Major Classes Of Natural Killer Cell Receptors

NK cell receptors act as sensors lor messages irofn o til si tie the cell. The NK cell uses these receptors, which are eategori/etl as either activating or inhibitory receptors, to monitor the lig-ands expressed by cells throughout the body. Activating receptors respond 10 ihe expression of exogenous ligands or ligands indicating that a target cell is pathologically "stressed" by initiating signals inside the NK ceil that enhance its ability to kill the target. These signals are referred to as positive signals because they drive NK cell cytolytic function. Inhibitory receptors respond to the expression of normal, endogenous ligands on target cells by initiating signals inside the NK ceil thai inhibit killing of (he targets. These signals are referred so as negative signals because they abrogate the signals initiated by activating receptors.

Once a message is defected by an NK cell receptor, ii must be relayed inside the cell. This signal transduction takes place as a series of interactions between both catalytieally-Scuve signaling molecules and catalyfically-inaciive adapter proteins. Protein-protein interactions arc mediated by domains within proteins that allow them to bind to specific motifs in other proteins. These motifs may be altered in the course of signaling, giving them more or less affinity for other members of the signaling pathway.

One of the most common interactions of this type is the binding of a Src homology 2 (SH2) domain-containing protein to a phosphorylated tyrosine [reviewed in 1 ]. For example, most of the pathways resulting in activation begin with receptor complexes that contain immunoreceptor tyrosine-based activation motifs (ITAM), while pathways resulting in inhibition usually begin with receptors that contain immunoreceptor tyrosine-based inhibitory motifs (ITIM) [reviewed in 2]. Both types of motifs become phosphorylated when the receptors bind their ligands, and this alteration increases their affinity for proteins containing SH2 domains that recognize the phosphorylated tyrosine in the context of the ITAM or the ITIM. The SH2 domain-containing proteins that bind these phosphorylated motifs are then activated by other modifications, and a signaling cascade is begun.

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