The mammalian protein disulphide-isomerase
(PDI) family encompasses several highly divergent proteins which
are involved in the processing
and maturation of secretory proteins in the ER by catalyzing the
rearrangement of disulphide bonds. PDI, which is an abundant protein
of the ER (>400uM), has a carboxy-terminal
retention signal sequence, KDEL, similar to that of BiP and Grp94. The PDI
proteins are characterized by the presence of one or more domains of ~95-110
amino acids related to the cytoplasmic protein thioredoxin. All but the PDI-D
subfamily are composed entirely of repeats of such domains, with at least one
domain containing and one domain lacking a redox-active-Cys-Xaa-Xaa-Cystetrapeptide.
In addition to their roles as redox catalysts
and isomerases, PDI proteins have other functions such as peptide
binding, cell
adhesion and perhaps chaperone activities. Platelet surface thiols
and disulphides play an important role in platelet responses. Catalytically
active PDI is found on platelet surfaces where it has been demonstrated
to mediate platelet aggregation and secretion possible by reducing
disulfide bonds thus leading to exposure of fibrinogen receptors
in platelets.
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