Phosphatidylserine Is the Signal for TAM Receptors and Their Ligands

Nature repeatedly repurposes, in that molecules that serve as metabolites, energy depots, or polymer subunits are atthe same time used to deliver signals within and between cells. The preeminent example of this repurposing is ATP, which functions as a building block for nucleic acids, an energy source for enzymatic reactions, a phosphate donorto regulate intracellular signaling, and a neurotransmitter to control the activity of neurons. A series of recent studies now consolidates the view that phosphatidylserine (PtdSer), a common phospholipid constituent of membrane bilayers, is similarly repurposed for use as a signal between cells and that the ligands and receptors of the Tyro3/Axl/Mer (TAM) fam ily of receptortyrosine kinases (RTKs) are prominent transducers of this signal.