Figure 1.

Structure of human HMGB1, a 25-kDa protein of 215 amino acids. Note that 20% of the residues are lysines and that the protein is organized in three domains made up by two positively charged DNA-binding structures (A and B box) and a negatively charged acidic tail composed of 30 glutamic and aspartic acids, exclusively. The A and B boxes are helical structures, partly covered by the tail, which is folded over the protein. There are two nuclear emigration signals in the proximal part of the A and B boxes, respectively, that can bind to the nuclear exportin CRM1. There are also two nuclear-localization signals, as indicated in the figure. The primary HMGB1 sequence is 98.5% identical in all mammals, and two of the three substitutions occur in the repetitive carboxyl terminus with switches of aspartic and glutamic acids. Truncation of the full-length HMGB1 demonstrates that the extracellular cytokine activity resides within the B box. This activity can be competitively inhibited by truncated A box protein. The cysteine in position 106 in the B box is indispensable for its cytokine role, given that oxidation or selective mutation of this residue abolishes the activity of HMGB1 signaling to activate cytokine release.