Analysis of a Bacterial Lipopolysaccharide‐Activated Serine Kinase That Phosphorylates p65/L‐Plastin in Macrophages

We previously identified p65/L-plastin as a phosphorylated protein in LPS-stimulated macrophages and determined its phosphorylation site. In vitro kinase assay using peptide substrates revealed that LPS-stimulated kinase activity selectively phosphorylated their serine-5 (Ser-5) residue. Kinase inhibitors for cAMP-dependent kinase such as H-89 inhibited the Ser-5 phosphorylation, but cAMP was not essential for the kinase activity. The LPS-stimulated kinase activity in cytosol fractions of macrophages was recovered as a sharp peak by anion exchange chromatography. These findings suggest that an as yet unknown H-89-sensitive serine kinase is rapidly activated by LPS stimulation and then phosphorylates p65/L-plastin, playing a vital role in macrophage activation.