Protein disulphide isomerase in platelet function

Platelet protein disulphide isomerase (PDI) has a role in platelet aggregation, probably targeting a thiol-containing platelet surface protein. The thiol-containing P2Y(12) ADP receptor is involved in aggregation induced by most agonists and may be the target of PDI. By excluding the P2Y(12) pathway and using the anti-PDI antibody RL90 this study showed that PDI targets a non-P2Y(12) thiol-protein in aggregation. Anti-PDI inhibited signalling-independent activation of the thiol-containing fibrinogen receptor alpha IIb beta 3 by Mn2+, suggesting that PDI directly interacts with alpha IIb beta 3. The thiol-containing form of PDI increased on the platelet surface with platelet activation, suggesting that active PDI readily becomes available for redox regulation of alpha IIb beta 3. Finally, using purified proteins PDI had greater ability to isomerize disulphide bonds than the alpha IIb beta 3 integrin, which also has PDI-like activity. In summary, a mechanism exists in platelets to increase the functional form of surface PDI and this PDI has a non-P2Y(12) target that may be alpha IIb beta 3.