Evidence that natural murine soluble interleukin 4 receptors may act as transport proteins.
The present studies were undertaken to determine whether the interleukin 4 binding proteins (IL-4BPs) previously identified in the biological fluids of mice are soluble forms of IL-4Rs. We also studied the binding properties of IL-4BPs in order to gain insight into their physiological role in vivo. Affinity-purified IL-4BPs and recombinant soluble IL-4Rs generated similar one-dimensional (Cleveland) peptide maps after digestion with either Staphylococcus aureus V8 protease or trypsin, indicating structural similarities. Furthermore, a rat mAb directed against the murine IL-4Rs immunoprecipitated the IL-4BPs and completely inhibited binding of 125I-IL-4 to a purified preparation of IL-4BPs. Taken together these data indicate that the IL-4BPs are soluble IL-4Rs. At 4 degrees C the IL-4BPs competitively inhibited the binding of IL-4 to membrane IL-4Rs but their ability to prevent binding of IL-4 to cells at 37 degrees C, at the same concentrations, was significantly reduced. Kinetic binding studies of soluble IL-4BPs vs. membrane IL-4Rs disclosed important differences in their rates of dissociation from IL-4. Whereas dissociation at 4 degrees C was slow for both, dissociation of IL-4 from IL-BPs at 37 degrees C was considerably faster (t 1/2 of 2 min) than dissociation of IL-4 from membrane IL-4Rs (t 1/2 of approximately 69 min). Temperature-dependent changes in dissociation kinetics were reversible, and could not be accounted for by either inactivation of the IL-4BPs at 37 degrees C or receptor internalization. Additional experiments also demonstrated that when IL-4BPs bind to IL-4 at 37 degrees C, the IL-4/IL-4BPs complex can rapidly dissociate, allowing IL-4 to bind to membrane IL-4Rs. In addition, binding of IL-4 by the IL-4BPs protects IL-4 from proteolytic degradation. Taken together, these results suggest that the IL-4BPs are naturally occurring forms of soluble IL-4Rs and that some of their properties (fast dissociation kinetics and protection of IL-4 from proteolysis) are consistent with a potential role as carrier proteins for IL-4 in the circulation.
Fernandez-Botran
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Department of Microbiology, University of Texas Southwestern Medical Center, Dallas 75235.
Vitetta
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D000818:Animals; D000911:Antibodies, Monoclonal / Q000276:immunology; D001201:Ascites; D001692:Biological Transport; D002352:Carrier Proteins / Q000378:metabolism; D002460:Cell Line; D010450:Endopeptidases / Q000494:pharmacology; D066298:In Vitro Techniques; D015847:Interleukin-4 / Q000737:chemistry / Q000276:immunology / Q000378:metabolism*; D007700:Kinetics; D008565:Membrane Proteins / Q000378:metabolism; D051379:Mice; D019948:Receptors, Interleukin-4; D011975:Receptors, Mitogen / Q000737:chemistry / Q000276:immunology / Q000302:isolation & purification / Q000378:metabolism*; D012995:Solubility; D013696:Temperature
D000911:Antibodies, Monoclonal; D002352:Carrier Proteins; D008565:Membrane Proteins; D019948:Receptors, Interleukin-4; D011975:Receptors, Mitogen; D015847:Interleukin-4; D010450:Endopeptidases