Interaction of linker for activation of T cells with multiple adapter proteins in platelets activated by the glycoprotein VI-selective ligand, convulxin.
The snake venom toxin convulxin activates platelets through the collagen receptor glycoprotein VI (GPVI)/Fc receptor gamma-chain (FcR gamma-chain) complex leading to tyrosine phosphorylation and activation of the tyrosine Syk and phospholipase Cgamma2 (PLCgamma2). In the present study, we demonstrate that convulxin is a considerably more powerful agonist than collagen or the GPVI-selective collagen-related peptide (CRP). Confirmation that the response to convulxin is mediated solely via Syk was provided by studies on Syk-deficient platelets. The increase in phosphorylation of the FcR gamma-chain is associated with marked increases in tyrosine phosphorylation of downstream proteins including Syk, linker for activation of T cells (LAT), SLP-76, and PLCgamma2. The transmembrane adapter LAT coprecipitates with SLP-76 and PLCgamma2, as well as with a number of other adapter proteins, some of which have not been previously described in platelets, including Cbl, Grb2, Gads, and SKAP-HOM. Gads is constitutively associated with SLP-76 and is probably the protein bridging its association with LAT. There was no detectable association between Grb2 and SLP-76 in control or stimulated cells, suggesting that the interaction of LAT with Grb2 is present in a separate complex to that of LAT-Gads-SLP-76. These results show that the trimeric convulxin stimulates a much greater phosphorylation of the FcR gamma-chain and subsequent downstream responses relative to CRP and collagen, presumably because of its ability to cause a greater degree of cross-linking of GPVI. The adapter LAT appears to play a critical role in recruiting a number of other adapter proteins to the surface membrane in response to activation of GPVI, presumably at sites of glycolipid-enriched microdomains, enabling an organized signaling cascade that leads to platelet activation.
Asazuma
N
N
Department of Pharmacology, University of Oxford, Mansfield Road, Oxford OX1 3QT, United Kingdom.
Wilde
J I
JI
Berlanga
O
O
Leduc
M
M
Leo
A
A
Schweighoffer
E
E
Tybulewicz
V
V
Bon
C
C
Liu
S K
SK
McGlade
C J
CJ
Schraven
B
B
Watson
S P
SP
D048868:Adaptor Proteins, Signal Transducing; D001792:Blood Platelets / Q000187:drug effects* / Q000378:metabolism; D002352:Carrier Proteins / Q000502:physiology*; D003094:Collagen / Q000494:pharmacology; D003435:Crotalid Venoms / Q000494:pharmacology*; D002373:Cyclic AMP Receptor Protein / Q000494:pharmacology; D051380:GRB2 Adaptor Protein; D006801:Humans; D016023:Integrins / Q000378:metabolism*; D047908:Intracellular Signaling Peptides and Proteins; D037181:Lectins, C-Type; D008565:Membrane Proteins; D010750:Phosphoproteins / Q000502:physiology*; D010766:Phosphorylation; D011506:Proteins / Q000502:physiology; D038981:Receptors, Collagen; D017452:Receptors, IgG / Q000502:physiology; D010738:Type C Phospholipases / Q000502:physiology
D048868:Adaptor Proteins, Signal Transducing; D002352:Carrier Proteins; D003435:Crotalid Venoms; D002373:Cyclic AMP Receptor Protein; C106957:FYB1 protein, human; D051380:GRB2 Adaptor Protein; C495948:GRB2 protein, human; D016023:Integrins; D047908:Intracellular Signaling Peptides and Proteins; C110810:LAT protein, human; D037181:Lectins, C-Type; D008565:Membrane Proteins; D010750:Phosphoproteins; D011506:Proteins; D038981:Receptors, Collagen; D017452:Receptors, IgG; C092947:SLP-76 signal Transducing adaptor proteins; C116431:src kinase associated phosphoprotein 2; C028890:convulxin; D003094:Collagen; D010738:Type C Phospholipases