Figure legends
Figure 1 –Schematic representation of the NK cell signalling
pathway. A) Upon binding of the killing activation receptor (KAR), the
ITAM motifs of the intracellular portion are phosphorylated by a member
of the Src family. Syk or ZAP70 are activated and further phosphorylate
the PI3K complex and activate VAV-1/-2/-3, PLCγ1 and PLCγ2. This will
further trigger a cytoskeleton reorganization and affect the calcium
flux into the cell, leading to an increase in Ca2+levels. This cascade results in the cytotoxic/cytolytic response of NK
cells, with the release of the cytotoxic granules (degranulation) and
production of cytokines/chemokines. B) Killing-inhibiting receptors
(KIRs), present on the surface of NK cells, may also be engaged by the
corresponding inhibiting ligands. These ITIM-bearing receptors are
subsequently phosphorylated by Src kinases. This leads to the
recruitment of SHP1/2 kinases that terminate the activation signalling
emanating from ITAMs, rendering the NK cells inactive. The ultimate
activation/inactivation result of these pathways is directly correlated
with the number of receptors engaged. Target cells expressing more
activation than inactivation ligands will engage more activation
receptors resulting in the transduction of more activation signals,
triggering the response of NK cells (Koch, Steinle, Watzl & Mandelboim,
2013; Linnartz-Gerlach, Kopatz & Neumann, 2014; Linnartz, Wang &
Neumann, 2010; Paul & Lal, 2017; Watzl & Long, 2010).
Figure 2 – NK cell
activation/inhibition equilibrium. The relative number of engaged
receptor-types dictates the response. A) Normal cells, expressing normal
MHC I molecules (inhibitory ligands) do not trigger the cytolytic
response of NK cells. B) Stressed cells, such as tumour cells, engaging
the same relative number of activating and inhibiting receptors of NK
cells, induce a dynamic equilibrium that results in no cytolytic
response of NK cells. C) If that equilibrium is tipped over by engaging
more activating receptors on NK cells, a cytolytic response will be
triggered against the cells presenting the corresponding ligands.
Figure 3 – Structure representation of NKp30. The two
antiparallel β-sheets (in yellow) and the two α-helixes (in red) are
shown. The two β-sheets are bridged by a disulphide bond between
residues Cys39 and Cys108. The stalk domain and the transmembrane
α-helix are not represented. Image generated from the published X-ray
structure of unbound NKp30 (PDB 3NOI) (Joyce, Tran, Zhuravleva,
Jaw, Colonna & Sun, 2011) .
Figure 4 – NKp30 molecular surface with A) B7-H6-contacting
residues (red) and N-glycosylation sites (blue) highlighted. Only one
possibly N-glycosylated residue (Asp68) is located near the identified
binding site, which comprises residues Ile50, Gly51, Ser52, Val53,
Leu80, Ser82, Phe85, Leu86, Glu111, Leu113 and Gly114, marked in red;
and B) B7-H6-contacting residues differentiated by type of amino acid
residue: polar residues (yellow) and hydrophobic residues (cyan).
Image generated with PyMOL(Schrodinger, 2015) from the published X-ray structure of unbound
NKp30 (PDB ID: 3NOI) (Joyce, Tran, Zhuravleva, Jaw, Colonna &
Sun, 2011) . NKp30 structure representation in its unbound (C, E) and
B7-H6-bound forms (D, F) are also shown. Isoleucine 50 is labelled as a
positioning reference. Conformation differences between the two forms
are observed in the distance between residues 52 and 82, labelled in
yellow, that differ by about 1 Å, and in the relative positions of
arginine 67 and asparagine 68. The loop composed by arginine 67 (blue)
and asparagine 68 (orange) seems to bend upon binding. Asparagine 68
moves away from proline 79 (in red) and arginine 67 bends down towards
the pocket, approaching valine 53 (not highlighted). Image generated
from the published X-ray structures of unbound NKp30 (PDB ID: 3NOI)(Joyce, Tran, Zhuravleva, Jaw, Colonna & Sun, 2011) and B7-H6
bound NKp30 (PDB ID: 3PV6) (Li, Wang & Mariuzza, 2011) .