We investigated the coupling of the fMLP (N-formyl-l-methionyl-l-leucyl-l-phenylalanine; ‘chemotactic peptide’) receptor with phosphorylation of the actin-binding protein l-plastin in neutrophils. Using two-dimensional IEF (isoelectric focusing)/PAGE and MALDI–TOF (matrix-assisted laser desorption ionization–time-of-flight)-MS, l-plastin was identified as a major phosphoprotein in fMLP-stimulated neutrophils whose phosphorylation was dependent on phosphoinositide 3-kinase, PLD (phospholipase D) and PKC (protein kinase C) activity. Two fMLP receptor subtypes were identified in neutrophils, characterized by a distinct sensitivity to fMLP and antagonistic peptides. Both receptor subtypes induced the phosphorylation of l-plastin. l-plastin phosphorylation induced by low-affinity fMLP receptors involves an action of phosphoinositide 3-kinase, PLD and PKC isotypes. In contrast, none of these intermediates are utilized by high-affinity fMLP receptors in the phosphorylation of l-plastin. However, the PKC inhibitor Ro-31-8220 inhibits l-plastin phosphorylation induced by the high-affinity fMLP receptor. Thus, an as yet unknown Ro-31-8220-sensitive kinase regulates l-plastin phosphorylation in response to the high-affinity fMLP receptor. The results suggest a model in which receptor subtypes induce a similar endpoint event through different signal-transduction intermediates. This may be relevant in the context of cell migration in which one receptor subpopulation may become desensitized in a concentration gradient of chemoattractant.

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