Phosphorylation of interleukin-6 at serine 54: An early event in the secretory pathway in human fibroblasts

• Published in: Biochemical and biophysical research communications Vol. 185; no. 2; pp. 524 - 530
• Main Authors: May, Lester T., Sehgal, Pravin B.
• Format: Journal Article
• Language: English
• Published: Elsevier Inc 1992
• ISSN: 0006-291X; 1090-2104
• DOI: 10.1016/0006-291X(92)91656-B

The cytokine interleukin-6 (IL-6) is the major phosphoprotein secreted by human fibroblasts induced with interleukin-1α (IL-1α). We have determined that Ser 54 is the predominant site of phosphorylation on the fibroblast-derived IL-6 polypeptide; the amino acid motif surrounding this site is reminiscent of the target site for the Golgi-associated protein (casein) kinase. It has been shown earlier that the IL-6 polypeptide follows the classical secretory pathway where N-linked glycosylation is detectable within the first 15 minutes of labeling with [ 35S]-methionine and O-linked glycosylation occurs between 15–30 minutes after the start of polypeptide synthesis. Pulse-chase experiments using [ 32P]-orthophosphate or [ 35S]-methionine as tracers indicated that phosphorylation of IL-6 occured prior to its O-glycosylation suggesting that de novo synthesized IL-6 polypeptide is rapidly, perhaps even cotranslationally, phosphorylated at an intravesicular site (in the endoplasmic reticulum and/or Golgi). When IL-1α-induced fibroblasts were exposed to cycloheximide there was enhancement of the net de novo synthesis and secretion of IL-6 as followed by [ 35S]-methionine labeling (“superinduction”) but the secreted cytokine was no longer phosphorylated as monitored by [ 32P] labeling. Thus, phosphorylation of the IL-6 polypeptide is not an obligatory requirement for secretion of this cytokine. Furthermore, IL-6 phosphorylation is inhibited by cycloheximide through a mechanism different from the drug's effects on polypeptide synthesis per se.