Structure of Human IL10

IL-10 is an intercalated dimer of two subunits,37-39 each consisting of six amphipathic helices A-F (Fig. 1A). The polypeptide chains of each subunit contribute to both parts of the dimer. Helices A-D of one subunit form a distinctive six-helix domain with helices E' and F' through 180° rotation around two-fold axis (Fig. 1A). In addition, helices A, C, D, F' and A', C', D' and F of each domain form left-handed four-helix bundles which are signature elements of all helical cytokines.40 The structure of the IL-10 subunit is stabilized by two intramolecular disulfide bridges (Fig. 1), Cys12-Cys108 and Cys62-Cys114. The disulfides hold together helices A, C and D, forming a frame with a long depression in the middle. The internal surface of the frame is very hydrophobic; therefore, when the amphipathic helices E' and F' cover the depression (Fig. 1B), almost all (86%) of the hydrophobic residues of IL-10 are involved in the formation of the intradomain hydrophobic cores. The domains are kept together by two flexible polypeptide links, separated by 15 A from each other, allowing some degree of freedom to change the elbow angle between the domains. Comparison of crystal structures of human

Lupus Structure
Figure 1B. Stereo diagram of the dimer of IL-10, subunits are shown in violet and green, helices of the symmetry related subunit are labeled as A'-F'.

IL-10 crystallized in different crystal forms,37,39 Epstein-Barr virus protein BCRF1,27 which is a very close homolog of human IL-10 (85% identity), and CMV IL-10,28 indicated that the elbow angle may change easily, even due to different crystal packing.

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