1H, 13C, and 15N NMR resonance assignments, secondary structure, and backbone topology of a variant of human interleukin-3.

Interleukin-3 (IL-3) is a cytokine which stimulates the proliferation and differentiation of hematopoietic progenitors into multiple cell lineages. The 1H, 15N, and 13C NMR resonances of a recombinant human IL-3 variant (SC-65369) have been assigned using two- and three-dimensional NMR techniques on uniformly 13C/15N-enriched protein. Five helical segments (residues 16-26, 42-50, 55-65, 73-82, and 104-120) and three reverse turns (residues 51-54, 68-71, and 87-90) were identified from the pattern of sequential NOE connectivities, NH(i)-C alpha H(i) scalar coupling constants (3JNH alpha), amide hydrogen exchange data, and the deviation of 13C alpha, 13C beta, 13CO, and C alpha H chemical shifts from random-coil values. Long-range NOEs indicate that the global folding pattern of human IL-3 is a four-helical bundle with an up-up-down-down arrangement of helices that is similar to that of other members of the cytokine family, such as granulocyte-macrophage colony stimulating factor (GM-CSF). A fifth short helix (helix A', residues 42-50) is located in the loop connecting the first and second helices. The absence of helix A' in the corresponding structures of GM-CSF and interleukin-5 suggests that it may be important for recognition of IL-3 by its receptor. The existence of at least two forms of the protein that differ in local conformation was implied from the observation of a limited set of doubled resonances in which each doublet partner had a similar pattern of short-, medium-, and long-range NOEs. The majority of the doubled resonances were close in sequence or space to a proline-rich sequence, which suggested that the different conformational forms of SC-65369 may be caused by slow cis-trans isomerization of proline peptide bonds.