The interleukin-4 site-2 epitope determining binding of the common receptor XXXX .

@entity1 @entity9597 @entity16409 @entity3646 @entity493
@entity1 @entity3646 ( @entity3646 ), one of the small four-helix-bundle cytokines, uses the specific @entity3646 receptor a chain together with a promiscuous subunit, the @entity16409 ( @entity16409 ) for transmembrane signaling. The ligand-binding properties of @entity16409 , which are presently poorly understood, were analysed by biosensor techniques employing recombinant ectodomains of @entity16409 and a receptor chains ( @entity3646 -BP). The formation of a ternary complex between solute @entity16409 ectodomain and @entity3646 saturated @entity3646 -BP could be established to exhibit a high dissociation constant Kd = 3 microM and a short half life tau1/2 = 7 s. This binding affinity resulted to the major part from the interaction of @entity16409 ectodomain with @entity3646 and not from a direct contact of the ectodomains, since binding between solute @entity16409 ectodomain and @entity3646 could be established (Kd about 150 microM), whereas no binding was found between the @entity16409 ectodomain and @entity3646 -BP in the absence of @entity3646 . The @entity3646 epitope involved in @entity16409 ectodomain interaction (site 2) was identified by means of an @entity493 -scanning mutational approach. The @entity3646 site 2 comprised residues I11 and N15 on helix A together with Y124 on helix D as major binding determinants. The @entity3646 @entity493 variants at site 2 generally showed only moderate defects in biological activity. Even the most affected variant [A124] @entity3646 retained a partial agonist activity of about 50% during a T-cell proliferation assay. The dosis leading to a half-maximal response (EC50) was not altered by site-2 substitutions. The present results are in accordance with a two-step-dimerisation mechanism for @entity9597 activation, where solute @entity3646 at physiological concentrations binds first via the high-affinity site 1 to the a chain only, since the affinity of @entity3646 site 2 for @entity16409 is too low. This site-2 affinity seems to be sufficient, however, to promote, in a second step, a productive association of @entity16409 to an @entity3646 chain complex in the membrane.
