Autophosphorylation docking site Tyr-867 in Mer receptor tyrosine kinase allows for dissociation of multiple signaling pathways for phagocytosis of apoptotic cells and down-modulation of lipopolysaccharide-inducible NF-kappaB transcriptional activation

J Biol Chem. 2008 Feb 8;283(6):3618-3627. doi: 10.1074/jbc.M706906200. Epub 2007 Nov 26.


Efficient clearance of apoptotic cells is essential for tissue homeostasis, allowing for cellular turnover without inflammatory consequences. The Mer (Nyk and c-Eyk) receptor tyrosine kinase (Mertk) is involved in two aspects of apoptotic cell clearance by acting as a receptor for Gas6, a gamma-carboxylated phosphatidylserine-binding protein that bridges apoptotic and viable cells. First, Mertk acts in a bona fide engulfment pathway in concert with alphavbeta5 integrin by regulating cytoskeletal assemblages, and second, it acts as a negative regulator for inflammation by down-modulating pro-inflammatory signals mediated from bacterial lipopolysaccharide-Toll-like receptor 4 (TLR4) signaling, and hence recapitulating anti-inflammatory immune modulation by apoptotic cells. Here we describe Mertk post-receptor events that govern phagocytosis and cytoskeletal signaling are principally mediated by autophosphorylation site Tyr-867. Using the Mertk Y867F mutant and pharmacological inhibitors, we show that Tyr-867 is required for phosphatidylinositol 3-kinase and phospholipase Cgamma2 activation; their activation in turn elicits protein kinase C-dependent signals that act on the actin cytoskeleton. Although Mertk(Y867F) blocked the tyrosine phosphorylation of FAK on Tyr-861 and p130(cas) and also abrogated the phagocytosis of apoptotic cells, this mutant did not suppress lipopolysaccharide-inducible NF-kappaB transcription, nor was NF-kappaB activation dependent on the protein kinase C inhibitor, calphostin C. Finally, unlike the cytoskeletal events associated with Tyr-867 autophosphorylation, the trans-inhibition of NF-kappaB occurred in a postnuclear-dependent fashion independent of cytosolic IkappaB phosphorylation and p65/RelA sequestration. Taken together, these data suggest that Mertk has distinct and separable effects for phagocytosis and for resolving inflammation, providing a molecular rationale for how immune licensing and inflammation can be dissociated from phagocytosis in a single phagocytic receptor.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Apoptosis
  • Cytoskeleton / metabolism
  • Enzyme Inhibitors / pharmacology
  • Gene Expression Regulation*
  • Humans
  • Lipopolysaccharides / metabolism*
  • Models, Biological
  • NF-kappa B / metabolism*
  • Naphthalenes / pharmacology
  • Phagocytosis
  • Phosphorylation
  • Proto-Oncogene Proteins / chemistry*
  • Proto-Oncogene Proteins / metabolism*
  • Receptor Protein-Tyrosine Kinases / chemistry*
  • Receptor Protein-Tyrosine Kinases / metabolism*
  • Signal Transduction
  • Transcriptional Activation*
  • Tyrosine / chemistry*
  • c-Mer Tyrosine Kinase


  • Enzyme Inhibitors
  • Lipopolysaccharides
  • NF-kappa B
  • Naphthalenes
  • Proto-Oncogene Proteins
  • Tyrosine
  • MERTK protein, human
  • Receptor Protein-Tyrosine Kinases
  • c-Mer Tyrosine Kinase
  • calphostin C