Killer activated receptor

Killer Activation Receptors (KARs) are receptors expressed on the plasmatic membrane of Natural Killer cells (NK cells). KARs work with inhibitory Killer-cell immunoglobulin-like receptors (KIRs), which inactivate them in order to regulate the NK cells functions on hosted or transformed cells. These two kinds of specific receptors have some morphological features in common, such as being transmembrane proteins. The similarities are specially found in the extracellular domains and, therefore, the differences tend to be in the intracellular domains. KARs and KIRs can only do their function in presence of immunoreceptors that contain tyrosine and have an activator or inhibitor function (they are called ITAMs and ITIMs).

At first, it was thought that there were only one KAR and one KIR (two-receptor model). In the last decade, lots of different KARs and KIRs, such as NKp46 or NKG2D, have been discovered (opposing-signals model).

There are two different kinds of surface receptors which are responsible for triggering NK-mediated natural cytotoxicity: the NK KARs (meaning: Killer Activation Receptors) and the NK KIRs (meaning: Killer Inhibitory Receptors). Such receptors have a broad binding specificity and, therefore, are able to broadcast opposite signals. It is the balance between these competing signals that determines whether or not the cytotoxic activity of the NK cell should get started.

As KARs and KIRs are receptors with antagonic effects on NK cells, they have some structural characteristics in common. Firstly, both of them are usually transmembrane proteins. Apart from that, the extracellular domains of these proteins tend to have similar molecular features and are responsible for ligand recognition.

Therefore, the opposing functions these receptors have must be attributed to differences in their intracellular domains. KARs proteins possess positively charged transmembrane residues and short cytoplasmic tails that contain few intracellular signaling domains. In contrast, KIRs proteins usually have long cytoplasmic tails.

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