Overall Structure. A. Schematic representation of the intermolecular contacts established by Ub monomer A (blue) and B (pink). Non-crystallographic (yellow) or crystallographic (green) contacts are marked under the corresponding residue. A capital letter marks an interaction <3.2Å and a small letter a contact <4Å. B. Secondary structure elements are indicated above the respective sequence. The relative accessibility (acc) is calculated by DSSP and rendered as blue-colored boxes (from dark-blue: fully accessible, to white: buried). The hydropathic character of a sequence is calculated according to the algorithm of Kyte and Doolittle with a window of 3 and is rendered as red (hydrophobic) to blue (hydrophilic) boxes. C. The Ub non-covalent crystallographic dimer using the color scheme in A). Side-chains of residues involved in the association are represented as yellow sticks and labeled. N- and C-terminal ends of each Ub chain are indicated.