Structure
Primary sequence analysis of OSM allocates it to the gp130 group of cytokines. OSM most resembles LIF, bearing 22% sequence identity and 30% similarity. Incidentally the genes for OSM and LIF occur in tandem on human chromosome 22. Both LIF and OSM genes have very similar gene structures sharing similar promoter elements and intron-exon structure. These data suggest that OSM and LIF arose relatively recently in evolutionary terms by gene duplication. Of the five cysteine residues within the human OSM sequence four participate in disulfide bridges, one of these disulfide bonds namely between helices A and B is necessary for OSM activity. The free cysteine residue does not appear to mediate dimerisation of OSM.
The three dimensional structure of human OSM has been solved to atomic resolution, confirming the predicted long chain four helix bundle topology. Comparing this structure with the known structures of other known LC cytokines shows it to be most closely related to LIF (RMSD of 2.1 Å across 145 equivalent Cα). A distinctive kink in the A helix arises from departure of the classical alpha helical H-bonding pattern, a feature shared with all known structures of LIFR utilising cytokines. This “kink” in effect results in a different special positioning of one extreme of the bundle to the other, significantly affecting the relative positioning of site III with sites I and II (see:Receptor recruitment sites)
Read more about this topic: Oncostatin M
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