Purification and characterization of protein disulphide-isomerase from the unicellular green alga Chlamydomonas reinhardii. A 120 kDa dimer antigenically distinct from the vertebrate enzyme.

Protein disulphide-isomerase (PDI) has been isolated from the unicellular green alga Chlamydomonas reinhardii and purified by (NH4)2SO4 precipitation, gel filtration and DEAE-Sephacel, hydroxyapatite and f.p.l.c. chromatography. The active algal enzyme is a 120 kDa dimer with a subunit molecular mass of 60 kDa when determined by SDS/PAGE. Although similar in size to the previously isolated vertebrate PDIs, the algal enzyme is antigenically distinct, polyclonal antibodies against the algal PDI showing no cross-reactivity with the vertebrate enzyme on immunoblots, and vice versa. The anti-(algal PDI) antiserum did not inhibit algal PDI activity, and C. reinhardii PDI could be immobilized on anti-PDI-Protein A-Sepharose in active form. In contrast with the situation in vertebrates, where PDI functions as a subunit of prolyl 4-hydroxylase, the C. reinhardii PDI is not associated with the algal prolyl 4-hydroxylase.