Functional analysis of tunicamycin-inducible gene A polypeptide from Aspergillus niger.

Tunicamycin-inducible gene A polypeptide (TIGA) is a member of the protein disulfide isomerase (PDI) family and is suggested to facilitate the folding of nascent polypeptides. The functional properties of TIGA were investigated here. TIGA acted as an isomerase, catalyzing the refolding of denatured and reduced ribonuclease A. TIGA also exhibited chaperone activity in the refolding of denatured prochymosin but not in the refolding of glyceraldehyde 3-phosphate dehydrogenase (GAPDH), indicating that it had substrate specificity with respect to chaperone activity. Detailed study with a series of thioredoxin-motif (trx-motif) mutants revealed that the 2 trx-motifs of TIGA were not equal in activity. The N-terminal trx-motif was more active than the C-terminal trx-motif, and the first cysteine in each trx-motif was necessary for isomerase activity.