Monomeric isomers of human interleukin 5 show that 1:1 receptor recruitment is sufficient for function.
The normally dimeric human interleukin 5 (IL-5) was re-engineered into two monomeric isomer forms to investigate mechanistic features of receptor recognition. One form, denoted GM1-IL-5, is a CD-loop expanded form, in which an 8-residue linker designed for flexibility was inserted between residues 85 and 86. The second, denoted DABC-IL-5, is a circularly permuted form of human IL-5 in which a chain discontinuity was introduced in the CD loop and the two consequent chain fragments were joined at the normal N and C termini by a di-glycyl linker. Both IL-5 isomers folded into stable monomers in solution as shown by sedimentation equilibrium and CD and formed an intrachain disulfide bond predicted from the structure of wild type IL-5. From titration microcalorimetry and optical biosensor analyses, both monomers were shown to interact with the IL-5 receptor alpha chain with 1:1 stoichiometry and affinities 30- to 40-fold weaker than for the dimeric wild type protein. And both monomers stimulated cell proliferation of human IL-5 receptor positive cells with a concentration dependence close to that of wild type. The data show that both monomeric and dimeric forms of IL-5 function through similar 1:1 receptor alpha chain recruitment processes and that it is the helical packing of the monomeric four-helix bundle unit in IL-5, rather than the helical connectivity itself, that appears to play the major role in presenting structural epitopes to trigger functional receptor activation.
Li
J
J
Department of Molecular Immunology, SmithKline Beecham Pharmaceuticals, 709 Swedeland Road, King of Prussia, PA 19406, USA.
Cook
R
R
Doyle
M L
ML
Hensley
P
P
McNulty
D E
DE
Chaiken
I
I
D001665:Binding Sites; D002460:Cell Line; D006801:Humans; D015848:Interleukin-5 / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D007536:Isomerism; D007700:Kinetics; D015202:Protein Engineering; D018123:Receptors, Interleukin / Q000378:metabolism*; D053649:Receptors, Interleukin-5
D015848:Interleukin-5; D018123:Receptors, Interleukin; D053649:Receptors, Interleukin-5