifs with viral and host proteins. Highlights The BRD3 ET domain binds to key peptide motifs of diverse host and viral proteins. These complexes reveal conformational plasticity in molecular recognition. NMR studies demonstrate restricted interdomain motion in the IN CTD / ET complex. A cost-effective approach is described for producing isotopically-labeled peptides. Etoc Blurb We address structurally how the MLV Integrase (IN) usurps the host function of the BET protein through comparative studies of the IN : Brd3 ET complex with that of the host NSD3. MLV integration and thus its pathogenesis is driven through protein interactions of the IN : BET family.