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Mutations in domain a' of protein disulfide isomerase affect the folding pathway of bovine pancreatic ribonuclease A

¹ Dipartimento di Biochimica e Biotecnologie Mediche,
² Dipartimento di Chimica Organica e Biochimica, School of Biotechnological Sciences, Università degli Studi di Napoli Federico II, Napoli, Italy
³ CEINGE, Biotecnologie Avanzate, scrl, Napoli, Italy
⁴ Dipartimento di Chimica, Università degli Studi di Salerno, Italy
⁵ Department of Medical Biochemistry and Biophysics, Karolinska Institutet, Stockholm, Sweden
⁶ Collagen Research Unit, Biocenter Oulu and Department of Medical Biochemistry and Molecular Biology, University of Oulu, Finland

Reprint requests to: Margherita Ruoppolo, Dipartimento di Biochimica e Biotecnologie Mediche, Facoltà di Medicina e Chirurgia, Università degli Studi di Napoli Federico II, Via Sergio Pansini 5, Napoli I-80131 Italy; e-mail: ruoppolo{at}dbbm.unina.it; fax: +39-081-7462404.

Protein disulfide isomerase (PDI, EC 5.3.4.1), an enzyme and chaperone, catalyses disulfide bond formation and rearrangements in protein folding. It is also a subunit in two proteins, the enzyme collagen prolyl 4-hydroxylase and the microsomal triglyceride transfer protein. It consists of two catalytically active domains, a and a', and two inactive ones, b and b', all four domains having the thioredoxin fold. Domain b' contains the primary peptide binding site, but a' is also critical for several of the major PDI functions. Mass spectrometry was used here to follow the folding pathway of bovine pancreatic ribonuclease A (RNase A) in the presence of three PDI mutants, F449R, 455–457, and abb', and the individual domains a and a'. The first two mutants contained alterations in the last helix of domain a', while the third lacked the entire domain a'. All mutants produced genuine, correctly folded RNase A, but the appearance rate of 50% of the product, as compared to wild-type PDI, was reduced 2.5-fold in the case of PDI 455–457, 7.5-fold to eightfold in the cases of PDI F449R and PDI abb', and over 15-fold in the cases of the individual domains a and a'. In addition, PDI F449R and PDI abb' affected the distribution of folding intermediates. Domains a and a' catalyzed the early steps in the folding but no disulfide rearrangements, and therefore the rate observed in the presence of these individual domains was similar to that of the spontaneous process.

Keywords: Mass spectrometry; PDI; protein folding; RNase A

Abbreviations: PDI, protein disulfide isomerase • F449R, PDI containing a point mutation F449R • 455–457, PDI lacking amino acids 455–457 • PDI abb', PDI lacking domain a' and the C-terminal extension c • RNase A, bovine pancreatic ribonuclease A • CD, circular dichroism • ESIMS, electrospray ionisation mass spectrometry • BPTI, bovine pancreatic trypsin inhibitor • GSH, reduced glutathione • GSSG, oxidized glutathione • IAM, iodoacetamide

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