Protein disulfide isomerase (PDI) and its degradation products were found i
n HepG2, COS-1, and CHO-K1 cells. Whether or not the products were formed t
hrough autodegradation of PDI was examined, since PDI contains the CGHC mot
if, which is the active center of proteolytic activity in ER-60 protease. C
ommercial bovine PDI was autodegraded to produce a trimmed PDI. In addition
, human recombinant PDI also had autodegradation activity. Mutant recombina
nt PDIs with CGHC motifs of which cysteine residues were replaced with seri
ne or alanine residues were prepared. However, they were not autodegraded,
suggesting the cysteine residues of motifs are necessary for autodegradatio
n.