ORGANELLE MARKERS - Endoplasmic Reticulum

PDI

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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