(a) Surface representation of a segment of HBc CLPs (Böttcher and Nassal, 2018), with the subunits in one asymmetric unit highlighted in color (blue chain A, cyan chain B, yellow chain C, red chain D). (b) Representation of the AB-dimer of the asymmetric unit. The naming of the helices follows Wynne et al., 1999. Inter-dimer contacts are formed between the hand-regions. Inner dimer contacts in HBc can be stabilized by a disulfide-bridge between Cys61 in HBc. (c) PFAM alignment of HBc and DHBc. The alignment corresponds to Conserved Protein Domain Family pfam00906 Hepatitis_core, except that the primary sequences were adapted (lower case green letters) to match the isolate-specific DHBc and HBc proteins used here (Uniprot IDs: P0C6J7.1 and P03146.1, respectively). Identical residues are shown in red, lower case grey letters indicate non-aligning residues. Grey bars show the positions of HBc helices α1 to α5 as in pdb: 1QGT; HBc domain borders are indicated in dark green. DHBc sequence 62–134 (highlighted in yellow) is predicted to replace HBc sequence 71–89, i.e. the spike tip comprising the C terminal part of α3, the connecting loop (the c/e1 epitope) and the N terminal part of α4. DHBc sequence 78–122 (light-green box) represents the biochemically predicted and here directly confirmed extension domain at the spike tip. (d) Conservation of the extension domain sequence among hepadnaviral large type CPs: The indicated CP sequences with their Uniprot and/or NCBI accession numbers were aligned using Clustal Omega as implemented in SnapGene v. 5. Tibet Frog c refers to one of three published Tibetan Frog HBV sequences. Conservation is indicated by colored blocks, with dark blue indicating the lowest and dark red the highest conservation. Residues differing from the consensus are shown with green background. The avian HBV CP sequences are highly similar to each other, including in the extension domain (red box around DHBV16 CP aa V77-T122). The frog virus sequence only shares some interspersed key motifs; however, the sequence predicted to align with the DHBc extension domain is also exceptionally rich in proline residues (10 in 48 aa total) which also cluster mostly in the 5'- proximal part.