In this study, we screened for protein disulfide isomerase (PDI)-binding proteins in bovine liver microsomes Under strict salt concentrations, using affinity column chromatography. One main hand observed Using SDS PAGE as identified its ERp57 (one of the PDI family proteins) by LC-MS/MS analysis. The K-D value of PDI binding to ERp57 was calculated as 5.46 x 10(6) M with the BIACORE system. The interactions between PDI and ERp57 Occurred specifically at their a and b domains. respectively, Interestingly, low concentrations of ERp57 enhanced the chaperone activity of PDI, while high concentrations interfered with chaperone activity. On the other hand, ERp57 did not affect the isomerase activity of PDI. Additionally following pre-incubation of ERp57 with calreticulin (CRT), decreased interactions were observed between ERp57 and PDI, and vice versa. Based oil the data. we propose that once ERp57 binds to PDI or CRT, the resultant complex inhibits further interactions. Therefore, ERp57 selectively forms a protein-folding complex with PDI or CRT in ER. © 2005 Elsevier Inc. All rights reserved.