Unraveling viral interleukin-6 binding to gp130 and activation of STAT-signaling pathways independently of the interleukin-6 receptor.
Human herpesvirus 8 encodes a viral version of interleukin-6 (vIL-6) which shows 25% sequence homology with human IL-6. In contrast to human IL-6, which first binds to the IL-6 receptor (IL-6R) and only subsequently associates with the signal transducing receptor subunit gp130, vIL-6 has been shown to directly bind to gp130 without the need of IL-6R. As a functional consequence, vIL-6 can activate far more target cells in the body since all cells express gp130, but only cells such as hepatocytes and some leukocytes express IL-6R. We sought to understand which amino acid sequences within the vIL-6 protein were responsible for its ability to bind and activate gp130 independent of IL-6R. As a first approach, we constructed chimeric IL-6 proteins in which all known gp130 interacting sites (sites II and III) were sequentially transferred from vIL-6 into the human IL-6 protein. To our surprise, human IL-6 carrying all gp130 interacting sites from vIL-6 did not show IL-6R-independent gp130 activation. Even more surprisingly, the loop between helix B and C of vIL-6, clearly shown in the crystal structure not to be in contact with gp130, is indispensable for direct binding to and activation of gp130. This points to an IL-6R induced change of site III conformation in human IL-6, which is already preformed in vIL-6. These data indicate a novel activation mechanism of human IL-6 by the IL-6R that will be important for the construction of novel hyperactive cytokine variants.
Adam
Nina
N
Institute of Biochemistry, Christian-Albrechts-University of Kiel, Olshausenstrasse 40, D-24098 Kiel, Germany.
Rabe
Björn
B
Suthaus
Jan
J
Grötzinger
Joachim
J
Rose-John
Stefan
S
Scheller
Jürgen
J
D000595:Amino Acid Sequence; D000818:Animals; D001665:Binding Sites; D019556:COS Cells; D002522:Chlorocebus aethiops; D050822:Cytokine Receptor gp130 / Q000378:metabolism*; D019288:Herpesvirus 8, Human / Q000235:genetics / Q000378:metabolism*; D006801:Humans; D015850:Interleukin-6 / Q000378:metabolism*; D051379:Mice; D008958:Models, Molecular; D011485:Protein Binding; D017434:Protein Structure, Tertiary; D019947:Receptors, Interleukin-6 / Q000378:metabolism*; D011993:Recombinant Fusion Proteins / Q000235:genetics / Q000378:metabolism; D014764:Viral Proteins / Q000235:genetics / Q000378:metabolism*
C508600:IL6 protein, human; C494199:IL6ST protein, human; D015850:Interleukin-6; D019947:Receptors, Interleukin-6; D011993:Recombinant Fusion Proteins; D014764:Viral Proteins; D050822:Cytokine Receptor gp130